Implement "arc work", to replace "arc feature"

Summary: Ref T13546. Fixes T2928. Adds a new "arc work" workflow which functions like the older "arc feature" workflow, but with modern infrastructure.

Test Plan: Used "arc work" to begin work on branches, bookmarks, and revisions in Git and Mercurial.

Maniphest Tasks: T13546, T2928

Differential Revision: https://secure.phabricator.com/D21336

src/__phutil_library_map__.php

@@ -225,6 +225,7 @@ phutil_register_library_map(array(
     'ArcanistGitRawCommitTestCase' => 'repository/raw/__tests__/ArcanistGitRawCommitTestCase.php',
     'ArcanistGitRepositoryMarkerQuery' => 'repository/marker/ArcanistGitRepositoryMarkerQuery.php',
     'ArcanistGitUpstreamPath' => 'repository/api/ArcanistGitUpstreamPath.php',
+    'ArcanistGitWorkEngine' => 'work/ArcanistGitWorkEngine.php',
     'ArcanistGitWorkingCopy' => 'workingcopy/ArcanistGitWorkingCopy.php',
     'ArcanistGitWorkingCopyRevisionHardpointQuery' => 'query/ArcanistGitWorkingCopyRevisionHardpointQuery.php',
     'ArcanistGlobalVariableXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistGlobalVariableXHPASTLinterRule.php',
@@ -336,6 +337,7 @@ phutil_register_library_map(array(
     'ArcanistMercurialParser' => 'repository/parser/ArcanistMercurialParser.php',
     'ArcanistMercurialParserTestCase' => 'repository/parser/__tests__/ArcanistMercurialParserTestCase.php',
     'ArcanistMercurialRepositoryMarkerQuery' => 'repository/marker/ArcanistMercurialRepositoryMarkerQuery.php',
+    'ArcanistMercurialWorkEngine' => 'work/ArcanistMercurialWorkEngine.php',
     'ArcanistMercurialWorkingCopy' => 'workingcopy/ArcanistMercurialWorkingCopy.php',
     'ArcanistMercurialWorkingCopyRevisionHardpointQuery' => 'query/ArcanistMercurialWorkingCopyRevisionHardpointQuery.php',
     'ArcanistMergeConflictLinter' => 'lint/linter/ArcanistMergeConflictLinter.php',
@@ -538,8 +540,11 @@ phutil_register_library_map(array(
     'ArcanistWeldWorkflow' => 'workflow/ArcanistWeldWorkflow.php',
     'ArcanistWhichWorkflow' => 'workflow/ArcanistWhichWorkflow.php',
     'ArcanistWildConfigOption' => 'config/option/ArcanistWildConfigOption.php',
+    'ArcanistWorkEngine' => 'work/ArcanistWorkEngine.php',
+    'ArcanistWorkWorkflow' => 'workflow/ArcanistWorkWorkflow.php',
     'ArcanistWorkflow' => 'workflow/ArcanistWorkflow.php',
     'ArcanistWorkflowArgument' => 'toolset/ArcanistWorkflowArgument.php',
+    'ArcanistWorkflowEngine' => 'engine/ArcanistWorkflowEngine.php',
     'ArcanistWorkflowGitHardpointQuery' => 'query/ArcanistWorkflowGitHardpointQuery.php',
     'ArcanistWorkflowInformation' => 'toolset/ArcanistWorkflowInformation.php',
     'ArcanistWorkflowMercurialHardpointQuery' => 'query/ArcanistWorkflowMercurialHardpointQuery.php',
@@ -1253,6 +1258,7 @@ phutil_register_library_map(array(
     'ArcanistGitRawCommitTestCase' => 'PhutilTestCase',
     'ArcanistGitRepositoryMarkerQuery' => 'ArcanistRepositoryMarkerQuery',
     'ArcanistGitUpstreamPath' => 'Phobject',
+    'ArcanistGitWorkEngine' => 'ArcanistWorkEngine',
     'ArcanistGitWorkingCopy' => 'ArcanistWorkingCopy',
     'ArcanistGitWorkingCopyRevisionHardpointQuery' => 'ArcanistWorkflowGitHardpointQuery',
     'ArcanistGlobalVariableXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
@@ -1322,7 +1328,7 @@ phutil_register_library_map(array(
     'ArcanistLambdaFuncFunctionXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistLandCommit' => 'Phobject',
     'ArcanistLandCommitSet' => 'Phobject',
-    'ArcanistLandEngine' => 'Phobject',
+    'ArcanistLandEngine' => 'ArcanistWorkflowEngine',
     'ArcanistLandSymbol' => 'Phobject',
     'ArcanistLandTarget' => 'Phobject',
     'ArcanistLandWorkflow' => 'ArcanistArcWorkflow',
@@ -1355,7 +1361,10 @@ phutil_register_library_map(array(
     'ArcanistLogicalOperatorsXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistLowercaseFunctionsXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistLowercaseFunctionsXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
-    'ArcanistMarkerRef' => 'ArcanistRef',
+    'ArcanistMarkerRef' => array(
+      'ArcanistRef',
+      'ArcanistDisplayRefInterface',
+    ),
     'ArcanistMarkersWorkflow' => 'ArcanistArcWorkflow',
     'ArcanistMercurialAPI' => 'ArcanistRepositoryAPI',
     'ArcanistMercurialCommitMessageHardpointQuery' => 'ArcanistWorkflowMercurialHardpointQuery',
@@ -1364,6 +1373,7 @@ phutil_register_library_map(array(
     'ArcanistMercurialParser' => 'Phobject',
     'ArcanistMercurialParserTestCase' => 'PhutilTestCase',
     'ArcanistMercurialRepositoryMarkerQuery' => 'ArcanistRepositoryMarkerQuery',
+    'ArcanistMercurialWorkEngine' => 'ArcanistWorkEngine',
     'ArcanistMercurialWorkingCopy' => 'ArcanistWorkingCopy',
     'ArcanistMercurialWorkingCopyRevisionHardpointQuery' => 'ArcanistWorkflowMercurialHardpointQuery',
     'ArcanistMergeConflictLinter' => 'ArcanistLinter',
@@ -1576,8 +1586,11 @@ phutil_register_library_map(array(
     'ArcanistWeldWorkflow' => 'ArcanistArcWorkflow',
     'ArcanistWhichWorkflow' => 'ArcanistWorkflow',
     'ArcanistWildConfigOption' => 'ArcanistConfigOption',
+    'ArcanistWorkEngine' => 'ArcanistWorkflowEngine',
+    'ArcanistWorkWorkflow' => 'ArcanistArcWorkflow',
     'ArcanistWorkflow' => 'Phobject',
     'ArcanistWorkflowArgument' => 'Phobject',
+    'ArcanistWorkflowEngine' => 'Phobject',
     'ArcanistWorkflowGitHardpointQuery' => 'ArcanistRuntimeHardpointQuery',
     'ArcanistWorkflowInformation' => 'Phobject',
     'ArcanistWorkflowMercurialHardpointQuery' => 'ArcanistRuntimeHardpointQuery',

