Substantially modernize the "arc land" workflow

Summary: Ref T13546. This has a lot of dangerously rough edges, but has managed to land at least one commit in each Git and Mercurial.

Test Plan:
  - Landed one commit under ideal conditions in Git and Mercurial.
  - See followups.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21315

src/__phutil_library_map__.php

@@ -218,7 +218,7 @@ phutil_register_library_map(array(
     'ArcanistGitAPI' => 'repository/api/ArcanistGitAPI.php',
     'ArcanistGitCommitMessageHardpointQuery' => 'query/ArcanistGitCommitMessageHardpointQuery.php',
     'ArcanistGitCommitSymbolCommitHardpointQuery' => 'ref/commit/ArcanistGitCommitSymbolCommitHardpointQuery.php',
-    'ArcanistGitLandEngine' => 'land/ArcanistGitLandEngine.php',
+    'ArcanistGitLandEngine' => 'land/engine/ArcanistGitLandEngine.php',
     'ArcanistGitLocalState' => 'repository/state/ArcanistGitLocalState.php',
     'ArcanistGitUpstreamPath' => 'repository/api/ArcanistGitUpstreamPath.php',
     'ArcanistGitWorkingCopy' => 'workingcopy/ArcanistGitWorkingCopy.php',
@@ -288,7 +288,11 @@ phutil_register_library_map(array(
     'ArcanistKeywordCasingXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistKeywordCasingXHPASTLinterRuleTestCase.php',
     'ArcanistLambdaFuncFunctionXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistLambdaFuncFunctionXHPASTLinterRule.php',
     'ArcanistLambdaFuncFunctionXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistLambdaFuncFunctionXHPASTLinterRuleTestCase.php',
-    'ArcanistLandEngine' => 'land/ArcanistLandEngine.php',
+    'ArcanistLandCommit' => 'land/ArcanistLandCommit.php',
+    'ArcanistLandCommitSet' => 'land/ArcanistLandCommitSet.php',
+    'ArcanistLandEngine' => 'land/engine/ArcanistLandEngine.php',
+    'ArcanistLandSymbol' => 'land/ArcanistLandSymbol.php',
+    'ArcanistLandTarget' => 'land/ArcanistLandTarget.php',
     'ArcanistLandWorkflow' => 'workflow/ArcanistLandWorkflow.php',
     'ArcanistLanguageConstructParenthesesXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistLanguageConstructParenthesesXHPASTLinterRule.php',
     'ArcanistLanguageConstructParenthesesXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistLanguageConstructParenthesesXHPASTLinterRuleTestCase.php',
@@ -320,9 +324,13 @@ phutil_register_library_map(array(
     'ArcanistLowercaseFunctionsXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistLowercaseFunctionsXHPASTLinterRule.php',
     'ArcanistLowercaseFunctionsXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistLowercaseFunctionsXHPASTLinterRuleTestCase.php',
     'ArcanistMercurialAPI' => 'repository/api/ArcanistMercurialAPI.php',
+    'ArcanistMercurialCommitMessageHardpointQuery' => 'query/ArcanistMercurialCommitMessageHardpointQuery.php',
+    'ArcanistMercurialLandEngine' => 'land/engine/ArcanistMercurialLandEngine.php',
+    'ArcanistMercurialLocalState' => 'repository/state/ArcanistMercurialLocalState.php',
     'ArcanistMercurialParser' => 'repository/parser/ArcanistMercurialParser.php',
     'ArcanistMercurialParserTestCase' => 'repository/parser/__tests__/ArcanistMercurialParserTestCase.php',
     'ArcanistMercurialWorkingCopy' => 'workingcopy/ArcanistMercurialWorkingCopy.php',
+    'ArcanistMercurialWorkingCopyRevisionHardpointQuery' => 'query/ArcanistMercurialWorkingCopyRevisionHardpointQuery.php',
     'ArcanistMergeConflictLinter' => 'lint/linter/ArcanistMergeConflictLinter.php',
     'ArcanistMergeConflictLinterTestCase' => 'lint/linter/__tests__/ArcanistMergeConflictLinterTestCase.php',
     'ArcanistMessageRevisionHardpointQuery' => 'query/ArcanistMessageRevisionHardpointQuery.php',
@@ -524,6 +532,7 @@ phutil_register_library_map(array(
     'ArcanistWorkflowArgument' => 'toolset/ArcanistWorkflowArgument.php',
     'ArcanistWorkflowGitHardpointQuery' => 'query/ArcanistWorkflowGitHardpointQuery.php',
     'ArcanistWorkflowInformation' => 'toolset/ArcanistWorkflowInformation.php',
+    'ArcanistWorkflowMercurialHardpointQuery' => 'query/ArcanistWorkflowMercurialHardpointQuery.php',
     'ArcanistWorkingCopy' => 'workingcopy/ArcanistWorkingCopy.php',
     'ArcanistWorkingCopyCommitHardpointQuery' => 'query/ArcanistWorkingCopyCommitHardpointQuery.php',
     'ArcanistWorkingCopyConfigurationSource' => 'config/source/ArcanistWorkingCopyConfigurationSource.php',
@@ -1298,8 +1307,12 @@ phutil_register_library_map(array(
     'ArcanistKeywordCasingXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistLambdaFuncFunctionXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistLambdaFuncFunctionXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
+    'ArcanistLandCommit' => 'Phobject',
+    'ArcanistLandCommitSet' => 'Phobject',
     'ArcanistLandEngine' => 'Phobject',
-    'ArcanistLandWorkflow' => 'ArcanistWorkflow',
+    'ArcanistLandSymbol' => 'Phobject',
+    'ArcanistLandTarget' => 'Phobject',
+    'ArcanistLandWorkflow' => 'ArcanistArcWorkflow',
     'ArcanistLanguageConstructParenthesesXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistLanguageConstructParenthesesXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistLesscLinter' => 'ArcanistExternalLinter',
@@ -1330,9 +1343,13 @@ phutil_register_library_map(array(
     'ArcanistLowercaseFunctionsXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistLowercaseFunctionsXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistMercurialAPI' => 'ArcanistRepositoryAPI',
+    'ArcanistMercurialCommitMessageHardpointQuery' => 'ArcanistWorkflowMercurialHardpointQuery',
+    'ArcanistMercurialLandEngine' => 'ArcanistLandEngine',
+    'ArcanistMercurialLocalState' => 'ArcanistRepositoryLocalState',
     'ArcanistMercurialParser' => 'Phobject',
     'ArcanistMercurialParserTestCase' => 'PhutilTestCase',
     'ArcanistMercurialWorkingCopy' => 'ArcanistWorkingCopy',
+    'ArcanistMercurialWorkingCopyRevisionHardpointQuery' => 'ArcanistWorkflowMercurialHardpointQuery',
     'ArcanistMergeConflictLinter' => 'ArcanistLinter',
     'ArcanistMergeConflictLinterTestCase' => 'ArcanistLinterTestCase',
     'ArcanistMessageRevisionHardpointQuery' => 'ArcanistRuntimeHardpointQuery',
@@ -1544,6 +1561,7 @@ phutil_register_library_map(array(
     'ArcanistWorkflowArgument' => 'Phobject',
     'ArcanistWorkflowGitHardpointQuery' => 'ArcanistRuntimeHardpointQuery',
     'ArcanistWorkflowInformation' => 'Phobject',
+    'ArcanistWorkflowMercurialHardpointQuery' => 'ArcanistRuntimeHardpointQuery',
     'ArcanistWorkingCopy' => 'Phobject',
     'ArcanistWorkingCopyCommitHardpointQuery' => 'ArcanistRuntimeHardpointQuery',
     'ArcanistWorkingCopyConfigurationSource' => 'ArcanistFilesystemConfigurationSource',

src/config/arc/ArcanistArcConfigurationEngineExtension.php

@@ -136,19 +136,15 @@ final class ArcanistArcConfigurationEngineExtension
           array(
             'origin',
           )),
-      id(new ArcanistBoolConfigOption())
+      id(new ArcanistStringConfigOption())
-        ->setKey('history.immutable')
+        ->setKey('arc.land.strategy')
         ->setSummary(
           pht(
-            'Configure use of history mutation operations like amends '.
+            'Configure a default merge strategy for "arc land".'))
-            'and rebases.'))
         ->setHelp(
           pht(
-            'If this option is set to "true", Arcanist will treat the '.
+            'Specifies the default behavior when "arc land" is run with '.
-            'repository history as immutable and will never issue '.
+            'no "--strategy" flag.')),
-            'commands which rewrite repository history (like amends or '.
-            'rebases). This option defaults to "true" in Mercurial, '.
-            '"false" in Git, and has no effect in Subversion.')),
     );
   }
 

