Preserve qualified merge reconstruction through admission transport

src/svrf/rules.py

@@ -180,9 +180,11 @@ def admission(row: dict, held: dict | None, watch_digest: str | None = None, car
 MERGE_CONFLICT = "merge:CONFLICT:"
 STALE_LINES = ("github:NOT_MERGEABLE",)
 # `history:` is the built-in tests-first check's own prefix; `reland:` is the one an
-# external admission.command uses to report the same order-only refusal (see
-# admission.py's PASSTHROUGH_PREFIXES). Both name the identical class of refusal.
-RELAND_LINE = re.compile(r"^(?:history|reland):REFUSED:(UNORDERED|MIXED)$")
+# external admission.command uses to report a qualified order-only refusal.
+# MERGE_LOSS requires that external qualification: the command must receive
+# preservation of the final tree before requesting its ordered reconstruction.
+RELAND_LINE = re.compile(
+    r"^(?:(?:history|reland):REFUSED:(UNORDERED|MIXED)|reland:REFUSED:(MERGE_LOSS))$")
 
 
 def repair_class(lines: list[str], is_union: Callable[[str], bool]) -> str | None:
@@ -200,15 +202,17 @@ def repair_class(lines: list[str], is_union: Callable[[str], bool]) -> str | Non
 
 
 def reland_class(lines: list[str]) -> str | None:
-    """UNORDERED or MIXED when every residual is a history-order refusal of one class:
-    the order of commits, not their content, which the train fixes by re-landing the same
-    final tree as an ordered history. Any other residual stays held for a person."""
+    """One received history-order class, including externally qualified merge repair.
+
+    The train re-lands the same final tree. Unqualified loss and mixed residual
+    classes retain their holds.
+    """
     if not lines:
         return None
     matches = [RELAND_LINE.match(line) for line in lines]
     if not all(matches):
         return None
-    classes = {m.group(1) for m in matches}
+    classes = {m.group(1) or m.group(2) for m in matches}
     return classes.pop() if len(classes) == 1 else None
 
 

tests/test_qualified_merge_reland.py

@@ -0,0 +1,37 @@
+"""An admission owner's qualified merge repair survives transport to the train."""
+from pathlib import Path
+from types import SimpleNamespace
+import tempfile
+import unittest
+
+from svrf.admission import Admission
+from svrf import rules
+
+
+class QualifiedMergeReland(unittest.TestCase):
+    def read(self, lines):
+        with tempfile.TemporaryDirectory() as temp:
+            git = SimpleNamespace(root=Path(temp), is_ancestor=lambda *a: False,
+                merge_preview=lambda *a: {'status': 'CLEAN', 'conflicts': []},
+                changed_paths=lambda *a: ['src/runtime.py'])
+            command = "printf '%s\\n' " + ' '.join("'"+line+"'" for line in lines) + '; exit 1'
+            return Admission(git, command=command)('head', 'base')
+
+    def test_qualified_command_result_relands_the_same_tree(self):
+        line = 'reland:REFUSED:MERGE_LOSS'
+        value = self.read([line])
+        self.assertEqual(value['residuals'], [line])
+        self.assertEqual(rules.admission_decision(value), ('RELAND', 'MERGE_LOSS'))
+        self.assertEqual(rules.admission_decision(value, reland=False), ('HOLD', None))
+
+    def test_unqualified_loss_and_other_failures_remain_held(self):
+        for line in ('history:REFUSED:MERGE_LOSS', 'dispositions:REFUSED:MERGE_LOSS',
+                     'check:MERGE_LOSS', 'MERGE_LOSS'):
+            with self.subTest(line=line):
+                self.assertIsNone(rules.reland_class([line]))
+                self.assertEqual(rules.admission_decision(self.read([line])), ('HOLD', None))
+        self.assertEqual(rules.admission_decision(self.read([
+            'reland:REFUSED:MERGE_LOSS', 'source mismatch'])), ('HOLD', None))
+
+
+if __name__ == '__main__': unittest.main()