Fix Gitaly N+1 calls with listing issues/MRs via APIsh-fix-issues-api-gitaly-nplusone

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

2 files changed, 8 insertions, 3 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 09253ab6b0e..5e66b4e76a5 100644
--- a/lib/api/entities.rb
+++ b/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
diff --git a/lib/api/issues.rb b/lib/api/issues.rb
index d687acf3423..7819c2de515 100644
--- a/lib/api/issues.rb
+++ b/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