Look for new branches more carefully

In certain cases, GitLab can miss a PostReceive invocation the first
time a branch is pushed. When this happens, the "branch created" hooks
are not run, which means various features don't work until the branch
is deleted and pushed again.

This MR changes the `Git::BranchPushService` so it checks the cache of
existing branches in addition to the `oldrev` reported for the branch.
If the branch name isn't in the cache, chances are we haven't run the
service yet (it's what refreshes the cache), so we can go ahead and
run it, even through `oldrev` is set.

If the cache has been cleared by some other means in the meantime, then
we'll still fail to run the hooks when we should. Fixing that in the
general case is a larger problem, and we'd need to devote significant
engineering effort to it.

There's a chance that we'll run the relevant hooks *multiple times*
with this change, if there's a race between the branch being created,
and the `PostReceive` worker being run multiple times, but this can
already happen, since Sidekiq is "at-least-once" execution of jobs. So,
this should be safe.

app/services/git/branch_hooks_service.rb

@@ -105,8 +105,14 @@ module Git
       CreateGpgSignatureWorker.perform_async(signable, project.id)
     end
 
+    # It's not sufficient to just check for a blank SHA as it's possible for the
+    # branch to be pushed, but for the `post-receive` hook to never run:
+    # https://gitlab.com/gitlab-org/gitlab-ce/issues/59257
     def creating_branch?
-      Gitlab::Git.blank_ref?(params[:oldrev])
+      strong_memoize(:creating_branch) do
+        Gitlab::Git.blank_ref?(params[:oldrev]) ||
+          !project.repository.branch_exists?(branch_name)
+      end
     end
 
     def updating_branch?

changelogs/unreleased/59257-find-new-branches-harder.yml

@@ -0,0 +1,5 @@
+---
+title: Look for new branches more carefully
+merge_request: 29761
+author:
+type: fixed

spec/services/git/branch_hooks_service_spec.rb

@@ -344,4 +344,38 @@ describe Git::BranchHooksService do
       end
     end
   end
+
+  describe 'New branch detection' do
+    let(:branch) { 'fix' }
+
+    context 'oldrev is the blank SHA' do
+      let(:oldrev) { Gitlab::Git::BLANK_SHA }
+
+      it 'is treated as a new branch' do
+        expect(service).to receive(:branch_create_hooks)
+
+        service.execute
+      end
+    end
+
+    context 'oldrev is set' do
+      context 'Gitaly does not know about the branch' do
+        it 'is treated as a new branch' do
+          allow(project.repository).to receive(:branch_names) { [] }
+
+          expect(service).to receive(:branch_create_hooks)
+
+          service.execute
+        end
+      end
+
+      context 'Gitaly knows about the branch' do
+        it 'is not treated as a new branch' do
+          expect(service).not_to receive(:branch_create_hooks)
+
+          service.execute
+        end
+      end
+    end
+  end
 end