Harden distributed pipeline

coordinator/migrations/0009_unique_partial_result_attempt.down.sql

@@ -0,0 +1,5 @@
+BEGIN;
+
+DROP INDEX IF EXISTS uq_partial_result_task_attempt;
+
+COMMIT;

coordinator/migrations/0009_unique_partial_result_attempt.up.sql

@@ -0,0 +1,26 @@
+BEGIN;
+
+-- Old deployments can contain more than one partial result because earlier
+-- versions accepted repeated PUTs. Preserve the one referenced by a completed
+-- task and discard stale rows; unfinished tasks must upload again after a
+-- deploy, just as they do after a lost lease.
+DELETE FROM artifacts AS a
+USING tasks AS t
+WHERE a.task_id = t.id
+  AND a.kind = 'partial_result'::artifact_kind
+  AND t.status <> 'completed'::task_status;
+
+DELETE FROM artifacts AS a
+USING tasks AS t
+WHERE a.task_id = t.id
+  AND a.kind = 'partial_result'::artifact_kind
+  AND t.status = 'completed'::task_status
+  AND a.id <> t.result_artifact_id;
+
+-- One lease attempt has one durable partial result. This makes an upload retry
+-- idempotent and prevents repeated uploads from accumulating orphan artifacts.
+CREATE UNIQUE INDEX uq_partial_result_task_attempt
+    ON artifacts (task_id, attempt)
+    WHERE kind = 'partial_result'::artifact_kind;
+
+COMMIT;