[IMP] runbot_merge: map PR commits to integrated commits

* when rebasing, store a map of rebased to source, that way it'll be
  possible to link cherry-picked forward ports to the originally
  integrated commit rather than just the one from the PR (which was
  likely not itself integrated as the straight merge mode is somewhat
  rare: as of 5600 PRs merged so far only 100 were straight merged)
* while at it, store the "merge head" of the PR (whether squashed,
  merged or rebased) and put *that* in the commit message

fixes #161

runbot_merge/github.py

@@ -172,7 +172,7 @@ class GH(object):
         """ Rebase pr's commits on top of dest, updates dest unless ``reset``
         is set.
 
-        Returns the hash of the rebased head.
+        Returns the hash of the rebased head and a map of all PR commits (to the PR they were rebased to)
         """
         logger = _logger.getChild('rebase')
         original_head = self.head(dest)
@@ -189,6 +189,7 @@ class GH(object):
             c['new_tree'] = self.merge(c['sha'], dest, tmp_msg)['tree']['sha']
 
         prev = original_head
+        mapping = {}
         for c in commits:
             copy = self('post', 'git/commits', json={
                 'message': c['commit']['message'],
@@ -198,7 +199,7 @@ class GH(object):
                 'committer': c['commit']['committer'],
             }, check={409: exceptions.MergeError}).json()
             logger.debug('copied %s to %s (parent: %s)', c['sha'], copy['sha'], prev)
-            prev = copy['sha']
+            prev = mapping[c['sha']] = copy['sha']
 
         if reset:
             self.set_ref(dest, original_head)
@@ -209,7 +210,7 @@ class GH(object):
                       self._repo, pr, dest, reset, len(commits),
                       prev)
         # prev is updated after each copy so it's the rebased PR head
-        return prev
+        return prev, mapping
 
     # fetch various bits of issues / prs to load them
     def pr(self, number):

runbot_merge/models/pull_requests.py

@@ -494,6 +494,7 @@ class PullRequests(models.Model):
     batch_id = fields.Many2one('runbot_merge.batch',compute='_compute_active_batch', store=True)
     batch_ids = fields.Many2many('runbot_merge.batch')
     staging_id = fields.Many2one(related='batch_id.staging_id', store=True)
+    commits_map = fields.Char(help="JSON-encoded mapping of PR commits to actually integrated commits. The integration head (either a merge commit or the PR's topmost) is mapped from the 'empty' pr commit (the key is an empty string, because you can't put a null key in json maps).", default='{}')
 
     link_warned = fields.Boolean(
         default=False, help="Whether we've already warned that this (ready)"
@@ -961,12 +962,16 @@ class PullRequests(models.Model):
         # on top of target
         msg = self._build_merge_message(commits[-1]['commit']['message'])
         commits[-1]['commit']['message'] = msg
-        return gh.rebase(self.number, target, commits=commits)
+        head, mapping = gh.rebase(self.number, target, commits=commits)
+        self.commits_map = json.dumps({**mapping, '': head})
+        return head
 
     def _stage_rebase_merge(self, gh, target, commits):
         msg = self._build_merge_message(self.message)
-        h = gh.rebase(self.number, target, reset=True, commits=commits)
-        return gh.merge(h, target, msg)['sha']
+        h, mapping = gh.rebase(self.number, target, reset=True, commits=commits)
+        merge_head = gh.merge(h, target, msg)['sha']
+        self.commits_map = json.dumps({**mapping, '': merge_head})
+        return merge_head
 
     def _stage_merge(self, gh, target, commits):
         pr_head = commits[-1] # oldest to newest
@@ -981,6 +986,7 @@ class PullRequests(models.Model):
             if len(merge) == 1:
                 [base_commit] = merge
 
+        commits_map = {c['sha']: c['sha'] for c in commits}
         if base_commit:
             # replicate pr_head with base_commit replaced by
             # the current head
@@ -996,11 +1002,18 @@ class PullRequests(models.Model):
                 'parents': new_parents,
             }).json()
             gh.set_ref(target, copy['sha'])
+            # merge commit *and old PR head* map to the pr head replica
+            commits_map[''] = commits_map[pr_head['sha']] = copy['sha']
+            self.commits_map = json.dumps(commits_map)
             return copy['sha']
         else:
             # otherwise do a regular merge
             msg = self._build_merge_message(self.message)
-            return gh.merge(self.head, target, msg)['sha']
+            merge_head = gh.merge(self.head, target, msg)['sha']
+            # and the merge commit is the normal merge head
+            commits_map[''] = merge_head
+            self.commits_map = json.dumps(commits_map)
+            return merge_head
 
