gh-120321: Add missing "return false" in gen_try_set_executing (gh-144291)

colesbury · web-flow · commit 6b4538192dda · 2026-01-27T16:38:50.000-05:00
We didn't catch this because of a combination of:

1) falling through to the if-statement below works
2) we only specialized FOR_ITER_GEN for uniquely referenced generators,
   so we didn't trigger the non-thread-safe behavior.
diff --git a/Python/ceval_macros.h b/Python/ceval_macros.h
@@ -529,6 +529,7 @@ gen_try_set_executing(PyGenObject *gen)
                 return true;
             }
         }
+        return false;
     }
 #endif
     // Use faster non-atomic modifications in the GIL-enabled build and when

Original file line number	Diff line number	Diff line change
`@@ -529,6 +529,7 @@ gen_try_set_executing(PyGenObject *gen)`
`529`	`529`	`return true;`
`530`	`530`	`}`
`531`	`531`	`}`
	`532`	`+ return false;`
`532`	`533`	`}`
`533`	`534`	`#endif`
`534`	`535`	`// Use faster non-atomic modifications in the GIL-enabled build and when`