src/engine/ArcanistWorkflowEngine.php

@@ -0,0 +1,48 @@
+<?php
+
+abstract class ArcanistWorkflowEngine
+  extends Phobject {
+
+  private $workflow;
+  private $viewer;
+  private $logEngine;
+  private $repositoryAPI;
+
+  final public function setViewer($viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  final public function getViewer() {
+    return $this->viewer;
+  }
+
+  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 setLogEngine(ArcanistLogEngine $log_engine) {
+    $this->logEngine = $log_engine;
+    return $this;
+  }
+
+  final public function getLogEngine() {
+    return $this->logEngine;
+  }
+
+}

src/hardpoint/ArcanistHardpointObject.php

@@ -5,6 +5,12 @@ abstract class ArcanistHardpointObject
 
   private $hardpointList;
 
+  public function __clone() {
+    if ($this->hardpointList) {
+      $this->hardpointList = clone $this->hardpointList;
+    }
+  }
+
   final public function getHardpoint($hardpoint) {
     return $this->getHardpointList()->getHardpoint(
       $this,

src/land/engine/ArcanistLandEngine.php

@@ -1,11 +1,7 @@
 <?php
 
-abstract class ArcanistLandEngine extends Phobject {
-
-  private $workflow;
-  private $viewer;
-  private $logEngine;
-  private $repositoryAPI;
+abstract class ArcanistLandEngine
+  extends ArcanistWorkflowEngine {
 
   private $sourceRefs;
   private $shouldHold;
@@ -33,15 +29,6 @@ abstract class ArcanistLandEngine extends Phobject {
 
   private $localState;
 
