[IMP] runbot_merge: tag PRs with a pseudo-branch on merge to master

If a PR got merged to master (or whatever the current development
branch is), there's no easy way to know what maintenance branch it
ended up landing in, except by asking git which branches contain the
commit (which can be rather slow).

Add a special case on merge which labels the PR with a pseudo-branch
patterned after the second-to-last branch of the project:

* if the branch ends with a number, increment the number by one
  e.g. 2.0 -> 2.1, 5 -> 5.1
* otherwise, just prefix with `post-` e.g. "maint" ->
  "post-maint" (that one doesn't sound very helpful, but I guess it's
  nice for the weirdoes who call their branches "natty narwhal" and
  shit)

Fixes #450

runbot_merge/models/pull_requests.py

@@ -1843,6 +1843,15 @@ class Stagings(models.Model):
                     self, prs
                 )
                 prs.write({'state': 'merged'})
+                pseudobranch = None
+                if self.target == project.branch_ids[:1] and project.branch_ids[1:2]:
+                    prev = project.branch_ids[1:2].name
+                    m = re.search(r'(\d+)(?:\.(\d+))?$', prev)
+                    if m:
+                        pseudobranch = "%s.%d" % (m[1], (int(m[2] or 0) + 1))
+                    else:
+                        pseudobranch = 'post-' + prev
+
                 for pr in prs:
                     self.env['runbot_merge.pull_requests.feedback'].create({
                         'repository': pr.repository.id,
@@ -1852,6 +1861,12 @@ class Stagings(models.Model):
                         }),
                         'close': True,
                     })
+                    if pseudobranch:
+                        self.env['runbot_merge.pull_requests.tagging'].create({
+                            'repository': pr.repository.id,
+                            'pull_request': pr.number,
+                            'tags_add': json.dumps([pseudobranch]),
+                        })
             finally:
                 self.batch_ids.write({'active': False})
                 self.write({'active': False})

runbot_merge/tests/test_by_branch.py

@@ -111,3 +111,68 @@ def test_status_skipped(env, project, repo, config):
         repo.post_status('staging.maintenance', 'success', 'ci')
     env.run_crons('runbot_merge.process_updated_commits')
     assert st.state == 'success'
+
+def test_pseudo_version_tag(env, project, make_repo, setreviewers, config):
+    """ Because the last branch in the sequence is "live", if a PR's merged in
+    it it's hard to know where it landed in terms of other branches.
+
+    Therefore if a PR is merged in one such branch, tag it using the previous
+    branch of the sequence:
+
+    * if that ends with a number, increment the number by 1
+    * otherwise add 'post-' prefix (I guess)
+    """
+    repo = make_repo('repo')
+    project.branch_ids.sequence = 1
+    project.write({
+        'repo_ids': [(0, 0, {'name': repo.name, 'required_statuses': 'ci'})],
+        'branch_ids': [
+            (0, 0, {'name': '2.0', 'sequence': 11}),
+            (0, 0, {'name': '1.0', 'sequence': 21})
+        ],
+    })
+    setreviewers(*project.repo_ids)
+
+    with repo:
+        [m] = repo.make_commits(None, Commit('c1', tree={'a': '1'}), ref='heads/master')
+        repo.make_ref('heads/1.0', m)
+        repo.make_ref('heads/2.0', m)
+        repo.make_ref('heads/bonk', m)
+
+    with repo:
+        repo.make_commits(m, Commit('pr1', tree={'b': '1'}), ref='heads/change')
+        pr = repo.make_pr(target='master', head='change')
+        repo.post_status(pr.ref, 'success', 'ci')
+        pr.post_comment('hansen r+', config['role_reviewer']['token'])
+    env.run_crons() # should create staging
+    pr_id = env['runbot_merge.pull_requests'].search([
+        ('repository.name', '=', repo.name),
+        ('number', '=', pr.number)
+    ])
+    assert pr_id.state == 'ready'
+    assert pr_id.staging_id
+    with repo:
+        repo.post_status('staging.master', 'success', 'ci')
+    env.run_crons() # should merge staging
+    assert pr_id.state == 'merged'
+    assert pr.labels >= {'2.1'}
+
+    # now the second branch is non-numeric, therefore the label should just be prefixed by "post-"
+    project.write({'branch_ids': [(0, 0, {'name': 'bonk', 'sequence': 6})]})
+    with repo:
+        repo.make_commits(m, Commit('pr2', tree={'c': '1'}), ref='heads/change2')
+        pr = repo.make_pr(target='master', head='change2')
+        repo.post_status(pr.ref, 'success', 'ci')
+        pr.post_comment('hansen r+', config['role_reviewer']['token'])
+    env.run_crons() # should create staging
+    pr_id = env['runbot_merge.pull_requests'].search([
+        ('repository.name', '=', repo.name),
+        ('number', '=', pr.number)
+    ])
+    assert pr_id.state == 'ready', pr.comments
+    assert pr_id.staging_id
+    with repo:
+        repo.post_status('staging.master', 'success', 'ci')
+    env.run_crons() # should merge staging
+    assert pr_id.state == 'merged'
+    assert pr.labels >= {'post-bonk'}