[Backport 1.9.latest] Fix erroneous additional batch execution

.changes/unreleased/Fixes-20241209-133317.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Fix running of extra "last" batch when there is only one batch
+time: 2024-12-09T13:33:17.253326-06:00
+custom:
+  Author: QMalcolm
+  Issue: "11112"

core/dbt/task/run.py

@@ -768,18 +768,21 @@
         # Wait until all submitted batches have completed
         while len(batch_results) != batch_idx:
             pass
-        # Final batch runs once all others complete to ensure post_hook runs at the end
-        self._submit_batch(
-            node=node,
-            adapter=runner.adapter,
-            relation_exists=relation_exists,
-            batches=batches,
-            batch_idx=batch_idx,
-            batch_results=batch_results,
-            pool=pool,
-            force_sequential_run=True,
-            skip=skip_batches,
-        )
+
+        # Only run "last" batch if there is more than one batch
+        if len(batches) != 1:
+            # Final batch runs once all others complete to ensure post_hook runs at the end
+            self._submit_batch(
+                node=node,
+                adapter=runner.adapter,
+                relation_exists=relation_exists,
+                batches=batches,
+                batch_idx=batch_idx,
+                batch_results=batch_results,
+                pool=pool,
+                force_sequential_run=True,
+                skip=skip_batches,
+            )
 
         # Finalize run: merge results, track model run, and print final result line
         runner.merge_batch_results(result, batch_results)

tests/functional/microbatch/test_microbatch.py

@@ -1057,11 +1057,20 @@ def pre_or_post_hook(event) -> bool:
 
         return EventCatcher(event_to_catch=JinjaLogDebug, predicate=pre_or_post_hook)  # type: ignore
 
+    @pytest.fixture
+    def generic_exception_catcher(self) -> EventCatcher:
+        return EventCatcher(event_to_catch=GenericExceptionOnRun)  # type: ignore
+
     def test_microbatch(
-        self, mocker: MockerFixture, project, batch_log_catcher: EventCatcher
+        self,
+        project,
+        batch_log_catcher: EventCatcher,
+        generic_exception_catcher: EventCatcher,
     ) -> None:
         with patch_microbatch_end_time("2020-01-01 13:57:00"):
-            _ = run_dbt(["run"], callbacks=[batch_log_catcher.catch])
+            _ = run_dbt(
+                ["run"], callbacks=[batch_log_catcher.catch, generic_exception_catcher.catch]
+            )
 
         # There should be two logs as the pre-hook and post-hook should
         # both only be run once
@@ -1071,3 +1080,6 @@ def test_microbatch(
         assert "pre-hook" in batch_log_catcher.caught_events[0].data.msg  # type: ignore
         assert "20200101" in batch_log_catcher.caught_events[1].data.msg  # type: ignore
         assert "post-hook" in batch_log_catcher.caught_events[1].data.msg  # type: ignore
+
+        # we had a bug where having only one batch caused a generic exception
+        assert len(generic_exception_catcher.caught_events) == 0