-  final public function setViewer($viewer) {
-    $this->viewer = $viewer;
-    return $this;
-  }
-
-  final public function getViewer() {
-    return $this->viewer;
-  }
-
   final public function setOntoRemote($onto_remote) {
     $this->ontoRemote = $onto_remote;
     return $this;
@@ -96,34 +83,6 @@ abstract class ArcanistLandEngine extends Phobject {
     return $this->intoLocal;
   }
 
-  final public function setWorkflow($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 setLogEngine(ArcanistLogEngine $log_engine) {
-    $this->logEngine = $log_engine;
-    return $this;
-  }
-
-  final public function getLogEngine() {
-    return $this->logEngine;
-  }
-
   final public function setShouldHold($should_hold) {
     $this->shouldHold = $should_hold;
     return $this;

src/repository/api/ArcanistGitAPI.php

@@ -1746,6 +1746,10 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
     return new ArcanistGitLandEngine();
   }
 
+  protected function newWorkEngine() {
+    return new ArcanistGitWorkEngine();
+  }
+
   public function newLocalState() {
     return id(new ArcanistGitLocalState())
       ->setRepositoryAPI($this);

src/repository/api/ArcanistMercurialAPI.php

@@ -1134,6 +1134,10 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     return new ArcanistMercurialLandEngine();
   }
 
+  protected function newWorkEngine() {
+    return new ArcanistMercurialWorkEngine();
+  }
+
   public function newLocalState() {
     return id(new ArcanistMercurialLocalState())
       ->setRepositoryAPI($this);

src/repository/api/ArcanistRepositoryAPI.php

@@ -763,6 +763,20 @@ abstract class ArcanistRepositoryAPI extends Phobject {
     return null;
   }
 
+  final public function getWorkEngine() {
+    $engine = $this->newWorkEngine();
+
+    if ($engine) {
+      $engine->setRepositoryAPI($this);
+    }
+
+    return $engine;
+  }
+
+  protected function newWorkEngine() {
+    return null;
+  }
+
   final public function getSupportedMarkerTypes() {
     return $this->newSupportedMarkerTypes();
   }
@@ -780,4 +794,8 @@ abstract class ArcanistRepositoryAPI extends Phobject {
     throw new PhutilMethodNotImplementedException();
   }
 
+  final public function getDisplayHash($hash) {
+    return substr($hash, 0, 12);
+  }
+
 }

src/repository/marker/ArcanistMarkerRef.php

@@ -1,9 +1,12 @@
 <?php
 
 final class ArcanistMarkerRef
-  extends ArcanistRef {
+  extends ArcanistRef
+  implements
+    ArcanistDisplayRefInterface {
 
-  const HARDPOINT_COMMITREF = 'commitRef';
+  const HARDPOINT_COMMITREF = 'arc.marker.commitRef';
+  const HARDPOINT_WORKINGCOPYSTATEREF = 'arc.marker.workingCopyStateRef';
 
   const TYPE_BRANCH = 'branch';
   const TYPE_BOOKMARK = 'bookmark';
@@ -13,18 +16,38 @@ final class ArcanistMarkerRef
   private $epoch;
   private $markerHash;
   private $commitHash;
+  private $displayHash;
   private $treeHash;
   private $summary;
   private $message;
   private $isActive = false;
 
   public function getRefDisplayName() {
-    return pht('Marker %s', $this->getName());
+    return $this->getDisplayRefObjectName();
+  }
+
+  public function getDisplayRefObjectName() {
+    switch ($this->getMarkerType()) {
+      case self::TYPE_BRANCH:
+        return pht('Branch "%s"', $this->getName());
+      case self::TYPE_BOOKMARK:
+        return pht('Bookmark "%s"', $this->getName());
+      default:
+        return pht('Marker "%s"', $this->getName());
+    }
+  }
+
+  public function getDisplayRefTitle() {
+    return pht(
+      '%s %s',
+      $this->getDisplayHash(),
+      $this->getSummary());
   }
 
   protected function newHardpoints() {
     return array(
       $this->newHardpoint(self::HARDPOINT_COMMITREF),
+      $this->newHardpoint(self::HARDPOINT_WORKINGCOPYSTATEREF),
     );
   }
 
