Fix Gitaly N+1 calls with listing issues/MRs via API

In GitLab 9.0,
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9661 removed the
`subscribed` flag from the API when the user requested a list of issues
or merge requests since calculating this value triggers extensive
Markdown processing.

In GitLab 12.0 via a4fbf39e, we accidentally reintroduced this
performance regression by changing `IssueBasic` to `Issue` in
`entities.rb`. This showed up as a Gitaly N+1 issue since the Markdown
processing would attempt to extract a commit if it detected a regex that
matched a commit.

We restore the prior behavior by once again removing the `subscribed`
flag for the bulk list of issues and merge requests and add a test to
ensure they aren't reintroduced.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66202

changelogs/unreleased/sh-fix-issues-api-gitaly-nplusone.yml

+---
+title: Fix Gitaly N+1 calls with listing issues/MRs via API
+merge_request: 31938
+author:
+type: performance

doc/api/issues.md

@@ -136,7 +136,6 @@ Example response:
          "award_emoji":"http://example.com/api/v4/projects/1/issues/76/award_emoji",
          "project":"http://example.com/api/v4/projects/1"
       },
-      "subscribed": false,
       "task_completion_status":{
          "count":0,
          "completed_count":0
@@ -441,7 +440,6 @@ Example response:
          "award_emoji":"http://example.com/api/v4/projects/4/issues/41/award_emoji",
          "project":"http://example.com/api/v4/projects/4"
       },
-      "subscribed": false,
       "task_completion_status":{
          "count":0,
          "completed_count":0

lib/api/entities.rb

@@ -645,7 +645,10 @@ module API
         end
       end
-      expose :subscribed do |issue, options|
+      # Calculating the value of subscribed field triggers Markdown
+      # processing. We can't do that for multiple issues / merge
+      # requests in a single API request.
+      expose :subscribed, if: -> (_, options) { options.fetch(:include_subscribed, true) } do |issue, options|
         issue.subscribed?(options[:current_user], options[:project] || issue.project)
       end
     end

lib/api/issues.rb

@@ -96,7 +96,8 @@ module API
           with: Entities::Issue,
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
-          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user)
+          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user),
+          include_subscribed: false
         }
         present issues, options
@@ -122,7 +123,8 @@ module API
           with: Entities::Issue,
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
-          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user)
+          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user),
+          include_subscribed: false
         }
         present issues, options

spec/requests/api/issues/get_group_issues_spec.rb

@@ -342,6 +342,14 @@ describe API::Issues do
         group_project.add_reporter(user)
       end
+      it 'exposes known attributes' do
+        get api(base_url, admin)
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response.last.keys).to include(*%w(id iid project_id title description))
+        expect(json_response.last).not_to have_key('subscribed')
+      end
+
       it 'returns all group issues (including opened and closed)' do
         get api(base_url, admin)

spec/requests/api/issues/get_project_issues_spec.rb

@@ -575,6 +575,7 @@ describe API::Issues do
       expect(json_response['assignee']).to be_a Hash
       expect(json_response['author']).to be_a Hash
       expect(json_response['confidential']).to be_falsy
+      expect(json_response['subscribed']).to be_truthy
     end
     it 'exposes the closed_at attribute' do

spec/requests/api/issues/issues_spec.rb

@@ -216,6 +216,10 @@ describe API::Issues do
         expect_paginated_array_response([issue.id, closed_issue.id])
         expect(json_response.first['title']).to eq(issue.title)
         expect(json_response.last).to have_key('web_url')
+        # Calculating the value of subscribed field triggers Markdown
+        # processing. We can't do that for multiple issues / merge
+        # requests in a single API request.
+        expect(json_response.last).not_to have_key('subscribed')
       end
       it 'returns an array of closed issues' do