Optimize branch lookups and force a repository reload for Repository#find_branchsh-reload-find-branch

If `git gc` runs and `Repository` has an instance to `Rugged::Repository`, a
bug in libgit2 may cause the instance to return a stale value or a missing
branch. This change not only optimizes the branch lookup so we don't have
to iterate through every branch, but it also works around the `git gc`
issue by forcing a repository reload every time `Repository#find_branch`
is called.

See: https://github.com/libgit2/libgit2/issues/1534

Closes #15392, #21470

1 files changed, 15 insertions, 2 deletions

diff --git a/app/models/repository.rb b/app/models/repository.rb
index 91bdafdac99..f891e8374d2 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -120,8 +120,21 @@ class Repository
     commits
   end
 
-  def find_branch(name)
-    raw_repository.branches.find { |branch| branch.name == name }
+  def find_branch(name, fresh_repo: true)
+    # Since the Repository object may have in-memory index changes, invalidating the memoized Repository object may
+    # cause unintended side effects. Because finding a branch is a read-only operation, we can safely instantiate
+    # a new repo here to ensure a consistent state to avoid a libgit2 bug where concurrent access (e.g. via git gc)
+    # may cause the branch to "disappear" erroneously or have the wrong SHA.
+    #
+    # See: https://github.com/libgit2/libgit2/issues/1534 and https://gitlab.com/gitlab-org/gitlab-ce/issues/15392
+    raw_repo =
+      if fresh_repo
+        Gitlab::Git::Repository.new(path_to_repo)
+      else
+        raw_repository
+      end
+
+    raw_repo.find_branch(name)
   end
 
   def find_tag(name)