src/land/ArcanistGitLandEngine.php

@@ -1,913 +0,0 @@
-<?php
-
-final class ArcanistGitLandEngine
-  extends ArcanistLandEngine {
-
-  private $localRef;
-  private $localCommit;
-  private $sourceCommit;
-  private $mergedRef;
-  private $restoreWhenDestroyed;
-  private $isGitPerforce;
-
-  private function setIsGitPerforce($is_git_perforce) {
-    $this->isGitPerforce = $is_git_perforce;
-    return $this;
-  }
-
-  private function getIsGitPerforce() {
-    return $this->isGitPerforce;
-  }
-
-  public function parseArguments() {
-    $api = $this->getRepositoryAPI();
-
-    $onto = $this->getEngineOnto();
-    $this->setTargetOnto($onto);
-
-    $remote = $this->getEngineRemote();
-
-    $is_pushable = $api->isPushableRemote($remote);
-    $is_perforce = $api->isPerforceRemote($remote);
-
-    if (!$is_pushable && !$is_perforce) {
-      throw new PhutilArgumentUsageException(
-        pht(
-          'No pushable remote "%s" exists. Use the "--remote" flag to choose '.
-          'a valid, pushable remote to land changes onto.',
-          $remote));
-    }
-
-    if ($is_perforce) {
-      $this->setIsGitPerforce(true);
-      $this->writeWarn(
-        pht('P4 MODE'),
-        pht(
-          'Operating in Git/Perforce mode after selecting a Perforce '.
-          'remote.'));
-
-      if (!$this->getShouldSquash()) {
-        throw new PhutilArgumentUsageException(
-          pht(
-            'Perforce mode does not support the "merge" land strategy. '.
-            'Use the "squash" land strategy when landing to a Perforce '.
-            'remote (you can use "--squash" to select this strategy).'));
-      }
-    }
-
-    $this->setTargetRemote($remote);
-  }
-
-  public function execute() {
-    $this->verifySourceAndTargetExist();
-    $this->fetchTarget();
-
-    $this->printLandingCommits();
-
-    if ($this->getShouldPreview()) {
-      $this->writeInfo(
-        pht('PREVIEW'),
-        pht('Completed preview of operation.'));
-      return;
-    }
-
-    $this->saveLocalState();
-
-    try {
-      $this->identifyRevision();
-      $this->updateWorkingCopy();
-
-      if ($this->getShouldHold()) {
-        $this->didHoldChanges();
-      } else {
-        $this->pushChange();
-        $this->reconcileLocalState();
-
-        $api = $this->getRepositoryAPI();
-        $api->execxLocal('submodule update --init --recursive');
-
-        if ($this->getShouldKeep()) {
-          echo tsprintf(
-            "%s\n",
-            pht('Keeping local branch.'));
-        } else {
-          $this->destroyLocalBranch();
-        }
-
-        $this->writeOkay(
-          pht('DONE'),
-          pht('Landed changes.'));
-      }
-
-      $this->restoreWhenDestroyed = false;
-    } catch (Exception $ex) {
-      $this->restoreLocalState();
-      throw $ex;
-    }
-  }
-
-  public function __destruct() {
-    if ($this->restoreWhenDestroyed) {
-      $this->writeWarn(
-        pht('INTERRUPTED!'),
-        pht('Restoring working copy to its original state.'));
-
-      $this->restoreLocalState();
-    }
-  }
-
-  protected function getLandingCommits() {
-    $api = $this->getRepositoryAPI();
-
-    list($out) = $api->execxLocal(
-      'log --oneline %s..%s --',
-      $this->getTargetFullRef(),
-      $this->sourceCommit);
-
-    $out = trim($out);
-
-    if (!strlen($out)) {
-      return array();
-    } else {
-      return phutil_split_lines($out, false);
-    }
-  }
-
-  private function identifyRevision() {
-    $api = $this->getRepositoryAPI();
-    $api->execxLocal('checkout %s --', $this->getSourceRef());
-    call_user_func($this->getBuildMessageCallback(), $this);
-  }
-
-  private function verifySourceAndTargetExist() {
-    $api = $this->getRepositoryAPI();
-
-    list($err) = $api->execManualLocal(
-      'rev-parse --verify %s',
-      $this->getTargetFullRef());
-
-    if ($err) {
-      $this->writeWarn(
-        pht('TARGET'),
-        pht(
-          'No local ref exists for branch "%s" in remote "%s", attempting '.
-          'fetch...',
-          $this->getTargetOnto(),
-          $this->getTargetRemote()));
-
-      $api->execManualLocal(
-        'fetch %s %s --',
-        $this->getTargetRemote(),
-        $this->getTargetOnto());
-
-      list($err) = $api->execManualLocal(
-        'rev-parse --verify %s',
-        $this->getTargetFullRef());
-      if ($err) {
-        throw new Exception(
-          pht(
-            'Branch "%s" does not exist in remote "%s".',
-            $this->getTargetOnto(),
-            $this->getTargetRemote()));
-      }
-
-      $this->writeInfo(
-        pht('FETCHED'),
-        pht(
-          'Fetched branch "%s" from remote "%s".',
-          $this->getTargetOnto(),
-          $this->getTargetRemote()));
-    }
-
-    list($err, $stdout) = $api->execManualLocal(
-      'rev-parse --verify %s',
-      $this->getSourceRef());
-
-    if ($err) {
-      throw new Exception(
-        pht(
-          'Branch "%s" does not exist in the local working copy.',
-          $this->getSourceRef()));
-    }
-
-    $this->sourceCommit = trim($stdout);
-  }
-
-  private function fetchTarget() {
-    $api = $this->getRepositoryAPI();
-
-    $ref = $this->getTargetFullRef();
-
-    // NOTE: Although this output isn't hugely useful, we need to passthru
-    // instead of using a subprocess here because `git fetch` may prompt the
-    // user to enter a password if they're fetching over HTTP with basic
-    // authentication. See T10314.
-
-    if ($this->getIsGitPerforce()) {
-      $this->writeInfo(
-        pht('P4 SYNC'),
-        pht('Synchronizing "%s" from Perforce...', $ref));
-
-      $sync_ref = sprintf(
-        'refs/remotes/%s/%s',
-        $this->getTargetRemote(),
-        $this->getTargetOnto());
-
-      $err = $api->execPassthru(
-        'p4 sync --silent --branch %R --',
-        $sync_ref);
-
-      if ($err) {
-        throw new ArcanistUsageException(
-          pht(
-            'Perforce sync failed! Fix the error and run "arc land" again.'));
-      }
-    } else {
-      $this->writeInfo(
-        pht('FETCH'),
-        pht('Fetching "%s"...', $ref));
-
-      $err = $api->execPassthru(
-        'fetch --quiet -- %s %s',
-        $this->getTargetRemote(),
-        $this->getTargetOnto());
-
-      if ($err) {
-        throw new ArcanistUsageException(
-          pht(
-            'Fetch failed! Fix the error and run "arc land" again.'));
-      }
-    }
-  }
-
-  private function updateWorkingCopy() {
-    $api = $this->getRepositoryAPI();
-    $source = $this->sourceCommit;
-
-    $api->execxLocal(
-      'checkout %s --',
-      $this->getTargetFullRef());
-
-    list($original_author, $original_date) = $this->getAuthorAndDate($source);
-
-    try {
-      if ($this->getShouldSquash()) {
-        // NOTE: We're explicitly specifying "--ff" to override the presence
-        // of "merge.ff" options in user configuration.
-
-        $api->execxLocal(
-          'merge --no-stat --no-commit --ff --squash -- %s',
-          $source);
-      } else {
-        $api->execxLocal(
-          'merge --no-stat --no-commit --no-ff -- %s',
-          $source);
-      }
-    } catch (Exception $ex) {
-      $api->execManualLocal('merge --abort');
-      $api->execManualLocal('reset --hard HEAD --');
-
-      throw new Exception(
-        pht(
-          'Local "%s" does not merge cleanly into "%s". Merge or rebase '.
-          'local changes so they can merge cleanly.',
-          $this->getSourceRef(),
-          $this->getTargetFullRef()));
-    }
-
-    // TODO: This could probably be cleaner by asking the API a question
-    // about working copy status instead of running a raw diff command. See
-    // discussion in T11435.
-    list($changes) = $api->execxLocal('diff --no-ext-diff HEAD --');
-    $changes = trim($changes);
-    if (!strlen($changes)) {
-      throw new Exception(
-        pht(
-          'Merging local "%s" into "%s" produces an empty diff. '.
-          'This usually means these changes have already landed.',
-          $this->getSourceRef(),
-          $this->getTargetFullRef()));
-    }
-
-    $api->execxLocal(
-      'commit --author %s --date %s -F %s --',
-      $original_author,
-      $original_date,
-      $this->getCommitMessageFile());
-
-    $this->getWorkflow()->didCommitMerge();
-
-    list($stdout) = $api->execxLocal(
-      'rev-parse --verify %s',
-      'HEAD');
-    $this->mergedRef = trim($stdout);
-  }
-
-  private function pushChange() {
-    $api = $this->getRepositoryAPI();
-
-    if ($this->getIsGitPerforce()) {
-      $this->writeInfo(
-        pht('SUBMITTING'),
-        pht('Submitting changes to "%s".', $this->getTargetFullRef()));
-
-      $config_argv = array();
-
-      // Skip the "git p4 submit" interactive editor workflow. We expect
-      // the commit message that "arc land" has built to be satisfactory.
-      $config_argv[] = '-c';
-      $config_argv[] = 'git-p4.skipSubmitEdit=true';
-
-      // Skip the "git p4 submit" confirmation prompt if the user does not edit
-      // the submit message.
-      $config_argv[] = '-c';
-      $config_argv[] = 'git-p4.skipSubmitEditCheck=true';