@@ -64,6 +87,17 @@ final class ArcanistMarkerRef
     return $this->markerHash;
   }
 
+  public function setDisplayHash($display_hash) {
+    $this->displayHash = $display_hash;
+    return $this;
+  }
+
+  public function getDisplayHash() {
+    return $this->displayHash;
+  }
+
+
+
   public function setCommitHash($commit_hash) {
     $this->commitHash = $commit_hash;
     return $this;
@@ -125,4 +159,12 @@ final class ArcanistMarkerRef
     return $this->getHardpoint(self::HARDPOINT_COMMITREF);
   }
 
+  public function attachWorkingCopyStateRef(ArcanistWorkingCopyStateRef $ref) {
+    return $this->attachHardpoint(self::HARDPOINT_WORKINGCOPYSTATEREF, $ref);
+  }
+
+  public function getWorkingCopyStateRef() {
+    return $this->getHardpoint(self::HARDPOINT_WORKINGCOPYSTATEREF);
+  }
+
 }

src/repository/marker/ArcanistMercurialRepositoryMarkerQuery.php

@@ -100,6 +100,7 @@ final class ArcanistMercurialRepositoryMarkerQuery
       }
 
       $message = $fields[4];
+      $message_lines = phutil_split_lines($message, false);
 
       $commit_ref = $api->newCommitRef()
         ->setCommitHash($node)
@@ -107,6 +108,7 @@ final class ArcanistMercurialRepositoryMarkerQuery
 
       $template = id(new ArcanistMarkerRef())
         ->setCommitHash($node)
+        ->setSummary(head($message_lines))
         ->attachCommitRef($commit_ref);
 
       if ($is_bookmarks) {

src/repository/marker/ArcanistRepositoryMarkerQuery.php

@@ -6,6 +6,7 @@ abstract class ArcanistRepositoryMarkerQuery
   private $repositoryAPI;
   private $isActive;
   private $markerTypes;
+  private $names;
   private $commitHashes;
   private $ancestorCommitHashes;
 
@@ -23,14 +24,47 @@ abstract class ArcanistRepositoryMarkerQuery
     return $this;
   }
 
+  final public function withNames(array $names) {
+    $this->names = array_fuse($names);
+    return $this;
+  }
+
   final public function withIsActive($active) {
     $this->isActive = $active;
     return $this;
   }
 
+  final public function executeOne() {
+    $markers = $this->execute();
+
+    if (!$markers) {
+      return null;
+    }
+
+    if (count($markers) > 1) {
+      throw new Exception(
+        pht(
+          'Query matched multiple markers, expected zero or one.'));
+    }
+
+    return head($markers);
+  }
+
   final public function execute() {
     $markers = $this->newRefMarkers();
 
+    $api = $this->getRepositoryAPI();
+    foreach ($markers as $marker) {
+      $state_ref = id(new ArcanistWorkingCopyStateRef())
+        ->setCommitRef($marker->getCommitRef());
+
+      $marker->attachWorkingCopyStateRef($state_ref);
+
+      $hash = $marker->getCommitHash();
+      $hash = $api->getDisplayHash($hash);
+      $marker->setDisplayHash($hash);
+    }
+
     $types = $this->markerTypes;
     if ($types !== null) {
       foreach ($markers as $key => $marker) {
@@ -40,6 +74,15 @@ abstract class ArcanistRepositoryMarkerQuery
       }
     }
 
+    $names = $this->names;
+    if ($names !== null) {
+      foreach ($markers as $key => $marker) {
+        if (!isset($names[$marker->getName()])) {
+          unset($markers[$key]);
+        }
+      }
+    }
+
     if ($this->isActive !== null) {
       foreach ($markers as $key => $marker) {
         if ($marker->getIsActive() !== $this->isActive) {
@@ -48,6 +91,7 @@ abstract class ArcanistRepositoryMarkerQuery
       }
     }
 
