Fix wrong commit count in push event payload

This fixes a regression in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20916.
We now only use the full commit count for the initial push to the default branch.
Otherwise, we rely on the number of commits that we calculated occurred in the push.

The original behavior in 11.1.4:

1. When a new branch is pushed, `@push_commits_count` was set to the total
number of refs available for the branch.
2. For other branches, `@push_commits_count` would remain `nil`.
3. `GitPushService#build_push_data` would build the push data with
`@push_commits_count`.
4. If this were `nil`, it would be set to the right value here:
https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.1.4/lib/gitlab/data_builder/push.rb#L60

Broken behavior:

1. `GitPushService#push_commits_count` is always called.
2. The total number of commits is therefore always equal to the total number of refs available.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49971

app/services/git_push_service.rb

@@ -158,7 +158,7 @@ class GitPushService < BaseService
   end
 
   def process_default_branch
-    offset = [push_commits_count - PROCESS_COMMIT_LIMIT, 0].max
+    offset = [push_commits_count_for_ref - PROCESS_COMMIT_LIMIT, 0].max
     @push_commits = project.repository.commits(params[:newrev], offset: offset, limit: PROCESS_COMMIT_LIMIT)
 
     project.after_create_default_branch
@@ -172,7 +172,7 @@ class GitPushService < BaseService
       params[:newrev],
       params[:ref],
       @push_commits,
-      commits_count: push_commits_count)
+      commits_count: commits_count)
   end
 
   def push_to_existing_branch?
@@ -213,8 +213,14 @@ class GitPushService < BaseService
     end
   end
 
-  def push_commits_count
-    strong_memoize(:push_commits_count) do
+  def commits_count
+    return push_commits_count_for_ref if default_branch? && push_to_new_branch?
+
+    Array(@push_commits).size
+  end
+
+  def push_commits_count_for_ref
+    strong_memoize(:push_commits_count_for_ref) do
       project.repository.commit_count_for_ref(params[:ref])
     end
   end

changelogs/unreleased/sh-fix-wrong-commit-count-in-push.yml

+---
+title: Fix wrong commit count in push event payload
+merge_request: 21338
+author:
+type: fixed

spec/services/git_push_service_spec.rb

@@ -204,16 +204,37 @@ describe GitPushService, services: true do
   end
 
   describe "Push Event" do
-    let!(:push_data) { push_data_from_service(project, user, oldrev, newrev, ref) }
-    let(:event) { Event.find_by_action(Event::PUSHED) }
+    context "with an existing branch" do
+      let!(:push_data) { push_data_from_service(project, user, oldrev, newrev, ref) }
+      let(:event) { Event.find_by_action(Event::PUSHED) }
 
-    it { expect(event).to be_an_instance_of(PushEvent) }
-    it { expect(event.project).to eq(project) }
-    it { expect(event.action).to eq(Event::PUSHED) }
-    it { expect(event.push_event_payload).to be_an_instance_of(PushEventPayload) }
-    it { expect(event.push_event_payload.commit_from).to eq(oldrev) }
-    it { expect(event.push_event_payload.commit_to).to eq(newrev) }
-    it { expect(event.push_event_payload.ref).to eq('master') }
+      it 'generates a push event with one commit' do
+        expect(event).to be_an_instance_of(PushEvent)
+        expect(event.project).to eq(project)
+        expect(event.action).to eq(Event::PUSHED)
+        expect(event.push_event_payload).to be_an_instance_of(PushEventPayload)
+        expect(event.push_event_payload.commit_from).to eq(oldrev)
+        expect(event.push_event_payload.commit_to).to eq(newrev)
+        expect(event.push_event_payload.ref).to eq('master')
+        expect(event.push_event_payload.commit_count).to eq(1)
+      end
+    end
+
+    context "with a new branch" do
+      let!(:new_branch_data) { push_data_from_service(project, user, Gitlab::Git::BLANK_SHA, newrev, ref) }
+      let(:event) { Event.find_by_action(Event::PUSHED) }
+
+      it 'generates a push event with more than one commit' do
+        expect(event).to be_an_instance_of(PushEvent)
+        expect(event.project).to eq(project)
+        expect(event.action).to eq(Event::PUSHED)
+        expect(event.push_event_payload).to be_an_instance_of(PushEventPayload)
+        expect(event.push_event_payload.commit_from).to be_nil
+        expect(event.push_event_payload.commit_to).to eq(newrev)
+        expect(event.push_event_payload.ref).to eq('master')
+        expect(event.push_event_payload.commit_count).to be > 1
+      end
+    end
 
     context "Updates merge requests" do
       it "when pushing a new branch for the first time" do
@@ -224,6 +245,7 @@ describe GitPushService, services: true do
     end
 
     describe 'system hooks' do
+      let!(:push_data) { push_data_from_service(project, user, oldrev, newrev, ref) }
       let(:system_hooks_service) { SystemHooksService.new }
 
       it "sends a system hook after pushing a branch" do