-
-      $flags_argv = array();
-
-      // Disable implicit "git p4 rebase" as part of submit. We're allowing
-      // the implicit "git p4 sync" to go through since this puts us in a
-      // state which is generally similar to the state after "git push", with
-      // updated remotes.
-
-      // We could do a manual "git p4 sync" with a more narrow "--branch"
-      // instead, but it's not clear that this is beneficial.
-      $flags_argv[] = '--disable-rebase';
-
-      // Detect moves and submit them to Perforce as move operations.
-      $flags_argv[] = '-M';
-
-      // If we run into a conflict, abort the operation. We expect users to
-      // fix conflicts and run "arc land" again.
-      $flags_argv[] = '--conflict=quit';
-
-      $err = $api->execPassthru(
-        '%LR p4 submit %LR --commit %R --',
-        $config_argv,
-        $flags_argv,
-        $this->mergedRef);
-
-      if ($err) {
-        throw new ArcanistUsageException(
-          pht(
-            'Submit failed! Fix the error and run "arc land" again.'));
-      }
-    } else {
-      $this->writeInfo(
-        pht('PUSHING'),
-        pht('Pushing changes to "%s".', $this->getTargetFullRef()));
-
-      $err = $api->execPassthru(
-        'push -- %s %s:%s',
-        $this->getTargetRemote(),
-        $this->mergedRef,
-        $this->getTargetOnto());
-
-      if ($err) {
-        throw new ArcanistUsageException(
-          pht(
-            'Push failed! Fix the error and run "arc land" again.'));
-      }
-    }
-  }
-
-  private function reconcileLocalState() {
-    $api = $this->getRepositoryAPI();
-
-    // Try to put the user into the best final state we can. This is very
-    // complicated because users are incredibly creative and their local
-    // branches may have the same names as branches in the remote but no
-    // relationship to them.
-
-    if ($this->localRef != $this->getSourceRef()) {
-      // The user ran `arc land X` but was on a different branch, so just put
-      // them back wherever they were before.
-      $this->writeInfo(
-        pht('RESTORE'),
-        pht('Switching back to "%s".', $this->localRef));
-      $this->restoreLocalState();
-      return;
-    }
-
-    // We're going to try to find a path to the upstream target branch. We
-    // try in two different ways:
-    //
-    //   - follow the source branch directly along tracking branches until
-    //     we reach the upstream; or
-    //   - follow a local branch with the same name as the target branch until
-    //     we reach the upstream.
-
-    // First, get the path from whatever we landed to wherever it goes.
-    $local_branch = $this->getSourceRef();
-
-    $path = $api->getPathToUpstream($local_branch);
-    if ($path->getLength()) {
-      // We may want to discard the thing we landed from the path, if we're
-      // going to delete it. In this case, we don't want to update it or worry
-      // if it's dirty.
-      if ($this->getSourceRef() == $this->getTargetOnto()) {
-        // In this case, we've done something like land "master" onto itself,
-        // so we do want to update the actual branch. We're going to use the
-        // entire path.
-      } else {
-        // Otherwise, we're going to delete the branch at the end of the
-        // workflow, so throw it away the most-local branch that isn't long
-        // for this world.
-        $path->removeUpstream($local_branch);
-
-        if (!$path->getLength()) {
-          // The local branch tracked upstream directly; however, it
-          // may not be the only one to do so.  If there's a local
-          // branch of the same name that tracks the remote, try
-          // switching to that.
-          $local_branch = $this->getTargetOnto();
-          list($err) = $api->execManualLocal(
-            'rev-parse --verify %s',
-            $local_branch);
-          if (!$err) {
-            $path = $api->getPathToUpstream($local_branch);
-          }
-          if (!$path->isConnectedToRemote()) {
-            $this->writeInfo(
-              pht('UPDATE'),
-              pht(
-                'Local branch "%s" directly tracks remote, staying on '.
-                'detached HEAD.',
-                $local_branch));
-            return;
-          }
-        }
-
-        $local_branch = head($path->getLocalBranches());
-      }
-    } else {
-      // The source branch has no upstream, so look for a local branch with
-      // the same name as the target branch. This corresponds to the common
-      // case where you have "master" and checkout local branches from it
-      // with "git checkout -b feature", then land onto "master".
-
-      $local_branch = $this->getTargetOnto();
-
-      list($err) = $api->execManualLocal(
-        'rev-parse --verify %s',
-        $local_branch);
-      if ($err) {
-        $this->writeInfo(
-          pht('UPDATE'),
-          pht(
-            'Local branch "%s" does not exist, staying on detached HEAD.',
-            $local_branch));
-        return;
-      }
-
-      $path = $api->getPathToUpstream($local_branch);
-    }
-
-    if ($path->getCycle()) {
-      $this->writeWarn(
-        pht('LOCAL CYCLE'),
-        pht(
-          'Local branch "%s" tracks an upstream but following it leads to '.
-          'a local cycle, staying on detached HEAD.',
-          $local_branch));
-      return;
-    }
-
-    $is_perforce = $this->getIsGitPerforce();
-
-    if ($is_perforce) {
-      // If we're in Perforce mode, we don't expect to have a meaningful
-      // path to the remote: the "p4" remote is not a real remote, and
-      // "git p4" commands do not configure branch upstreams to provide
-      // a path.
-
-      // Just pretend the target branch is connected directly to the remote,
-      // since this is effectively the behavior of Perforce and appears to
-      // do the right thing.
-      $cascade_branches = array($local_branch);
-    } else {
-      if (!$path->isConnectedToRemote()) {
-        $this->writeInfo(
-          pht('UPDATE'),
-          pht(
-            'Local branch "%s" is not connected to a remote, staying on '.
-            'detached HEAD.',
-            $local_branch));
-        return;
-      }
-
-      $remote_remote = $path->getRemoteRemoteName();
-      $remote_branch = $path->getRemoteBranchName();
-
-      $remote_actual = $remote_remote.'/'.$remote_branch;
-      $remote_expect = $this->getTargetFullRef();
-      if ($remote_actual != $remote_expect) {
-        $this->writeInfo(
-          pht('UPDATE'),
-          pht(
-            'Local branch "%s" is connected to a remote ("%s") other than '.
-            'the target remote ("%s"), staying on detached HEAD.',
-            $local_branch,
-            $remote_actual,
-            $remote_expect));
-        return;
-      }
-
-      // If we get this far, we have a sequence of branches which ultimately
-      // connect to the remote. We're going to try to update them all in reverse
-      // order, from most-upstream to most-local.
-
-      $cascade_branches = $path->getLocalBranches();
-      $cascade_branches = array_reverse($cascade_branches);
-    }
-
-    // First, check if any of them are ahead of the remote.
-
-    $ahead_of_remote = array();
-    foreach ($cascade_branches as $cascade_branch) {
-      list($stdout) = $api->execxLocal(
-        'log %s..%s --',
-        $this->mergedRef,
-        $cascade_branch);
-      $stdout = trim($stdout);
-
-      if (strlen($stdout)) {
-        $ahead_of_remote[$cascade_branch] = $cascade_branch;
-      }
-    }
-
-    // We're going to handle the last branch (the thing we ultimately intend
-    // to check out) differently. It's OK if it's ahead of the remote, as long
-    // as we just landed it.
-
-    $local_ahead = isset($ahead_of_remote[$local_branch]);
-    unset($ahead_of_remote[$local_branch]);
-    $land_self = ($this->getTargetOnto() === $this->getSourceRef());
-
-    // We aren't going to pull anything if anything upstream from us is ahead
-    // of the remote, or the local is ahead of the remote and we didn't land
-    // it onto itself.
-    $skip_pull = ($ahead_of_remote || ($local_ahead && !$land_self));
-
-    if ($skip_pull) {
-      $this->writeInfo(
-        pht('UPDATE'),
-        pht(
-          'Local "%s" is ahead of remote "%s". Checking out "%s" but '.
-          'not pulling changes.',
-          nonempty(head($ahead_of_remote), $local_branch),
-          $this->getTargetFullRef(),
-          $local_branch));
-
-      $this->writeInfo(
-        pht('CHECKOUT'),
-        pht(
-          'Checking out "%s".',
-          $local_branch));
-
-      $api->execxLocal('checkout %s --', $local_branch);
-
-      return;
-    }
-
-    // If nothing upstream from our nearest branch is ahead of the remote,
-    // pull it all.
-
-    $cascade_targets = array();
-    if (!$ahead_of_remote) {
-      foreach ($cascade_branches as $cascade_branch) {
-        if ($local_ahead && ($local_branch == $cascade_branch)) {
-          continue;
-        }
-        $cascade_targets[] = $cascade_branch;
-      }
-    }
-
-    if ($is_perforce) {
