git: use force fetch and update for target ref. This solves a case

when in PRs a target is force updated and is out of sync. Before
we used a pull which --ff-only fails obviosly because two are out of
sync. This change uses new logic that resets the target branch according to the
source target branch allowing smooth merge simulation.

rhodecode/apps/repository/tests/test_repo_pullrequests.py

@@ -747,6 +747,69 b' class TestPullrequestsView(object):'
         assert 'Pull request updated to' in response.body
         assert 'with 1 added, 1 removed commits.' in response.body
 
+    def test_update_target_revision_with_removal_of_1_commit_git(self, backend_git, csrf_token):
+        backend = backend_git
+        commits = [
+            {'message': 'master-commit-1'},
+            {'message': 'master-commit-2-change-1'},
+            {'message': 'master-commit-3-change-2'},
+
+            {'message': 'feat-commit-1', 'parents': ['master-commit-1']},
+            {'message': 'feat-commit-2'},
+        ]
+        commit_ids = backend.create_master_repo(commits)
+        target = backend.create_repo(heads=['master-commit-3-change-2'])
+        source = backend.create_repo(heads=['feat-commit-2'])
+
+        # create pr from a in source to A in target
+        pull_request = PullRequest()
+        pull_request.source_repo = source
+        # TODO: johbo: Make sure that we write the source ref this way!
+        pull_request.source_ref = 'branch:{branch}:{commit_id}'.format(
+            branch=backend.default_branch_name,
+            commit_id=commit_ids['master-commit-3-change-2'])
+
+        pull_request.target_repo = target
+        # TODO: johbo: Target ref should be branch based, since tip can jump
+        # from branch to branch
+        pull_request.target_ref = 'branch:{branch}:{commit_id}'.format(
+            branch=backend.default_branch_name,
+            commit_id=commit_ids['feat-commit-2'])
+
+        pull_request.revisions = [
+            commit_ids['feat-commit-1'],
+            commit_ids['feat-commit-2']
+        ]
+        pull_request.title = u"Test"
+        pull_request.description = u"Description"
+        pull_request.author = UserModel().get_by_username(
+            TEST_USER_ADMIN_LOGIN)
+        Session().add(pull_request)
+        Session().commit()
+        pull_request_id = pull_request.pull_request_id
+
+        # PR is created, now we simulate a force-push into target,
+        # that drops a 2 last commits
+        vcsrepo = target.scm_instance()
+        vcsrepo.config.clear_section('hooks')
+        vcsrepo.run_git_command(['reset', '--soft', 'HEAD~2'])
+
+        # update PR
+        self.app.post(
+            route_path('pullrequest_update',
+                repo_name=target.repo_name,
+                pull_request_id=pull_request_id),
+            params={'update_commits': 'true',
+                    'csrf_token': csrf_token},
+            status=200)
+
+        response = self.app.get(route_path(
+            'pullrequest_new',
+            repo_name=target.repo_name))
+        assert response.status_int == 200
+        response.mustcontain('Pull request updated to')
+        response.mustcontain('with 0 added, 0 removed commits.')
+
     def test_update_of_ancestor_reference(self, backend, csrf_token):
         commits = [
             {'message': 'ancestor'},

rhodecode/lib/vcs/backends/git/repository.py

@@ -757,7 +757,7 b' class GitRepository(BaseRepository):'
                 cmd = ['fetch', self.path, source_branch]
                 self.run_git_command(cmd, fail_on_stderr=False)
 
-    def _local_fetch(self, repository_path, branch_name):
+    def _local_fetch(self, repository_path, branch_name, use_origin=False):
         """
         Fetch a branch from a local repository.
         """
@@ -765,7 +765,17 b' class GitRepository(BaseRepository):'
         if repository_path == self.path:
             raise ValueError('Cannot fetch from the same repository')
 
-        cmd = ['fetch', '--no-tags', repository_path, branch_name]
+        if use_origin:
+            branch_name = '+{branch}:refs/heads/{branch}'.format(
+                branch=branch_name)
+
+        cmd = ['fetch', '--no-tags', '--update-head-ok',
+               repository_path, branch_name]
+        self.run_git_command(cmd, fail_on_stderr=False)
+
+    def _local_reset(self, branch_name):
+        branch_name = '{}'.format(branch_name)
+        cmd = ['reset', '--hard', branch_name]
         self.run_git_command(cmd, fail_on_stderr=False)
 
     def _last_fetch_heads(self):
@@ -840,7 +850,7 b' class GitRepository(BaseRepository):'
                'merge', '--no-ff', '-m', safe_str(merge_message)]
         cmd.extend(heads)
         try:
-            self.run_git_command(cmd, fail_on_stderr=False)
+            output = self.run_git_command(cmd, fail_on_stderr=False)
         except RepositoryError:
             # Cleanup any merge leftovers
             self.run_git_command(['merge', '--abort'], fail_on_stderr=False)
@@ -897,6 +907,8 b' class GitRepository(BaseRepository):'
                     merger_name, merger_email, dry_run=False,
                     use_rebase=False, close_branch=False):
         if target_ref.commit_id != self.branches[target_ref.name]:
+            log.warning('Target ref %s commit mismatch %s vs %s', target_ref,
+                        target_ref.commit_id, self.branches[target_ref.name])
             return MergeResponse(
                 False, False, None, MergeFailureReason.TARGET_IS_NOT_HEAD)
 
@@ -907,20 +919,29 b' class GitRepository(BaseRepository):'
             if shadow_repo.get_remote_ref(source_ref.name):
                 shadow_repo._checkout(source_ref.name, force=True)
 
-        # checkout target
+        # checkout target, and fetch changes
         shadow_repo._checkout(target_ref.name, force=True)
-        shadow_repo._local_pull(self.path, target_ref.name)
+
+        # fetch/reset pull the target, in case it is changed
+        # this handles even force changes
+        shadow_repo._local_fetch(self.path, target_ref.name, use_origin=True)
+        shadow_repo._local_reset(target_ref.name)
 
         # Need to reload repo to invalidate the cache, or otherwise we cannot
         # retrieve the last target commit.
         shadow_repo = GitRepository(shadow_repository_path)
         if target_ref.commit_id != shadow_repo.branches[target_ref.name]:
+            log.warning('Shadow Target ref %s commit mismatch %s vs %s',
+                        target_ref, target_ref.commit_id,
+                        shadow_repo.branches[target_ref.name])
             return MergeResponse(
                 False, False, None, MergeFailureReason.TARGET_IS_NOT_HEAD)
 
+        # calculate new branch
         pr_branch = shadow_repo._get_new_pr_branch(
             source_ref.name, target_ref.name)
         log.debug('using pull-request merge branch: `%s`', pr_branch)
+        # checkout to temp branch, and fetch changes
         shadow_repo._checkout(pr_branch, create=True)
         try:
             shadow_repo._local_fetch(source_repo.path, source_ref.name)

rhodecode/templates/pullrequests/pullrequest_show.mako

@@ -417,6 +417,7 b''
                       <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}
                     </div>
                   </div>
                 </div>