 # state changes on reviews
 RPLUS = {
@@ -1363,7 +1376,7 @@ class Stagings(models.Model):
                     self.env['runbot_merge.pull_requests.feedback'].create({
                         'repository': pr.repository.id,
                         'pull_request': pr.number,
-                        'message': "Merged, thanks!",
+                        'message': "Merged at %s, thanks!" % json.loads(pr.commits_map)[''],
                         'close': True,
                     })
             finally:

runbot_merge/tests/test_basic.py

@@ -975,11 +975,16 @@ class TestMergeMethod:
         repo.post_status(staging.id, 'success', 'legal/cla')
         repo.post_status(staging.id, 'success', 'ci/runbot')
         run_crons(env)
-        assert env['runbot_merge.pull_requests'].search([
+        pr = env['runbot_merge.pull_requests'].search([
             ('repository.name', '=', repo.name),
             ('number', '=', prx.number)
-        ]).state == 'merged'
+        ])
+        assert pr.state == 'merged'
         assert prx.state == 'closed'
+        assert json.loads(pr.commits_map) == {
+            c1: actual_sha,
+            '': actual_sha,
+        }, "for a squash, the one PR commit should be mapped to the one rebased commit"
 
     def test_pr_update_to_many_commits(self, repo, env):
         """
@@ -1157,16 +1162,26 @@ class TestMergeMethod:
         repo.post_status('heads/staging.master', 'success', 'ci/runbot')
         run_crons(env)
 
-        assert env['runbot_merge.pull_requests'].search([
+        pr = env['runbot_merge.pull_requests'].search([
             ('repository.name', '=', repo.name),
             ('number', '=', prx.number),
-        ]).state == 'merged'
+        ])
+        assert pr.state == 'merged'
 
         # check that the dummy commit is not in the final master
         master = log_to_node(repo.log('heads/master'))
         assert master == merge_head
-        final_tree = repo.read_tree(repo.commit('heads/master'))
+        head = repo.commit('heads/master')
+        final_tree = repo.read_tree(head)
         assert final_tree == {'m': b'2', 'b': b'1'}, "sanity check of final tree"
+        r1 = repo.commit(head.parents[1])
+        r0 = repo.commit(r1.parents[0])
+        assert json.loads(pr.commits_map) == {
+            b0: r0.id,
+            b1: r1.id,
+            '': head.id,
+        }
+        assert r0.parents == [m2]
 
     def test_pr_rebase_ff(self, repo, env, users):
         """ test result on rebase-merge
@@ -1220,17 +1235,28 @@ class TestMergeMethod:
         repo.post_status('heads/staging.master', 'success', 'ci/runbot')
         run_crons(env)
 
-        assert env['runbot_merge.pull_requests'].search([
+        pr = env['runbot_merge.pull_requests'].search([
             ('repository.name', '=', repo.name),
             ('number', '=', prx.number),
-        ]).state == 'merged'
+        ])
+        assert pr.state == 'merged'
 
         # check that the dummy commit is not in the final master
         master = log_to_node(repo.log('heads/master'))
         assert master == nb1
-        final_tree = repo.read_tree(repo.commit('heads/master'))
+        head = repo.commit('heads/master')
+        final_tree = repo.read_tree(head)
         assert final_tree == {'m': b'2', 'b': b'1'}, "sanity check of final tree"
 
+        m1 = head
+        m0 = repo.commit(m1.parents[0])
+        assert json.loads(pr.commits_map) == {
+            '': m1.id, # merge commit
+            b1: m1.id, # second PR's commit
+            b0: m0.id, # first PR's commit
+        }
+        assert m0.parents == [m2], "can't hurt to check the parent of our root commit"
+
     @pytest.mark.skip(reason="what do if the PR contains merge commits???")
     def test_pr_contains_merges(self, repo, env):
         pass
@@ -1272,6 +1298,14 @@ class TestMergeMethod:
         expected = node('gibberish\n\nblahblah\n\ncloses {}#{}'
                         '\n\nSigned-off-by: {}'.format(repo.name, prx.number, reviewer), m, c0)
         assert log_to_node(repo.log('heads/master')), expected
+        pr = env['runbot_merge.pull_requests'].search([
+            ('repository.name', '=', repo.name),
+            ('number', '=', prx.number),
+        ])
+        assert json.loads(pr.commits_map) == {
+            prx.head: prx.head,
+            '': master.id
+        }
 
     def test_unrebase_emptymessage(self, repo, env, users):
         """ When merging between master branches (e.g. forward port), the PR