+
     return $this->sortMarkers($markers);
   }
 

src/repository/state/ArcanistGitLocalState.php

@@ -42,7 +42,7 @@ final class ArcanistGitLocalState
     }
 
     $log = $this->getWorkflow()->getLogEngine();
-    $log->writeStatus(pht('SAVE STATE'), $where);
+    $log->writeTrace(pht('SAVE STATE'), $where);
   }
 
   protected function executeRestoreLocalState() {

src/work/ArcanistGitWorkEngine.php

@@ -0,0 +1,57 @@
+<?php
+
+final class ArcanistGitWorkEngine
+  extends ArcanistWorkEngine {
+
+  protected function getDefaultStartSymbol() {
+    $api = $this->getRepositoryAPI();
+
+    // NOTE: In Git, we're trying to find the current branch name because the
+    // behavior of "--track" depends on the symbol we pass.
+
+    $marker = $api->newMarkerRefQuery()
+      ->withIsActive(true)
+      ->withMarkerTypes(array(ArcanistMarkerRef::TYPE_BRANCH))
+      ->executeOne();
+    if ($marker) {
+      return $marker->getName();
+    }
+
+    return $api->getWorkingCopyRevision();
+  }
+
+  protected function newMarker($symbol, $start) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    $log->writeStatus(
+      pht('NEW BRANCH'),
+      pht(
+        'Creating new branch "%s" from "%s".',
+        $symbol,
+        $start));
+
+    $future = $api->newFuture(
+      'checkout --track -b %s %s --',
+      $symbol,
+      $start);
+    $future->resolve();
+  }
+
+  protected function moveToMarker(ArcanistMarkerRef $marker) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    $log->writeStatus(
+      pht('BRANCH'),
+      pht(
+        'Checking out branch "%s".',
+        $marker->getName()));
+
+    $future = $api->newFuture(
+      'checkout %s --',
+      $marker->getName());
+    $future->resolve();
+  }
+
+}

src/work/ArcanistMercurialWorkEngine.php

@@ -0,0 +1,56 @@
+<?php
+
+final class ArcanistMercurialWorkEngine
+  extends ArcanistWorkEngine {
+
+  protected function getDefaultStartSymbol() {
+    $api = $this->getRepositoryAPI();
+    return $api->getWorkingCopyRevision();
+  }
+
+  protected function newMarker($symbol, $start) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    $log->writeStatus(
+      pht('NEW BOOKMARK'),
+      pht(
+        'Creating new bookmark "%s" from "%s".',
+        $symbol,
+        $start));
+
+    if ($start !== $this->getDefaultStartSymbol()) {
+      $future = $api->newFuture('update -- %s', $start);
+      $future->resolve();
+    }
+
+    $future = $api->newFuture('bookmark %s --', $symbol);
+    $future->resolve();
+  }
+
+  protected function moveToMarker(ArcanistMarkerRef $marker) {
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    if ($marker->isBookmark()) {
+      $log->writeStatus(
+        pht('BOOKMARK'),
+        pht(
+          'Checking out bookmark "%s".',
+          $marker->getName()));
+    } else {
+      $log->writeStatus(
+        pht('BRANCH'),
+        pht(
+          'Checking out branch "%s".',
+          $marker->getName()));
+    }
+
+    $future = $api->newFuture(
+      'checkout %s --',
+      $marker->getName());
+
+    $future->resolve();
+  }
+
+}

