commits: detect missing commits on diffsets from new PR ancestor logic.

- this would not crash the server and allow people to update some old PRs
- fixes  #5622

rhodecode/apps/repository/views/repo_pull_requests.py

@@ -568,6 +568,7 b' class RepoPullRequestsView(RepoAppView, '
             c.commit_ranges.append(comm)
 
         c.missing_requirements = missing_requirements
+
         c.ancestor_commit = ancestor_commit
         c.statuses = source_repo.statuses(
             [x.raw_id for x in c.commit_ranges])
@@ -596,6 +597,7 b' class RepoPullRequestsView(RepoAppView, '
             if not force_recache and has_proper_diff_cache:
                 c.diffset = cached_diff['diff']
             else:
+                try:
                 c.diffset = self._get_diffset(
                     c.source_repo.repo_name, commits_source_repo,
                     c.ancestor_commit,
@@ -603,10 +605,14 b' class RepoPullRequestsView(RepoAppView, '
                     target_commit, source_commit,
                     diff_limit, file_limit, c.fulldiff,
                     hide_whitespace_changes, diff_context)
-
                 # save cached diff
                 if caching_enabled:
                     cache_diff(cache_file_path, c.diffset, diff_commit_cache)
+                except CommitDoesNotExistError:
+                    log.exception('Failed to generate diffset')
+                    c.missing_commits = True
+
+            if not c.missing_commits:
 
             c.limited_diff = c.diffset.limited_diff
 

rhodecode/templates/pullrequests/pullrequest_show.mako

@@ -465,9 +465,9 b''
                   <div class="alert alert-warning">
                     <div>
                       <strong>${_('Missing commits')}:</strong>
-                        ${_('This pull request cannot be displayed, because one or more commits no longer exist in the source repository.')}
-                        ${_('Please update this pull request, push the commits back into the source repository, or consider closing this pull request.')}
-                        ${_('Consider doing a {force_refresh_url} in case you think this is an error.').format(force_refresh_url=h.link_to('force refresh', h.current_route_path(request, force_refresh='1')))|n}
+                        ${_('This pull request cannot be displayed, because one or more commits no longer exist in the source repository.')}<br/>
+                        ${_('Please update this pull request, push the commits back into the source repository, or consider closing this pull request.')}<br/>
+                        ${_('Consider doing a `force update commits` in case you think this is an error.')}
                     </div>
                   </div>
                 </div>