ActionView::Template::Error: undefined method `sha' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/2735/

NoMethodError: undefined method `sha' for nil:NilClass
  app/models/merge_request.rb:575:in `target_sha'
    @target_sha ||= target_project.repository.commit(target_branch).sha
  app/models/merge_request.rb:637:in `diverged_commits_count'
    target_sha: target_sha,
  lib/gitlab/metrics/instrumentation.rb:138:in `diverged_commits_count'
    retval   = #{send_signature}
  app/models/merge_request.rb:651:in `diverged_from_target_branch?'
    diverged_commits_count > 0
  lib/gitlab/metrics/instrumentation.rb:138:in `diverged_from_target_branch?'
    retval   = #{send_signature}
...
(148 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `sha' for nil:NilClass
  app/models/merge_request.rb:575:in `target_sha'
    @target_sha ||= target_project.repository.commit(target_branch).sha
  app/models/merge_request.rb:637:in `diverged_commits_count'
    target_sha: target_sha,
  lib/gitlab/metrics/instrumentation.rb:138:in `diverged_commits_count'
    retval   = #{send_signature}
  app/models/merge_request.rb:651:in `diverged_from_target_branch?'
    diverged_commits_count > 0
  lib/gitlab/metrics/instrumentation.rb:138:in `diverged_from_target_branch?'
    retval   = #{send_signature}
...
(148 additional frame(s) were not displayed)

In that case, the user opened a MR master23 -> test and then deleted the test branch. I can reproduce this issue locally.

Also reported at https://gitlab.com/gitlab-org/gitlab-ce/issues/14305#note_4316457.

Added regression label

mentioned in issue #14305 (moved)

Milestone changed to 8.6

mentioned in issue #14303 (closed)

mentioned in merge request !3135 (merged)

I found the issue: before displaying the "diverged commits" note, we're checking if the MR is open, but we should check if it's mergeable instead because this check ensure the source and target branches exist: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/projects/merge_requests/_show.html.haml#L37

I'll open an MR.

mentioned in merge request !3294 (merged)

mentioned in commit 4a8a8282

Status changed to closed by commit 4a8a8282

mentioned in commit 49534a65

mentioned in commit 17418461