Remove rescue of Gitlab::Git::CommitError at user_merge_to_ref RPC

We've been experiencing transient Gitlab::Git::CommitError's
that were being rescued at gitaly-ruby, specially at
UserMergeToRef RPC.

This commit removes these rescues and also make the error
messages more detailed for further investigation at Sentry.

4 files changed, 28 insertions, 7 deletions

diff --git a/changelogs/unreleased/osw-bubble-up-conflicts-error-to-sentry.yml b/changelogs/unreleased/osw-bubble-up-conflicts-error-to-sentry.yml
new file mode 100644
index 000000000..ae09c85f8
--- /dev/null
+++ b/changelogs/unreleased/osw-bubble-up-conflicts-error-to-sentry.yml
@@ -0,0 +1,5 @@
+---
+title: Remove rescue of Gitlab::Git::CommitError at UserMergeToRef RPC
+merge_request: 1372
+author:
+type: other
diff --git a/ruby/lib/gitlab/git/operation_service.rb b/ruby/lib/gitlab/git/operation_service.rb
index c2869039f..1235a1c01 100644
--- a/ruby/lib/gitlab/git/operation_service.rb
+++ b/ruby/lib/gitlab/git/operation_service.rb
@@ -110,15 +110,21 @@ module Gitlab
       #
       # ref - The target ref path we're committing to.
       # from_ref - The ref we're taking the HEAD commit from.
-      def commit_ref(ref, from_ref:)
+      def commit_ref(ref, source_sha, from_ref:)
         update_autocrlf_option
 
-        repository.write_ref(ref, from_ref.target)
+        target_sha = from_ref.target
+        repository.write_ref(ref, target_sha)
 
         # Make commit
         newrev = yield
 
-        raise Gitlab::Git::CommitError.new('Failed to create commit') unless newrev
+        unless newrev
+          error = "Failed to create merge commit for source_sha #{source_sha} and" \
+                  " target_sha #{target_sha} at #{ref}"
+
+          raise Gitlab::Git::CommitError.new(error)
+        end
 
         oldrev = from_ref.target
 
@@ -201,8 +207,10 @@ module Gitlab
 
         unless status.zero?
           Gitlab::GitLogger.error("'git update-ref' in #{repository.path}: #{output}")
+          ref_name = Gitlab::Git.branch_name(ref) || ref
+
           raise Gitlab::Git::CommitError.new(
-            "Could not update branch #{Gitlab::Git.branch_name(ref)}." \
+            "Could not update #{ref_name}." \
             " Please refresh and try again."
           )
         end
diff --git a/ruby/lib/gitlab/git/repository.rb b/ruby/lib/gitlab/git/repository.rb
index f98886877..ad14b35bb 100644
--- a/ruby/lib/gitlab/git/repository.rb
+++ b/ruby/lib/gitlab/git/repository.rb
@@ -277,14 +277,12 @@ module Gitlab
 
         raise InvalidRef unless ref
 
-        OperationService.new(user, self).commit_ref(target_ref, from_ref: ref) do
+        OperationService.new(user, self).commit_ref(target_ref, source_sha, from_ref: ref) do
           our_commit = ref.target
           their_commit = source_sha
 
           create_merge_commit(user, our_commit, their_commit, message)
         end
-      rescue Gitlab::Git::CommitError # when merge_index.conflicts?
-        nil
       rescue Rugged::ReferenceError, InvalidRef
         raise ArgumentError, 'Invalid merge source'
       end
diff --git a/ruby/spec/lib/gitlab/git/repository_spec.rb b/ruby/spec/lib/gitlab/git/repository_spec.rb
index 8cb8558a6..c7cedbdb7 100644
--- a/ruby/spec/lib/gitlab/git/repository_spec.rb
+++ b/ruby/spec/lib/gitlab/git/repository_spec.rb
@@ -543,6 +543,16 @@ describe Gitlab::Git::Repository do # rubocop:disable Metrics/BlockLength
         let(:arg_first_parent_ref) {}
       end
     end
+
+    context 'when conflicts detected' do
+      it 'raises Gitlab::Git::CommitError' do
+        allow(repository.rugged).to receive_message_chain(:merge_commits, :conflicts?) { true }
+
+        expect { repository.merge_to_ref(user, source_sha, arg_branch, target_ref, 'foo', arg_first_parent_ref) }
+          .to raise_error(Gitlab::Git::CommitError, "Failed to create merge commit for source_sha #{source_sha} and" \
+                                                    " target_sha #{branch_head} at #{target_ref}")
+      end
+    end
   end
 
   describe '#ff_merge' do