Fix reuse of dtor fiber during shutdown (#16026)

Zend/tests/fibers/destructors_011.phpt

@@ -0,0 +1,70 @@
+--TEST--
+Fibers in destructors 011: gc collection after the dtor fiber is dtor
+--FILE--
+<?php
+
+class SimpleCycle {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        printf("%s\n", __METHOD__);
+    }
+}
+
+class CreateGarbageInDtor {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        printf("%s\n", __METHOD__);
+        // Create an object whose dtor will be called after the dtor fiber's
+        new CollectCyclesInFiberInDtor();
+    }
+}
+
+class CollectCyclesInFiberInDtor {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        printf("%s\n", __METHOD__);
+        new SimpleCycle();
+        print "Collecting cycles\n";
+        $f = new Fiber(function () {
+            gc_collect_cycles();
+        });
+        $f->start();
+        print "Done collecting cycles\n";
+    }
+}
+
+register_shutdown_function(function () {
+    print "Shutdown\n";
+});
+
+// Create a cycle
+new SimpleCycle();
+
+// Collect cycles to create the dtor fiber
+$f = new Fiber(function () {
+    gc_collect_cycles();
+});
+$f->start();
+
+// Create an object whose dtor will be called during shutdown
+// (by zend_objects_store_call_destructors)
+new CreateGarbageInDtor();
+
+?>
+--EXPECT--
+SimpleCycle::__destruct
+Shutdown
+CreateGarbageInDtor::__destruct
+CollectCyclesInFiberInDtor::__destruct
+Collecting cycles
+SimpleCycle::__destruct
+Done collecting cycles

Zend/zend_gc.c

@@ -2261,6 +2261,9 @@ static ZEND_FUNCTION(gc_destructor_fiber)
 
 		if (UNEXPECTED(fiber->flags & ZEND_FIBER_FLAG_DESTROYED)) {
 			/* Fiber is being destroyed by shutdown sequence */
+			if (GC_G(dtor_fiber) == fiber) {
+				GC_G(dtor_fiber) = NULL;
+			}
 			GC_DELREF(&fiber->std);
 			gc_check_possible_root((zend_refcounted*)&fiber->std.gc);
 			return;