Add support to marking commits as UNREACHABLE for Mercurial

Summary:
When previously known commits have been destroyed in a Mercurial repository, Phabricator does not end up marking the commits as unreachable. This results in daemon tasks which continuously fail and retry.

This updates `PhabricatorRepositoryDiscoveryEngine` and `PhabricatorManagementRepositoryMarkReachableWorkflow` to include support of marking commits as unreachable for Mercurial repositories.

The `PhabricatorMercurialGraphStream` also needed updated to support a stream with no starting commit.

Refs T13634

Test Plan:
1. I set up a hosted Mercurial repository.
2. I removed the head commit from the on-disk repository state.
3. I attempted to load the repository page and saw an exception due to a missing commit.
4. I went to `/manage` for the repository and scheduled an update of the repository.
5. After an updated performed, I went to the repository main page and saw there was no exception and the history view properly did not have the commit I had removed.
6. I checked the phd logs and verified there were no exceptions related to the repository.
7. I ran the `./bin/repository mark-reachable` command on the Mercurial repository and it reported that it marked the commit as unreachable.
8. I pushed the same commit back upstream and verified that the commit was found and displayed in the history view of the repository page and `mark-unreachable` did not identify it as being unreachable.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T13634

Differential Revision: https://secure.phabricator.com/D21715

src/applications/repository/daemon/PhabricatorMercurialGraphStream.php

@@ -16,13 +16,23 @@ final class PhabricatorMercurialGraphStream
   private $local          = array();
   private $localParents   = array();
 
-  public function __construct(PhabricatorRepository $repository, $commit) {
+  public function __construct(PhabricatorRepository $repository,
+    $start_commit = null) {
+
     $this->repository = $repository;
 
+    $command = 'log --template %s --rev %s';
+    $template = '{rev}\1{node}\1{date}\1{parents}\2';
+    if ($start_commit !== null) {
+      $revset = hgsprintf('reverse(ancestors(%s))', $start_commit);
+    } else {
+      $revset = 'reverse(all())';
+    }
+
     $future = $repository->getLocalCommandFuture(
-      'log --template %s --rev %s',
-      '{rev}\1{node}\1{date}\1{parents}\2',
-      hgsprintf('reverse(ancestors(%s))', $commit));
+      $command,
+      $template,
+      $revset);
 
     $this->iterator = new LinesOfALargeExecFuture($future);
     $this->iterator->setDelimiter("\2");

src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php

@@ -780,8 +780,7 @@ final class PhabricatorRepositoryDiscoveryEngine
   }
 
   private function markUnreachableCommits(PhabricatorRepository $repository) {
-    // For now, this is only supported for Git.
-    if (!$repository->isGit()) {
+    if (!$repository->isGit() && !$repository->isHg()) {
       return;
     }
 
@@ -799,7 +798,11 @@ final class PhabricatorRepositoryDiscoveryEngine
     }
 
     // We can share a single graph stream across all the checks we need to do.
-    $stream = new PhabricatorGitGraphStream($repository);
+    if ($repository->isGit()) {
+      $stream = new PhabricatorGitGraphStream($repository);
+    } else if ($repository->isHg()) {
+      $stream = new PhabricatorMercurialGraphStream($repository);
+    }
 
     foreach ($old_refs as $old_ref) {
       $identifier = $old_ref->getCommitIdentifier();
@@ -812,7 +815,7 @@ final class PhabricatorRepositoryDiscoveryEngine
 
   private function markUnreachableFrom(
     PhabricatorRepository $repository,
-    PhabricatorGitGraphStream $stream,
+    PhabricatorRepositoryGraphStream $stream,
     $identifier) {
 
     $unreachable = array();

src/applications/repository/management/PhabricatorRepositoryManagementMarkReachableWorkflow.php

@@ -48,11 +48,11 @@ final class PhabricatorRepositoryManagementMarkReachableWorkflow
   }
 
   private function markReachable(PhabricatorRepository $repository) {
-    if (!$repository->isGit()) {
+    if (!$repository->isGit() && !$repository->isHg()) {
       throw new PhutilArgumentUsageException(
         pht(
-          'Only Git repositories are supported, this repository ("%s") is '.
-          'not a Git repository.',
+          'Only Git and Mercurial repositories are supported, unable to '.
+          'operate on this repository ("%s").',
           $repository->getDisplayName()));
     }
 
@@ -65,7 +65,12 @@ final class PhabricatorRepositoryManagementMarkReachableWorkflow
 
     $flag = PhabricatorRepositoryCommit::IMPORTED_UNREACHABLE;
 
-    $graph = new PhabricatorGitGraphStream($repository);
+    if ($repository->isGit()) {
+      $graph = new PhabricatorGitGraphStream($repository);
+    } else if ($repository->isHg()) {
+      $graph = new PhabricatorMercurialGraphStream($repository);
+    }
+
     foreach ($commits as $commit) {
       $identifier = $commit->getCommitIdentifier();