Short-circuit get_gc for currently running generator

Zend/tests/generators/gc_running_generator.phpt

@@ -0,0 +1,25 @@
+--TEST--
+GC on running generator
+--FILE--
+<?php
+
+function gen() {
+    yield;
+    // Trigger GC while $v is being reassigned.
+    $ary = [new stdClass, new stdClass, new stdClass];
+    $ary[0]->foo = $ary;
+    foreach ($ary as &$v) { }
+}
+
+for ($i = 0; $i < 10000; $i++) {
+    // Make sure gen is registered as a GC root.
+    $gen = gen();
+    $gen2 = $gen;
+    unset($gen);
+    foreach ($gen2 as $v) {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/zend_generators.c

@@ -340,6 +340,16 @@ static HashTable *zend_generator_get_gc(zval *object, zval **table, int *n) /* {
 		return NULL;
 	}
 
+	if (generator->flags & ZEND_GENERATOR_CURRENTLY_RUNNING) {
+		/* If the generator is currently running, we certainly won't be able to GC any values it
+		 * holds on to. The execute_data state might be inconsistent during execution (e.g. because
+		 * GC has been triggered in the middle of a variable reassignment), so we should not try
+		 * to inspect it here. */
+		*table = NULL;
+		*n = 0;
+		return NULL;
+	}
+
 	op_array = &EX(func)->op_array;
 	gc_buffer_size = calc_gc_buffer_size(generator);
 	if (generator->gc_buffer_size < gc_buffer_size) {