Don't error when trying to mirror or observe an empty repository

Summary:
Fixes T5965.

Fixes two issues:

  - Observing an empty repository could write a warning to the log.
  - Mirroring an empty repository to a remote could fail.

For observing:

If newly-created with `git init --bare`, `git ls-remote` will
return the empty string.  Properly return an empty set of refs, rather
than attempting to parse the single "line" that is produced by
splitting that on newlines:

```
[2018-01-23 18:47:00] ERROR 8: Undefined offset: 1 at [/phab_path/phabricator/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:405]
arcanist(head=master, ref.master=5634f8410176), phabricator(head=master, ref.master=12551a1055ce), phutil(head=master, ref.master=4755785517cf)
  #0 PhabricatorRepositoryPullEngine::loadGitRemoteRefs(PhabricatorRepository) called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:343]
  #1 PhabricatorRepositoryPullEngine::executeGitUpdate() called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:126]
  #2 PhabricatorRepositoryPullEngine::pullRepositoryWithLock() called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:40]
  #3 PhabricatorRepositoryPullEngine::pullRepository() called at [<phabricator>/src/applications/repository/management/PhabricatorRepositoryManagementUpdateWorkflow.php:59]
  ...
```

For mirroring:

`git` treats `git push --mirror` specially when a repository is empty. Detect this case by seeing if `git for-each-ref --count 1` does anything. If the repository is empty, just bail.

Test Plan:
  - Observed an empty and non-empty repository.
  - Mirrored an empty and non-empty repository.

Reviewers: alexmv, amckinley

Reviewed By: alexmv

Subscribers: Korvin, epriestley

Maniphest Tasks: T5965

Differential Revision: https://secure.phabricator.com/D18920

Author: epriestley

src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php

@@ -76,6 +76,20 @@ private function pushToGitRepository(
     PhabricatorRepository $repository,
     PhabricatorRepositoryURI $mirror_uri) {
 
+    // See T5965. Test if we have any refs to mirror. If we have nothing, git
+    // will exit with an error ("No refs in common and none specified; ...")
+    // when we run "git push --mirror".
+
+    // If we don't have any refs, we just bail out. (This is arguably sort of
+    // the wrong behavior: to mirror an empty repository faithfully we should
+    // delete everything in the remote.)
+
+    list($stdout) = $repository->execxLocalCommand(
+      'for-each-ref --count 1 --');
+    if (!strlen($stdout)) {
+      return;
+    }
+
     $argv = array(
       'push --verbose --mirror -- %P',
       $mirror_uri->getURIEnvelope(),

src/applications/repository/engine/PhabricatorRepositoryPullEngine.php

@@ -399,6 +399,11 @@ private function loadGitRemoteRefs(PhabricatorRepository $repository) {
       'ls-remote %P',
       $remote_envelope);
 
+    // Empty repositories don't have any refs.
+    if (!strlen(rtrim($stdout))) {
+      return array();
+    }
+
     $map = array();
     $lines = phutil_split_lines($stdout, false);
     foreach ($lines as $line) {