src/work/ArcanistWorkEngine.php

@@ -0,0 +1,215 @@
+<?php
+
+abstract class ArcanistWorkEngine
+  extends ArcanistWorkflowEngine {
+
+  private $symbolArgument;
+  private $startArgument;
+
+  final public function setSymbolArgument($symbol_argument) {
+    $this->symbolArgument = $symbol_argument;
+    return $this;
+  }
+
+  final public function getSymbolArgument() {
+    return $this->symbolArgument;
+  }
+
+  final public function setStartArgument($start_argument) {
+    $this->startArgument = $start_argument;
+    return $this;
+  }
+
+  final public function getStartArgument() {
+    return $this->startArgument;
+  }
+
+  final public function execute() {
+    $workflow = $this->getWorkflow();
+    $api = $this->getRepositoryAPI();
+
+    $local_state = $api->newLocalState()
+      ->setWorkflow($workflow)
+      ->saveLocalState();
+
+    $symbol = $this->getSymbolArgument();
+
+    $markers = $api->newMarkerRefQuery()
+      ->withNames(array($symbol))
+      ->execute();
+
+    if ($markers) {
+      if (count($markers) > 1) {
+
+        // TODO: This almost certainly means the symbol is a Mercurial branch
+        // with multiple heads. We can pick some head.
+
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Symbol "%s" is ambiguous.',
+            $symbol));
+      }
+
+      $target = head($markers);
+      $this->moveToMarker($target);
+      $local_state->discardLocalState();
+      return;
+    }
+
+    $revision_marker = $this->workOnRevision($symbol);
+    if ($revision_marker) {
+      $this->moveToMarker($revision_marker);
+      $local_state->discardLocalState();
+      return;
+    }
+
+    $task_marker = $this->workOnTask($symbol);
+    if ($task_marker) {
+      $this->moveToMarker($task_marker);
+      $local_state->discardLocalState();
+      return;
+    }
+
+    // NOTE: We're resolving this symbol so we can raise an error message if
+    // it's bogus, but we're using the symbol (not the resolved version) to
+    // actually create the new marker. This matters in Git because it impacts
+    // the behavior of "--track" when we pass a branch name.
+
+    $start = $this->getStartArgument();
+    if ($start !== null) {
+      $start_commit = $api->getCanonicalRevisionName($start);
+      if (!$start_commit) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Unable to resolve startpoint "%s".',
+            $start));
+      }
+    } else {
+      $start = $this->getDefaultStartSymbol();
+    }
+
+    $this->newMarker($symbol, $start);
+    $local_state->discardLocalState();
+  }
+
+  abstract protected function newMarker($symbol, $start);
+  abstract protected function moveToMarker(ArcanistMarkerRef $marker);
+  abstract protected function getDefaultStartSymbol();
+
+  private function workOnRevision($symbol) {
+    $workflow = $this->getWorkflow();
+    $api = $this->getRepositoryAPI();
+    $log = $this->getLogEngine();
+
+    try {
+      $revision_symbol = id(new ArcanistRevisionSymbolRef())
+        ->setSymbol($symbol);
+    } catch (Exception $ex) {
+      return;
+    }
+
+    $workflow->loadHardpoints(
+      $revision_symbol,
+      ArcanistSymbolRef::HARDPOINT_OBJECT);
+
+    $revision_ref = $revision_symbol->getObject();
+    if (!$revision_ref) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'No revision "%s" exists, or you do not have permission to '.
+          'view it.',
+          $symbol));
+    }
+
+    $markers = $api->newMarkerRefQuery()
+      ->execute();
+
+    $state_refs = mpull($markers, 'getWorkingCopyStateRef');
+
+    $workflow->loadHardpoints(
+      $state_refs,
+      ArcanistWorkingCopyStateRef::HARDPOINT_REVISIONREFS);
+
+    $selected = array();
+    foreach ($markers as $marker) {
+      $state_ref = $marker->getWorkingCopyStateRef();
+      $revision_refs = $state_ref->getRevisionRefs();
+      $revision_refs = mpull($revision_refs, null, 'getPHID');
+
+      if (isset($revision_refs[$revision_ref->getPHID()])) {
+        $selected[] = $marker;
+      }
+    }
+
+    if (!$selected) {
+
+      // TODO: We could patch/load here.
+
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Revision "%s" was not found anywhere in this working copy.',
+          $revision_ref->getMonogram()));
+    }
+
+    if (count($selected) > 1) {
+      $selected = msort($selected, 'getEpoch');
+
+      echo tsprintf(
+        "\n%!\n%W\n\n",
+        pht('AMBIGUOUS MARKER'),
+        pht(
+          'More than one marker in the local working copy is associated '.
+          'with the revision "%s", using the most recent one.',
+          $revision_ref->getMonogram()));
+
+      foreach ($selected as $marker) {
+        echo tsprintf('%s', $marker->newDisplayRef());
+      }
+
+      echo tsprintf("\n");
+
+      $target = last($selected);
+    } else {
+      $target = head($selected);
+    }
+
+    $log->writeStatus(
+      pht('REVISION'),
+      pht('Resuming work on revision:'));
+
+    echo tsprintf('%s', $revision_ref->newDisplayRef());
+    echo tsprintf("\n");
+
+    return $target;
+  }
+
+  private function workOnTask($symbol) {
+    $workflow = $this->getWorkflow();
+
+    try {
+      $task_symbol = id(new ArcanistTaskSymbolRef())
+        ->setSymbol($symbol);
+    } catch (Exception $ex) {
+      return;
+    }
+
+    $workflow->loadHardpoints(
+      $task_symbol,
+      ArcanistSymbolRef::HARDPOINT_OBJECT);
+
+    $task_ref = $task_symbol->getObject();
+    if (!$task_ref) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'No task "%s" exists, or you do not have permission to view it.',
+          $symbol));
+    }
+
+    throw new Exception(pht('TODO: Implement this workflow.'));
+
+    $this->loadHardpoints(
+      $task_ref,
+      ArcanistTaskRef::HARDPOINT_REVISIONREFS);
+  }
+
+}