-      // In Perforce, we've already set the remote to the right state with an
-      // implicit "git p4 sync" during "git p4 submit", and "git pull" isn't a
-      // meaningful operation. We're going to skip this step and jump down to
-      // the "git reset --hard" below to get everything into the right state.
-    } else if ($cascade_targets) {
-      $this->writeInfo(
-        pht('UPDATE'),
-        pht(
-          'Local "%s" tracks target remote "%s", checking out and '.
-          'pulling changes.',
-          $local_branch,
-          $this->getTargetFullRef()));
-
-      foreach ($cascade_targets as $cascade_branch) {
-        $this->writeInfo(
-          pht('PULL'),
-          pht(
-            'Checking out and pulling "%s".',
-            $cascade_branch));
-
-        $api->execxLocal('checkout %s --', $cascade_branch);
-        $api->execxLocal(
-          'pull %s %s --',
-          $this->getTargetRemote(),
-          $cascade_branch);
-      }
-
-      if (!$local_ahead) {
-        return;
-      }
-    }
-
-    // In this case, the user did something like land a branch onto itself,
-    // and the branch is tracking the correct remote. We're going to discard
-    // the local state and reset it to the state we just pushed.
-
-    $this->writeInfo(
-      pht('RESET'),
-      pht(
-        'Local "%s" landed into remote "%s", resetting local branch to '.
-        'remote state.',
-        $this->getTargetOnto(),
-        $this->getTargetFullRef()));
-
-    $api->execxLocal('checkout %s --', $local_branch);
-    $api->execxLocal('reset --hard %s --', $this->getTargetFullRef());
-
-    return;
-  }
-
-  private function destroyLocalBranch() {
-    $api = $this->getRepositoryAPI();
-    $source_ref = $this->getSourceRef();
-
-    if ($source_ref == $this->getTargetOnto()) {
-      // If we landed a branch into a branch with the same name, so don't
-      // destroy it. This prevents us from cleaning up "master" if you're
-      // landing master into itself.
-      return;
-    }
-
-    // TODO: Maybe this should also recover the proper upstream?
-
-    // See T10321. If we were not landing a branch, don't try to clean it up.
-    // This happens most often when landing from a detached HEAD.
-    $is_branch = $this->isBranch($source_ref);
-    if (!$is_branch) {
-      echo tsprintf(
-        "%s\n",
-        pht(
-          '(Source "%s" is not a branch, leaving working copy as-is.)',
-          $source_ref));
-      return;
-    }
-
-    $recovery_command = csprintf(
-      'git checkout -b %R %R',
-      $source_ref,
-      $this->sourceCommit);
-
-    echo tsprintf(
-      "%s\n",
-      pht('Cleaning up branch "%s"...', $source_ref));
-
-    echo tsprintf(
-      "%s\n",
-      pht('(Use `%s` if you want it back.)', $recovery_command));
-
-    $api->execxLocal('branch -D -- %s', $source_ref);
-  }
-
-  /**
-   * Save the local working copy state so we can restore it later.
-   */
-  private function saveLocalState() {
-    $api = $this->getRepositoryAPI();
-
-    $this->localCommit = $api->getWorkingCopyRevision();
-
-    list($ref) = $api->execxLocal('rev-parse --abbrev-ref HEAD');
-    $ref = trim($ref);
-    if ($ref === 'HEAD') {
-      $ref = $this->localCommit;
-    }
-
-    $this->localRef = $ref;
-
-    $this->restoreWhenDestroyed = true;
-  }
-
-  /**
-   * Restore the working copy to the state it was in before we started
-   * performing writes.
-   */
-  private function restoreLocalState() {
-    $api = $this->getRepositoryAPI();
-
-    $api->execxLocal('checkout %s --', $this->localRef);
-    $api->execxLocal('reset --hard %s --', $this->localCommit);
-    $api->execxLocal('submodule update --init --recursive');
-
-    $this->restoreWhenDestroyed = false;
-  }
-
-  private function getTargetFullRef() {
-    return $this->getTargetRemote().'/'.$this->getTargetOnto();
-  }
-
-  private function getAuthorAndDate($commit) {
-    $api = $this->getRepositoryAPI();
-
-    // TODO: This is working around Windows escaping problems, see T8298.
-
-    list($info) = $api->execxLocal(
-      'log -n1 --format=%C %s --',
-      '%aD%n%an%n%ae',
-      $commit);
-
-    $info = trim($info);
-    list($date, $author, $email) = explode("\n", $info, 3);
-
-    return array(
-      "$author <{$email}>",
-      $date,
-    );
-  }
-
-  private function didHoldChanges() {
-    if ($this->getIsGitPerforce()) {
-      $this->writeInfo(
-        pht('HOLD'),
-        pht(
-          'Holding change locally, it has not been submitted.'));
-
-      $push_command = csprintf(
-        '$ git p4 submit -M --commit %R --',
-        $this->mergedRef);
-    } else {
-      $this->writeInfo(
-        pht('HOLD'),
-        pht(
-          'Holding change locally, it has not been pushed.'));
-
-      $push_command = csprintf(
-        '$ git push -- %R %R:%R',
-        $this->getTargetRemote(),
-        $this->mergedRef,
-        $this->getTargetOnto());
-    }
-
-    $restore_command = csprintf(
-      '$ git checkout %R --',
-      $this->localRef);
-
-    echo tsprintf(
-      "\n%s\n\n".
-      "%s\n\n".
-      "    **%s**\n\n".
-      "%s\n\n".
-      "    **%s**\n\n".
-      "%s\n",
-      pht(
-        'This local working copy now contains the merged changes in a '.
-        'detached state.'),
-      pht('You can push the changes manually with this command:'),
-      $push_command,
-      pht(
-        'You can go back to how things were before you ran "arc land" with '.
-        'this command:'),
-      $restore_command,
-      pht(
-        'Local branches have not been changed, and are still in exactly the '.
-        'same state as before.'));
-  }
-
-  private function isBranch($ref) {
-    $api = $this->getRepositoryAPI();
-
-    list($err) = $api->execManualLocal(
-      'show-ref --verify --quiet -- %R',
-      'refs/heads/'.$ref);
-
-    return !$err;
-  }
-
-  private function getEngineOnto() {
-    $source_ref = $this->getSourceRef();
-
-    $onto = $this->getOntoArgument();
-    if ($onto !== null) {
-      $this->writeInfo(
-        pht('TARGET'),
-        pht(
-          'Landing onto "%s", selected with the "--onto" flag.',
-          $onto));
-      return $onto;
-    }
-
-    $api = $this->getRepositoryAPI();
-    $path = $api->getPathToUpstream($source_ref);
-
-    if ($path->getLength()) {
-      $cycle = $path->getCycle();
-      if ($cycle) {
-        $this->writeWarn(
-          pht('LOCAL CYCLE'),
-          pht(
-            'Local branch tracks an upstream, but following it leads to a '.
-            'local cycle; ignoring branch upstream.'));
-
-        echo tsprintf(
-          "\n    %s\n\n",
-          implode(' -> ', $cycle));
-
-      } else {
-        if ($path->isConnectedToRemote()) {
-          $onto = $path->getRemoteBranchName();
-          $this->writeInfo(
-            pht('TARGET'),
-            pht(
-              'Landing onto "%s", selected by following tracking branches '.
-              'upstream to the closest remote.',
-              $onto));
-          return $onto;
-        } else {
-          $this->writeInfo(
-            pht('NO PATH TO UPSTREAM'),
-            pht(
-              'Local branch tracks an upstream, but there is no path '.
-              'to a remote; ignoring branch upstream.'));
-        }
-      }
-    }
-
-    $workflow = $this->getWorkflow();
-
-    $config_key = 'arc.land.onto.default';
-    $onto = $workflow->getConfigFromAnySource($config_key);
-    if ($onto !== null) {
-      $this->writeInfo(
-        pht('TARGET'),
-        pht(
-          'Landing onto "%s", selected by "%s" configuration.',
-          $onto,
-          $config_key));
-      return $onto;
-    }
-
-    $onto = 'master';
-    $this->writeInfo(
-      pht('TARGET'),
-      pht(
-        'Landing onto "%s", the default target under git.',
-        $onto));
-
-    return $onto;
-  }
-
-  private function getEngineRemote() {
-    $source_ref = $this->getSourceRef();
-
-    $remote = $this->getRemoteArgument();
-    if ($remote !== null) {
-      $this->writeInfo(
-        pht('REMOTE'),
-        pht(
-          'Using remote "%s", selected with the "--remote" flag.',
-          $remote));
-      return $remote;
-    }
-
-    $api = $this->getRepositoryAPI();
-    $path = $api->getPathToUpstream($source_ref);
-
-    $remote = $path->getRemoteRemoteName();
-    if ($remote !== null) {
-      $this->writeInfo(
-        pht('REMOTE'),
-        pht(
-          'Using remote "%s", selected by following tracking branches '.
-          'upstream to the closest remote.',
-          $remote));
-      return $remote;
-    }
-
-    $remote = 'p4';
-    if ($api->isPerforceRemote($remote)) {
-      $this->writeInfo(
-        pht('REMOTE'),
-        pht(
-          'Using Perforce remote "%s". The existence of this remote implies '.
-          'this working copy was synchronized from a Perforce repository.',
-          $remote));
-      return $remote;
-    }
-
-    $remote = 'origin';
-    $this->writeInfo(
-      pht('REMOTE'),
-      pht(
-        'Using remote "%s", the default remote under Git.',
-        $remote));
-
-    return $remote;
-  }
-
-}

