[IMP] runbot_merge: recombine remaining batches after culprit PR

When a multi-batch staging fails and a culprit PR is later identified
in a single-batch staging, cancel pending sibling stagings/splits from
the same split root and create a new split with all remaining active
batches. This lets the scheduler stage them together again, reducing CI
cycles and accelerating merges while preserving safety (bisect still
applies if more culprits/interactions remain).

Author: MiquelRForgeFlow

runbot_merge/models/pull_requests.py

@@ -2304,6 +2304,41 @@ def fail(self, message, prs=None):
         })
         return True
 
+    def recombine_remaining_after_culprit(self, culprit_pr):
+        """Combines all the remaining batches of the same split group
+        (same source_id) except for the batch of the culprit PR and the closed ones,
+        creating a single Split with the rest in order to test them together.
+        """
+        culprit_batch = culprit_pr.batch_id
+        if not culprit_batch:
+            _logger.info("No culprit batch found for PR %s; skipping recombine", culprit_pr)
+            return False
+
+        Split = self.env['runbot_merge.split']
+        source = (self.parent_id or self)
+
+        sibling_splits = Split.search([('source_id', '=', source.id)])
+        if not sibling_splits:
+            _logger.info("No sibling splits for root staging %s; skipping recombine", source)
+            return False
+
+        candidate_batches = sibling_splits.mapped('batch_ids').filtered(lambda b: b.id != culprit_batch.id and b.active)
+        if len(candidate_batches) <= 1:
+            _logger.info("Nothing to recombine (remaining=%s) for source %s after culprit %s",
+                         len(candidate_batches), source, culprit_pr)
+            return False
+
+        new_split = Split.create({
+            'target': self.target.id,
+            'source_id': source.id,
+            'batch_ids': [Command.link(b.id) for b in candidate_batches],
+        })
+        _logger.info(
+            "Recombined %d batches after culprit PR#%s into %s (source=%s)",
+            len(candidate_batches), culprit_pr.number, new_split, source
+        )
+        return True
+
     def try_splitting(self):
         batches = len(self.batch_ids)
         if batches > 1:
@@ -2357,11 +2392,15 @@ def try_splitting(self):
                 viewmore = ' (view more at %(target_url)s)' % status
             if pr:
                 self.fail("%s%s" % (reason, viewmore), pr)
+                try:
+                    self.recombine_remaining_after_culprit(pr)
+                except Exception:
+                    _logger.exception("Failed to recombine after culprit PR %s", pr)
             else:
                 self.fail('%s on %s%s' % (reason, head.commit_id.sha, viewmore))
             return False
 
-        # the staging failed but we don't have a specific culprit, fail
+        # the staging failed, but we don't have a specific culprit, fail
         # everything
         self.fail("unknown reason")
 
@@ -2527,6 +2566,7 @@ def for_commits(self, *heads):
 
 class Split(models.Model):
     _name = _description = 'runbot_merge.split'
+    _order = 'id desc'
 
     target = fields.Many2one('runbot_merge.branch', required=True)
     source_id = fields.Many2one('runbot_merge.stagings', required=True)