src/workflow/ArcanistWorkWorkflow.php

@@ -0,0 +1,95 @@
+<?php
+
+final class ArcanistWorkWorkflow
+  extends ArcanistArcWorkflow {
+
+  public function getWorkflowName() {
+    return 'work';
+  }
+
+  public function getWorkflowArguments() {
+    return array(
+      $this->newWorkflowArgument('start')
+        ->setParameter('symbol')
+        ->setHelp(
+          pht(
+            'When creating a new branch or bookmark, use this as the '.
+            'branch point.')),
+      $this->newWorkflowArgument('symbol')
+        ->setWildcard(true),
+    );
+  }
+
+  public function getWorkflowInformation() {
+    $help = pht(<<<EOHELP
+Begin or resume work on a branch, bookmark, task, or revision.
+
+The __symbol__ may be a branch or bookmark name, a revision name (like "D123"),
+a task name (like "T123"), or a new symbol.
+
+If you provide a symbol which currently does not identify any ongoing work,
+Arcanist will create a new branch or bookmark with the name you provide.
+
+If you provide the name of an existing branch or bookmark, Arcanist will switch
+to that branch or bookmark.
+
+If you provide the name of a revision or task, Arcanist will look for a related
+branch or bookmark that exists in the working copy. If it finds one, it will
+switch to it. If it does not find one, it will attempt to create a new branch
+or bookmark.
+
+When "arc work" creates a branch or bookmark, it will use "--start" as the
+branchpoint if it is provided. Otherwise, the current working copy state will
+serve as the starting point.
+EOHELP
+      );
+
+    return $this->newWorkflowInformation()
+      ->setSynopsis(pht('Begin or resume work.'))
+      ->addExample(pht('**work** [--start __start__] __symbol__'))
+      ->setHelp($help);
+  }
+
+  public function runWorkflow() {
+    $api = $this->getRepositoryAPI();
+
+    $work_engine = $api->getWorkEngine();
+    if (!$work_engine) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          '"arc work" must be run in a Git or Mercurial working copy.'));
+    }
+
+    $argv = $this->getArgument('symbol');
+    if (count($argv) === 0) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Provide a branch, bookmark, task, or revision name to begin '.
+          'or resume work on.'));
+    } else if (count($argv) === 1) {
+      $symbol_argument = $argv[0];
+      if (!strlen($symbol_argument)) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Provide a nonempty symbol to begin or resume work on.'));
+      }
+    } else {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Too many arguments: provide exactly one argument.'));
+    }
+
+    $start_argument = $this->getArgument('start');
+
+    $work_engine
+      ->setViewer($this->getViewer())
+      ->setWorkflow($this)
+      ->setLogEngine($this->getLogEngine())
+      ->setSymbolArgument($symbol_argument)
+      ->setStartArgument($start_argument)
+      ->execute();
+
+    return 0;
+  }
+
+}