src/land/ArcanistLandCommit.php

@@ -0,0 +1,149 @@
+<?php
+
+final class ArcanistLandCommit
+  extends Phobject {
+
+  private $hash;
+  private $summary;
+  private $displaySummary;
+  private $parents;
+  private $symbols = array();
+  private $explicitRevisionRef;
+  private $revisionRef = false;
+  private $parentCommits;
+  private $isHeadCommit;
+  private $isImplicitCommit;
+
+  public function setHash($hash) {
+    $this->hash = $hash;
+    return $this;
+  }
+
+  public function getHash() {
+    return $this->hash;
+  }
+
+  public function setSummary($summary) {
+    $this->summary = $summary;
+    return $this;
+  }
+
+  public function getSummary() {
+    return $this->summary;
+  }
+
+  public function getDisplaySummary() {
+    if ($this->displaySummary === null) {
+      $this->displaySummary = id(new PhutilUTF8StringTruncator())
+        ->setMaximumGlyphs(64)
+        ->truncateString($this->getSummary());
+    }
+    return $this->displaySummary;
+  }
+
+  public function setParents(array $parents) {
+    $this->parents = $parents;
+    return $this;
+  }
+
+  public function getParents() {
+    return $this->parents;
+  }
+
+  public function addSymbol(ArcanistLandSymbol $symbol) {
+    $this->symbols[] = $symbol;
+    return $this;
+  }
+
+  public function getSymbols() {
+    return $this->symbols;
+  }
+
+  public function setExplicitRevisionref(ArcanistRevisionRef $ref) {
+    $this->explicitRevisionRef = $ref;
+    return $this;
+  }
+
+  public function getExplicitRevisionref() {
+    return $this->explicitRevisionRef;
+  }
+
+  public function setParentCommits(array $parent_commits) {
+    $this->parentCommits = $parent_commits;
+    return $this;
+  }
+
+  public function getParentCommits() {
+    return $this->parentCommits;
+  }
+
+  public function setIsHeadCommit($is_head_commit) {
+    $this->isHeadCommit = $is_head_commit;
+    return $this;
+  }
+
+  public function getIsHeadCommit() {
+    return $this->isHeadCommit;
+  }
+
+  public function setIsImplicitCommit($is_implicit_commit) {
+    $this->isImplicitCommit = $is_implicit_commit;
+    return $this;
+  }
+
+  public function getIsImplicitCommit() {
+    return $this->isImplicitCommit;
+  }
+
+  public function getAncestorRevisionPHIDs() {
+    $phids = array();
+
+    foreach ($this->getParentCommits() as $parent_commit) {
+      $phids += $parent_commit->getAncestorRevisionPHIDs();
+    }
+
+    $revision_ref = $this->getRevisionRef();
+    if ($revision_ref) {
+      $phids[$revision_ref->getPHID()] = $revision_ref->getPHID();
+    }
+
+    return $phids;
+  }
+
+  public function getRevisionRef() {
+    if ($this->revisionRef === false) {
+      $this->revisionRef = $this->newRevisionRef();
+    }
+
+    return $this->revisionRef;
+  }
+
+  private function newRevisionRef() {
+    $revision_ref = $this->getExplicitRevisionRef();
+    if ($revision_ref) {
+      return $revision_ref;
+    }
+
+    $parent_refs = array();
+    foreach ($this->getParentCommits() as $parent_commit) {
+      $parent_ref = $parent_commit->getRevisionRef();
+      if ($parent_ref) {
+        $parent_refs[$parent_ref->getPHID()] = $parent_ref;
+      }
+    }
+
+    if (count($parent_refs) > 1) {
+      throw new Exception(
+        pht(
+          'Too many distinct parent refs!'));
+    }
+
+    if ($parent_refs) {
+      return head($parent_refs);
+    }
+
+    return null;
+  }
+
+
+}

src/land/ArcanistLandCommitSet.php

@@ -0,0 +1,52 @@
+<?php
+
+final class ArcanistLandCommitSet
+  extends Phobject {
+
+  private $revisionRef;
+  private $commits;
+
+  public function setRevisionRef(ArcanistRevisionRef $revision_ref) {
+    $this->revisionRef = $revision_ref;
+    return $this;
+  }
+
+  public function getRevisionRef() {
+    return $this->revisionRef;
+  }
+
+  public function setCommits(array $commits) {
+    assert_instances_of($commits, 'ArcanistLandCommit');
+    $this->commits = $commits;
+
+    $revision_phid = $this->getRevisionRef()->getPHID();
+    foreach ($commits as $commit) {
+      $revision_ref = $commit->getExplicitRevisionRef();
+
+      if ($revision_ref) {
+        if ($revision_ref->getPHID() === $revision_phid) {
+          continue;
+        }
+      }
+
+      $commit->setIsImplicitCommit(true);
+    }
+
+    return $this;
+  }
+
+  public function getCommits() {
+    return $this->commits;
+  }
+
+  public function hasImplicitCommits() {
+    foreach ($this->commits as $commit) {
+      if ($commit->getIsImplicitCommit()) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+}

src/land/ArcanistLandEngine.php

@@ -1,182 +0,0 @@
-<?php
-
-abstract class ArcanistLandEngine extends Phobject {
-
-  private $workflow;
-  private $repositoryAPI;
-  private $targetRemote;
-  private $targetOnto;
-  private $sourceRef;
-  private $commitMessageFile;
-  private $shouldHold;
-  private $shouldKeep;
-  private $shouldSquash;
-  private $shouldDeleteRemote;
-  private $shouldPreview;
-  private $remoteArgument;
-  private $ontoArgument;
-
-  // TODO: This is really grotesque.
-  private $buildMessageCallback;
-
-  final public function setWorkflow(ArcanistWorkflow $workflow) {
-    $this->workflow = $workflow;
-    return $this;
-  }
-
-  final public function getWorkflow() {
-    return $this->workflow;
-  }
-
-  final public function setRepositoryAPI(
-    ArcanistRepositoryAPI $repository_api) {
-    $this->repositoryAPI = $repository_api;
-    return $this;
-  }
-
-  final public function getRepositoryAPI() {
-    return $this->repositoryAPI;
-  }
-
-  final public function setShouldHold($should_hold) {
-    $this->shouldHold = $should_hold;
-    return $this;
-  }
-
-  final public function getShouldHold() {
-    return $this->shouldHold;
-  }
-
-  final public function setShouldKeep($should_keep) {
-    $this->shouldKeep = $should_keep;
-    return $this;
-  }
-
-  final public function getShouldKeep() {
-    return $this->shouldKeep;
-  }
-
-  final public function setShouldSquash($should_squash) {
-    $this->shouldSquash = $should_squash;
-    return $this;
-  }
-
-  final public function getShouldSquash() {
-    return $this->shouldSquash;
-  }
-
-  final public function setShouldPreview($should_preview) {
-    $this->shouldPreview = $should_preview;
-    return $this;
-  }
-
-  final public function getShouldPreview() {
-    return $this->shouldPreview;
-  }
-
-  final public function setTargetRemote($target_remote) {
-    $this->targetRemote = $target_remote;
-    return $this;
-  }
-
-  final public function getTargetRemote() {
-    return $this->targetRemote;
-  }
-
-  final public function setTargetOnto($target_onto) {
-    $this->targetOnto = $target_onto;
-    return $this;
-  }
-
-  final public function getTargetOnto() {
-    return $this->targetOnto;
-  }
-
-  final public function setSourceRef($source_ref) {
-    $this->sourceRef = $source_ref;
-    return $this;
-  }
-
-  final public function getSourceRef() {
-    return $this->sourceRef;
-  }
-
-  final public function setBuildMessageCallback($build_message_callback) {
-    $this->buildMessageCallback = $build_message_callback;
-    return $this;
-  }
-
-  final public function getBuildMessageCallback() {
-    return $this->buildMessageCallback;
-  }
-
-  final public function setCommitMessageFile($commit_message_file) {
-    $this->commitMessageFile = $commit_message_file;
-    return $this;
-  }
-
-  final public function getCommitMessageFile() {
-    return $this->commitMessageFile;
-  }
-
-  final public function setRemoteArgument($remote_argument) {
-    $this->remoteArgument = $remote_argument;
-    return $this;
-  }
-
-  final public function getRemoteArgument() {
-    return $this->remoteArgument;
-  }
-
-  final public function setOntoArgument($onto_argument) {
-    $this->ontoArgument = $onto_argument;
-    return $this;
-  }
-
-  final public function getOntoArgument() {
-    return $this->ontoArgument;
-  }
-
-  abstract public function parseArguments();
-  abstract public function execute();
-
-  abstract protected function getLandingCommits();
-
-  protected function printLandingCommits() {
-    $logs = $this->getLandingCommits();
-
-    if (!$logs) {
-      throw new ArcanistUsageException(
-        pht(
-          'There are no commits on "%s" which are not already present on '.
-          'the target.',
-          $this->getSourceRef()));
-    }
-
-    $list = id(new PhutilConsoleList())
-      ->setWrap(false)
-      ->addItems($logs);
-
-    id(new PhutilConsoleBlock())
-      ->addParagraph(
-        pht(
-          'These %s commit(s) will be landed:',
-          new PhutilNumber(count($logs))))
-      ->addList($list)
-      ->draw();
-  }
-
-  protected function writeWarn($title, $message) {
-    return $this->getWorkflow()->writeWarn($title, $message);
-  }
-
-  protected function writeInfo($title, $message) {
-    return $this->getWorkflow()->writeInfo($title, $message);
-  }
-
-  protected function writeOkay($title, $message) {
-    return $this->getWorkflow()->writeOkay($title, $message);
-  }
-
-
-}

src/land/ArcanistLandSymbol.php

@@ -0,0 +1,27 @@
+<?php
+
+final class ArcanistLandSymbol
+  extends Phobject {
+
+  private $symbol;
+  private $commit;
+
+  public function setSymbol($symbol) {
+    $this->symbol = $symbol;
+    return $this;
+  }
+
+  public function getSymbol() {
+    return $this->symbol;
+  }
+
+  public function setCommit($commit) {
+    $this->commit = $commit;
+    return $this;
+  }
+
+  public function getCommit() {
+    return $this->commit;
+  }
+
+}

src/land/ArcanistLandTarget.php

@@ -0,0 +1,41 @@
+<?php
+
+final class ArcanistLandTarget
+  extends Phobject {
+
+  private $remote;
+  private $ref;
+  private $commit;
+
+  public function setRemote($remote) {
+    $this->remote = $remote;
+    return $this;
+  }
+
+  public function getRemote() {
+    return $this->remote;
+  }
+
+  public function setRef($ref) {
+    $this->ref = $ref;
+    return $this;
+  }
+
+  public function getRef() {
+    return $this->ref;
+  }
+
+  public function getLandTargetKey() {
+    return sprintf('%s/%s', $this->getRemote(), $this->getRef());
+  }
+
+  public function setLandTargetCommit($commit) {
+    $this->commit = $commit;
+    return $this;
+  }
+
+  public function getLandTargetCommit() {
+    return $this->commit;
+  }
+
+}

