Add logging: trace blocking_operation through execute/exception/cancel paths

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: samuel-williams-shopify

ext/io/event/worker_pool.c

@@ -15,7 +15,7 @@
 #include <string.h>
 
 enum {
-	DEBUG = 1,
+	DEBUG = 0,
 };
 
 static VALUE IO_Event_WorkerPool;
@@ -146,9 +146,11 @@ static void worker_unblock_func(void *_worker) {
 	pthread_mutex_unlock(&pool->mutex);
 
 	// If there's a currently executing blocking operation, cancel it
-	// if (worker->current_blocking_operation) {
-	// 	rb_fiber_scheduler_blocking_operation_cancel(worker->current_blocking_operation);
-	// }
+	if (worker->current_blocking_operation) {
+		fprintf(stderr, "[worker_pool] unblock: cancelling blocking_operation=%p\n",
+			(void*)worker->current_blocking_operation);
+		rb_fiber_scheduler_blocking_operation_cancel(worker->current_blocking_operation);
+	}
 }
 
 // Function to wait for work and execute it without GVL.
@@ -177,6 +179,8 @@ static void *worker_wait_and_execute(void *_worker) {
 
 		// Execute work WITHOUT GVL (this is the whole point!)
 		if (work) {
+			fprintf(stderr, "[worker_pool] execute: blocking_operation=%p\n",
+				(void*)work->blocking_operation);
 			worker->current_blocking_operation = work->blocking_operation;
 			rb_fiber_scheduler_blocking_operation_execute(work->blocking_operation);
 			worker->current_blocking_operation = NULL;
@@ -376,6 +380,11 @@ static VALUE worker_pool_call(VALUE self, VALUE _blocking_operation) {
 	if (DEBUG) fprintf(stderr, "<- worker_pool_call:work completed=%d, state=%d\n", work.completed, state);
 
 	if (state) {
+		// Exception path: cleanup in rb_fiber_scheduler_blocking_operation_wait is
+		// bypassed (operation->state becomes a dangling pointer to our caller's stack).
+		// Log the blocking_operation so we can correlate with any later cancel calls.
+		fprintf(stderr, "[worker_pool] exception path: blocking_operation=%p state_tag=%d\n",
+			(void*)blocking_operation, state);
 		rb_jump_tag(state);
 	} else {
 		return Qtrue;