src/land/engine/ArcanistMercurialLandEngine.php

@@ -0,0 +1,603 @@
+<?php
+
+final class ArcanistMercurialLandEngine
+  extends ArcanistLandEngine {
+
+  protected function getDefaultSymbols() {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    $bookmark = $api->getActiveBookmark();
+    if ($bookmark !== null) {
+
+      $log->writeStatus(
+        pht('SOURCE'),
+        pht(
+          'Landing the active bookmark, "%s".',
+          $bookmark));
+
+      return array($bookmark);
+    }
+
+    $branch = $api->getBranchName();
+    if ($branch !== null) {
+
+      $log->writeStatus(
+        pht('SOURCE'),
+        pht(
+          'Landing the current branch, "%s".',
+          $branch));
+
+      return array($branch);
+    }
+
+    throw new Exception(pht('TODO: Operate on raw revision.'));
+  }
+
+  protected function resolveSymbols(array $symbols) {
+    assert_instances_of($symbols, 'ArcanistLandSymbol');
+    $api = $this->getRepositoryAPI();
+
+    foreach ($symbols as $symbol) {
+      $raw_symbol = $symbol->getSymbol();
+
+      if ($api->isBookmark($raw_symbol)) {
+        $hash = $api->getBookmarkCommitHash($raw_symbol);
+        $symbol->setCommit($hash);
+
+        // TODO: Set that this is a bookmark?
+
+        continue;
+      }
+
+      if ($api->isBranch($raw_symbol)) {
+        $hash = $api->getBranchCommitHash($raw_symbol);
+        $symbol->setCommit($hash);
+
+        // TODO: Set that this is a branch?
+
+        continue;
+      }
+
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Symbol "%s" is not a bookmark or branch name.',
+          $raw_symbol));
+    }
+  }
+
+  protected function selectOntoRemote(array $symbols) {
+    assert_instances_of($symbols, 'ArcanistLandSymbol');
+    $remote = $this->newOntoRemote($symbols);
+
+    // TODO: Verify this remote actually exists.
+
+    return $remote;
+  }
+
+  private function newOntoRemote(array $symbols) {
+    assert_instances_of($symbols, 'ArcanistLandSymbol');
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    $remote = $this->getOntoRemoteArgument();
+    if ($remote !== null) {
+
+      $log->writeStatus(
+        pht('ONTO REMOTE'),
+        pht(
+          'Remote "%s" was selected with the "--onto-remote" flag.',
+          $remote));
+
+      return $remote;
+    }
+
+    $remote = $this->getOntoRemoteFromConfiguration();
+    if ($remote !== null) {
+      $remote_key = $this->getOntoRemoteConfigurationKey();
+
+      $log->writeStatus(
+        pht('ONTO REMOTE'),
+        pht(
+          'Remote "%s" was selected by reading "%s" configuration.',
+          $remote,
+          $remote_key));
+
+      return $remote;
+    }
+
+    $api = $this->getRepositoryAPI();
+
+    $default_remote = 'default';
+
+    $log->writeStatus(
+      pht('ONTO REMOTE'),
+      pht(
+        'Landing onto remote "%s", the default remote under Mercurial.',
+        $default_remote));
+
+    return $default_remote;
+  }
+
+  protected function selectOntoRefs(array $symbols) {
+    assert_instances_of($symbols, 'ArcanistLandSymbol');
+    $log = $this->getLogEngine();
+
+    $onto = $this->getOntoArguments();
+    if ($onto) {
+
+      $log->writeStatus(
+        pht('ONTO TARGET'),
+        pht(
+          'Refs were selected with the "--onto" flag: %s.',
+          implode(', ', $onto)));
+
+      return $onto;
+    }
+
+    $onto = $this->getOntoFromConfiguration();
+    if ($onto) {
+      $onto_key = $this->getOntoConfigurationKey();
+
+      $log->writeStatus(
+        pht('ONTO TARGET'),
+        pht(
+          'Refs were selected by reading "%s" configuration: %s.',
+          $onto_key,
+          implode(', ', $onto)));
+
+      return $onto;
+    }
+
+    $api = $this->getRepositoryAPI();
+
+    $default_onto = 'default';
+
+    $log->writeStatus(
+      pht('ONTO TARGET'),
+      pht(
+        'Landing onto target "%s", the default target under Mercurial.',
+        $default_onto));
+
+    return array($default_onto);
+  }
+
+  protected function confirmOntoRefs(array $onto_refs) {
+    foreach ($onto_refs as $onto_ref) {
+      if (!strlen($onto_ref)) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Selected "onto" ref "%s" is invalid: the empty string is not '.
+            'a valid ref.',
+            $onto_ref));
+      }
+    }
+  }
+
+  protected function selectIntoRemote() {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    if ($this->getIntoEmptyArgument()) {
+      $this->setIntoEmpty(true);
+
+      $log->writeStatus(
+        pht('INTO REMOTE'),
+        pht(
+          'Will merge into empty state, selected with the "--into-empty" '.
+          'flag.'));
+
+      return;
+    }
+
+    if ($this->getIntoLocalArgument()) {
+      $this->setIntoLocal(true);
+
+      $log->writeStatus(
+        pht('INTO REMOTE'),
+        pht(
+          'Will merge into local state, selected with the "--into-local" '.
+          'flag.'));
+
+      return;
+    }
+
+    $into = $this->getIntoRemoteArgument();
+    if ($into !== null) {
+
+      // TODO: Verify that this is a valid path.
+      // TODO: Allow a raw URI?
+
+      $this->setIntoRemote($into);
+
+      $log->writeStatus(
+        pht('INTO REMOTE'),
+        pht(
+          'Will merge into remote "%s", selected with the "--into" flag.',
+          $into));
+
+      return;
+    }
+
+    $onto = $this->getOntoRemote();
+    $this->setIntoRemote($onto);
+
+    $log->writeStatus(
+      pht('INTO REMOTE'),
+      pht(
+        'Will merge into remote "%s" by default, because this is the remote '.
+        'the change is landing onto.',
+        $onto));
+  }
+
+  protected function selectIntoRef() {
+    $log = $this->getLogEngine();
+
+    if ($this->getIntoEmptyArgument()) {
+      $log->writeStatus(
+        pht('INTO TARGET'),
+        pht(
+          'Will merge into empty state, selected with the "--into-empty" '.
+          'flag.'));
+
+      return;
+    }
+
+    $into = $this->getIntoArgument();
+    if ($into !== null) {
+      $this->setIntoRef($into);
+
+      $log->writeStatus(
+        pht('INTO TARGET'),
+        pht(
+          'Will merge into target "%s", selected with the "--into" flag.',
+          $into));
+
+      return;
+    }
+
+    $ontos = $this->getOntoRefs();
+    $onto = head($ontos);
+
+    $this->setIntoRef($onto);
+    if (count($ontos) > 1) {
+      $log->writeStatus(
+        pht('INTO TARGET'),
+        pht(
+          'Will merge into target "%s" by default, because this is the first '.
+          '"onto" target.',
+          $onto));
+    } else {
+      $log->writeStatus(
+        pht('INTO TARGET'),
+        pht(
+          'Will merge into target "%s" by default, because this is the "onto" '.
+          'target.',
+          $onto));
+    }
+  }
+
+  protected function selectIntoCommit() {
+    // Make sure that our "into" target is valid.
+    $log = $this->getLogEngine();
+
+    if ($this->getIntoEmpty()) {
+      // If we're running under "--into-empty", we don't have to do anything.
+
+      $log->writeStatus(
+        pht('INTO COMMIT'),
+        pht('Preparing merge into the empty state.'));
+
+      return null;
+    }
+
+    if ($this->getIntoLocal()) {
+      // If we're running under "--into-local", just make sure that the
+      // target identifies some actual commit.
+      $api = $this->getRepositoryAPI();
+      $local_ref = $this->getIntoRef();
+
+      // TODO: This error handling could probably be cleaner.
+
+      $into_commit = $api->getCanonicalRevisionName($local_ref);
+
+      $log->writeStatus(
+        pht('INTO COMMIT'),
+        pht(
+          'Preparing merge into local target "%s", at commit "%s".',
+          $local_ref,
+          $this->getDisplayHash($into_commit)));
+
+      return $into_commit;
+    }
+
+    $target = id(new ArcanistLandTarget())
+      ->setRemote($this->getIntoRemote())
+      ->setRef($this->getIntoRef());
+
+    $commit = $this->fetchTarget($target);
+    if ($commit !== null) {
+      $log->writeStatus(
+        pht('INTO COMMIT'),
+        pht(
+          'Preparing merge into "%s" from remote "%s", at commit "%s".',
+          $target->getRef(),
+          $target->getRemote(),
+          $this->getDisplayHash($commit)));
+      return $commit;
+    }
+
+    // If we have no valid target and the user passed "--into" explicitly,
+    // treat this as an error. For example, "arc land --into Q --onto Q",
+    // where "Q" does not exist, is an error.
+    if ($this->getIntoArgument()) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Ref "%s" does not exist in remote "%s".',
+          $target->getRef(),
+          $target->getRemote()));
+    }
+
+    // Otherwise, treat this as implying "--into-empty". For example,
+    // "arc land --onto Q", where "Q" does not exist, is equivalent to
+    // "arc land --into-empty --onto Q".
+    $this->setIntoEmpty(true);
+
+    $log->writeStatus(
+      pht('INTO COMMIT'),
+      pht(
+        'Preparing merge into the empty state to create target "%s" '.
+        'in remote "%s".',
+        $target->getRef(),
+        $target->getRemote()));
+
+    return null;
+  }
+
+  private function fetchTarget(ArcanistLandTarget $target) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    // TODO: Support bookmarks.
+    // TODO: Deal with bookmark save/restore behavior.
+    // TODO: Format this nicely with passthru.
+    // TODO: Raise a good error message when the ref does not exist.
+
+    $api->execPassthru(
+      'pull -b %s -- %s',
+      $target->getRef(),
+      $target->getRemote());
+
+    // TODO: Deal with multiple branch heads.
+
+    list($stdout) = $api->execxLocal(
+      'log --rev %s --template %s --',
+      hgsprintf(
+        'last(ancestors(%s) and !outgoing(%s))',
+        $target->getRef(),
+        $target->getRemote()),
+      '{node}');
+
+    return trim($stdout);
+  }
+
+  protected function selectCommits($into_commit, array $symbols) {
+    assert_instances_of($symbols, 'ArcanistLandSymbol');
+    $api = $this->getRepositoryAPI();
+
+    $commit_map = array();
+    foreach ($symbols as $symbol) {
+      $symbol_commit = $symbol->getCommit();
+      $template = '{node}-{parents}-';
+
+      if ($into_commit === null) {
+        list($commits) = $api->execxLocal(
+          'log --rev %s --template %s --',
+          hgsprintf('reverse(ancestors(%s))', $into_commit),
+          $template);
+      } else {
+        list($commits) = $api->execxLocal(
+          'log --rev %s --template %s --',
+          hgsprintf(
+            'reverse(ancestors(%s) - ancestors(%s))',
+            $symbol_commit,
+            $into_commit),
+          $template);
+      }
+
+      $commits = phutil_split_lines($commits, false);
+      foreach ($commits as $line) {
+        if (!strlen($line)) {
+          continue;
+        }
+
+        $parts = explode('-', $line, 3);
+        if (count($parts) < 3) {
+          throw new Exception(
+            pht(
+              'Unexpected output from "hg log ...": %s',
+              $line));
+        }
+
+        $hash = $parts[0];
+        if (!isset($commit_map[$hash])) {
+          $parents = $parts[1];
+          $parents = trim($parents);
+          if (strlen($parents)) {
+            $parents = explode(' ', $parents);
+          } else {
+            $parents = array();
+          }
+
+          $summary = $parts[2];
+
+          $commit_map[$hash] = id(new ArcanistLandCommit())
+            ->setHash($hash)
+            ->setParents($parents)
+            ->setSummary($summary);
+        }
+
+        $commit = $commit_map[$hash];
+        $commit->addSymbol($symbol);
+      }
+    }
+
+    return $this->confirmCommits($into_commit, $symbols, $commit_map);
+  }
+
+  protected function executeMerge(ArcanistLandCommitSet $set, $into_commit) {
+    $api = $this->getRepositoryAPI();
+
+    if ($this->getStrategy() !== 'squash') {
+      throw new Exception(pht('TODO: Support merge strategies'));
+    }
+
+    // TODO: Add a Mercurial version check requiring 2.1.1 or newer.
+
+    $api->execxLocal(
+      'update --rev %s',
+      hgsprintf('%s', $into_commit));
+
+    $commits = $set->getCommits();
+
+    $min_commit = last($commits)->getHash();
+    $max_commit = head($commits)->getHash();
+
+    $revision_ref = $set->getRevisionRef();
+    $commit_message = $revision_ref->getCommitMessage();
+
+    try {
+      $argv = array();
+      $argv[] = '--dest';
+      $argv[] = hgsprintf('%s', $into_commit);
+
+      $argv[] = '--rev';
+      $argv[] = hgsprintf('%s..%s', $min_commit, $max_commit);
+
+      $argv[] = '--logfile';
+      $argv[] = '-';
+
+      $argv[] = '--keep';
+      $argv[] = '--collapse';
+
+      $future = $api->execFutureLocal('rebase %Ls', $argv);
+      $future->write($commit_message);
+      $future->resolvex();
+
+    } catch (CommandException $ex) {
+      // TODO
+      // $api->execManualLocal('rebase --abort');
+      throw $ex;
+    }
+
+    list($stdout) = $api->execxLocal('log --rev tip --template %s', '{node}');
+    $new_cursor = trim($stdout);
+
+    return $new_cursor;
+  }
+
+  protected function pushChange($into_commit) {
+    $api = $this->getRepositoryAPI();
+
+    // TODO: This does not respect "--into" or "--onto" properly.
+
+    $api->execxLocal(
+      'push --rev %s -- %s',
+      $into_commit,
+      $this->getOntoRemote());
+  }
+
+  protected function cascadeState(ArcanistLandCommitSet $set, $into_commit) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    // This has no effect when we're executing a merge strategy.
+    if (!$this->isSquashStrategy()) {
+      return;
+    }
+
+    $old_commit = last($set->getCommits())->getHash();
+    $new_commit = $into_commit;
+
+    list($output) = $api->execxLocal(
+      'log --rev %s --template %s',
+      hgsprintf('children(%s)', $old_commit),
+      '{node}\n');
+    $child_hashes = phutil_split_lines($output, false);
+
+    foreach ($child_hashes as $child_hash) {
+      if (!strlen($child_hash)) {
+        continue;
+      }
+
+      // TODO: If the only heads which are descendants of this child will
+      // be deleted, we can skip this rebase?
+
+      try {
+        $api->execxLocal(
+          'rebase --source %s --dest %s --keep --keepbranches',
+          $child_hash,
+          $new_commit);
+      } catch (CommandException $ex) {
+        // TODO: Recover state.
+        throw $ex;
+      }
+    }
+  }
+
+
+  protected function pruneBranches(array $sets) {
+    assert_instances_of($sets, 'ArcanistLandCommitSet');
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    // This has no effect when we're executing a merge strategy.
+    if (!$this->isSquashStrategy()) {
+      return;
+    }
+
+    $strip = array();
+
+    // We've rebased all descendants already, so we can safely delete all
+    // of these commits.
+
+    $sets = array_reverse($sets);
+    foreach ($sets as $set) {
+      $commits = $set->getCommits();
+
+      $min_commit = head($commits)->getHash();
+      $max_commit = last($commits)->getHash();
+
+      $strip[] = hgsprintf('%s::%s', $min_commit, $max_commit);
+    }
+
+    $rev_set = '('.implode(') or (', $strip).')';
+
+    // See PHI45. If we have "hg evolve", get rid of old commits using
+    // "hg prune" instead of "hg strip".
+
+    // If we "hg strip" a commit which has an obsolete predecessor, it
+    // removes the obsolescence marker and revives the predecessor. This is
+    // not desirable: we want to destroy all predecessors of these commits.
+
+    try {
+      $api->execxLocal(
+        '--config extensions.evolve= prune --rev %s',
+        $rev_set);
+    } catch (CommandException $ex) {
+      $api->execxLocal(
+        '--config extensions.strip= strip --rev %s',
+        $rev_set);
+    }
+  }
+
+  protected function reconcileLocalState(
+    $into_commit,
+    ArcanistRepositoryLocalState $state) {
+
+    // TODO: For now, just leave users wherever they ended up.
+
+    $state->discardLocalState();
+  }
+
+}

src/query/ArcanistMercurialCommitMessageHardpointQuery.php

@@ -0,0 +1,36 @@
+<?php
+
+final class ArcanistMercurialCommitMessageHardpointQuery
+  extends ArcanistWorkflowMercurialHardpointQuery {
+
+  public function getHardpoints() {
+    return array(
+      ArcanistCommitRef::HARDPOINT_MESSAGE,
+    );
+  }
+
+  protected function canLoadRef(ArcanistRef $ref) {
+    return ($ref instanceof ArcanistCommitRef);
+  }
+
+  public function loadHardpoint(array $refs, $hardpoint) {
+    $api = $this->getRepositoryAPI();
+
+    $hashes = mpull($refs, 'getCommitHash');
+    $unique_hashes = array_fuse($hashes);
+
+    // TODO: Batch this properly and make it future oriented.
+
+    $messages = array();
+    foreach ($unique_hashes as $unique_hash) {
+      $messages[$unique_hash] = $api->getCommitMessage($unique_hash);
+    }
+
+    foreach ($hashes as $ref_key => $hash) {
+      $hashes[$ref_key] = $messages[$hash];
+    }
+
+    yield $this->yieldMap($hashes);
+  }
+
+}

src/query/ArcanistMercurialWorkingCopyRevisionHardpointQuery.php

@@ -0,0 +1,76 @@
+<?php
+
+final class ArcanistMercurialWorkingCopyRevisionHardpointQuery
+  extends ArcanistWorkflowMercurialHardpointQuery {
+
+  public function getHardpoints() {
+    return array(
+      ArcanistWorkingCopyStateRef::HARDPOINT_REVISIONREFS,
+    );
+  }
+
+  protected function canLoadRef(ArcanistRef $ref) {
+    return ($ref instanceof ArcanistWorkingCopyStateRef);
+  }
+
+  public function loadHardpoint(array $refs, $hardpoint) {
+    yield $this->yieldRequests(
+      $refs,
+      array(
+        ArcanistWorkingCopyStateRef::HARDPOINT_COMMITREF,
+      ));
+
+    // TODO: This has a lot in common with the Git query in the same role.
+
+    $hashes = array();
+    $map = array();
+    foreach ($refs as $ref_key => $ref) {
+      $commit = $ref->getCommitRef();
+
+      $commit_hashes = array();
+
+      $commit_hashes[] = array(
+        'hgcm',
+        $commit->getCommitHash(),
+      );
+
+      foreach ($commit_hashes as $hash) {
+        $hashes[] = $hash;
+        $hash_key = $this->getHashKey($hash);
+        $map[$hash_key][$ref_key] = $ref;
+      }
+    }
+
+    $results = array_fill_keys(array_keys($refs), array());
+    if ($hashes) {
+      $revisions = (yield $this->yieldConduit(
+        'differential.query',
+        array(
+          'commitHashes' => $hashes,
+        )));
+
+      foreach ($revisions as $dict) {
+        $revision_hashes = idx($dict, 'hashes');
+        if (!$revision_hashes) {
+          continue;
+        }
+
+        $revision_ref = ArcanistRevisionRef::newFromConduitQuery($dict);
+        foreach ($revision_hashes as $revision_hash) {
+          $hash_key = $this->getHashKey($revision_hash);
+          $state_refs = idx($map, $hash_key, array());
+          foreach ($state_refs as $ref_key => $state_ref) {
+            $results[$ref_key][] = $revision_ref;
+          }
+        }
+      }
+    }
+
+    yield $this->yieldMap($results);
+  }
+
+  private function getHashKey(array $hash) {
+    return $hash[0].':'.$hash[1];
+  }
+
+}

src/query/ArcanistWorkflowMercurialHardpointQuery.php

@@ -0,0 +1,11 @@
+<?php
+
+abstract class ArcanistWorkflowMercurialHardpointQuery
+  extends ArcanistRuntimeHardpointQuery {
+
+  final protected function canLoadHardpoint() {
+    $api = $this->getRepositoryAPI();
+    return ($api instanceof ArcanistMercurialAPI);
+  }
+
+}

src/ref/revision/ArcanistRevisionBuildableHardpointQuery.php

@@ -29,7 +29,7 @@ final class ArcanistRevisionBuildableHardpointQuery
     $buildables = (yield $this->yieldConduitSearch(
       'harbormaster.buildable.search',
       array(
-        'objectPHIDs' => $diff_map,
+        'objectPHIDs' => array_values($diff_map),
         'manual' => false,
       )));
 

src/repository/api/ArcanistGitAPI.php

@@ -1564,6 +1564,11 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
     return ($uri !== null);
   }
 
+  public function isFetchableRemote($remote_name) {
+    $uri = $this->getGitRemoteFetchURI($remote_name);
+    return ($uri !== null);
+  }
+
   private function getGitRemoteFetchURI($remote_name) {
     return $this->getGitRemoteURI($remote_name, $for_push = false);
   }
@@ -1739,4 +1744,13 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
     return false;
   }
 
+  protected function newLandEngine() {
+    return new ArcanistGitLandEngine();
+  }
+
+  public function newLocalState() {
+    return id(new ArcanistGitLocalState())
+      ->setRepositoryAPI($this);
+  }
+
 }

src/repository/api/ArcanistMercurialAPI.php

@@ -534,6 +534,8 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
   }
 
   public function getAllBranches() {
+    // TODO: This is wrong, and returns bookmarks.
+
     list($branch_info) = $this->execxLocal('bookmarks');
     if (trim($branch_info) == 'no bookmarks set') {
       return array();
@@ -548,10 +550,14 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
 
     $return = array();
     foreach ($matches as $match) {
-      list(, $current, $name) = $match;
+      list(, $current, $name, $hash) = $match;
+
+      list($id, $hash) = explode(':', $hash);
+
       $return[] = array(
         'current' => (bool)$current,
         'name'    => rtrim($name),
+        'hash' => $hash,
       );
     }
     return $return;
@@ -562,9 +568,13 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
 
     $refs = array();
     foreach ($branches as $branch) {
+      $commit_ref = $this->newCommitRef()
+        ->setCommitHash($branch['hash']);
+
       $refs[] = $this->newBranchRef()
         ->setBranchName($branch['name'])
-        ->setIsCurrentBranch($branch['current']);
+        ->setIsCurrentBranch($branch['current'])
+        ->attachCommitRef($commit_ref);
     }
 
     return $refs;
@@ -1064,6 +1074,46 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     return $bookmarks;
   }
 
+  public function getBookmarkCommitHash($name) {
+    // TODO: Cache this.
+
+    $bookmarks = $this->getBookmarks($name);
+    $bookmarks = ipull($bookmarks, null, 'name');
+
+    foreach ($bookmarks as $bookmark) {
+      if ($bookmark['name'] === $name) {
+        return $bookmark['revision'];
+      }
+    }
+
+    throw new Exception(pht('No bookmark "%s".', $name));
+  }
+
+  public function getBranchCommitHash($name) {
+    // TODO: Cache this.
+    // TODO: This won't work when there are multiple branch heads with the
+    // same name.
+
+    $branches = $this->getBranches($name);
+
+    $heads = array();
+    foreach ($branches as $branch) {
+      if ($branch['name'] === $name) {
+        $heads[] = $branch;
+      }
+    }
+
+    if (count($heads) === 1) {
+      return idx(head($heads), 'revision');
+    }
+
+    if (!$heads) {
+      throw new Exception(pht('No branch "%s".', $name));
+    }
+
+    throw new Exception(pht('Too many branch heads for "%s".', $name));
+  }
+
   private function splitBranchOrBookmarkLine($line) {
     // branches and bookmarks are printed in the format:
     // default                 0:a5ead76cdf85 (inactive)
@@ -1108,4 +1158,14 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     return $env;
   }
 
+  protected function newLandEngine() {
+    return new ArcanistMercurialLandEngine();
+  }
+
+  public function newLocalState() {
+    return id(new ArcanistMercurialLocalState())
+      ->setRepositoryAPI($this);
+  }
+
+
 }

src/repository/api/ArcanistRepositoryAPI.php

@@ -734,4 +734,19 @@ abstract class ArcanistRepositoryAPI extends Phobject {
   final public function newBranchRef() {
     return new ArcanistBranchRef();
   }
+
+  final public function getLandEngine() {
+    $engine = $this->newLandEngine();
+
+    if ($engine) {
+      $engine->setRepositoryAPI($this);
+    }
+
+    return $engine;
+  }
+
+  protected function newLandEngine() {
+    return null;
+  }
+
 }

src/repository/state/ArcanistMercurialLocalState.php

@@ -0,0 +1,60 @@
+<?php
+
+final class ArcanistMercurialLocalState
+  extends ArcanistRepositoryLocalState {
+
+  private $localCommit;
+  private $localRef;
+  private $localPath;
+
+  public function getLocalRef() {
+    return $this->localRef;
+  }
+
+  public function getLocalPath() {
+    return $this->localPath;
+  }
+
+  protected function executeSaveLocalState() {
+    $api = $this->getRepositoryAPI();
+    // TODO: Fix this.
+  }
+
+  protected function executeRestoreLocalState() {
+    $api = $this->getRepositoryAPI();
+    // TODO: Fix this.
+
+    // TODO: In Mercurial, we may want to discard commits we've created.
+    // $repository_api->execxLocal(
+    //   '--config extensions.mq= strip %s',
+    //   $this->onto);
+
+  }
+
+  protected function executeDiscardLocalState() {
+    // TODO: Fix this.
+  }
+
+  protected function canStashChanges() {
+    // Depends on stash extension.
+    return false;
+  }
+
+  protected function getIgnoreHints() {
+    // TODO: Provide this.
+    return array();
+  }
+
+  protected function saveStash() {
+    return null;
+  }
+
+  protected function restoreStash($stash_ref) {
+    return null;
+  }
+
+  protected function discardStash($stash_ref) {
+    return null;
+  }
+
+}

src/repository/state/ArcanistRepositoryLocalState.php

@@ -151,6 +151,9 @@ abstract class ArcanistRepositoryLocalState
     $this->executeSaveLocalState();
     $this->shouldRestore = true;
 
+    // TODO: Detect when we're in the middle of a rebase.
+    // TODO: Detect when we're in the middle of a cherry-pick.
+
     return $this;
   }
 

src/workflow/ArcanistWorkflow.php

@@ -244,7 +244,7 @@ abstract class ArcanistWorkflow extends Phobject {
     return $err;
   }
 
-  final protected function getLogEngine() {
+  final public function getLogEngine() {
     return $this->getRuntime()->getLogEngine();
   }
 
@@ -2357,6 +2357,15 @@ abstract class ArcanistWorkflow extends Phobject {
       $prompts = $this->newPrompts();
       assert_instances_of($prompts, 'ArcanistPrompt');
 
+      $prompts[] = $this->newPrompt('arc.state.stash')
+        ->setDescription(
+          pht(
+            'Prompts the user to stash changes and continue when the '.
+            'working copy has untracked, uncommitted. or unstaged '.
+            'changes.'));
+
+      // TODO: Swap to ArrayCheck?
+
       $map = array();
       foreach ($prompts as $prompt) {
         $key = $prompt->getKey();
@@ -2380,7 +2389,7 @@ abstract class ArcanistWorkflow extends Phobject {
     return $this->promptMap;
   }
 
-  protected function getPrompt($key) {
+  final public function getPrompt($key) {
     $map = $this->getPromptMap();
 
     $prompt = idx($map, $key);