Prevent throwing in running generator

Generator::throw() on a running generator is not allowed. It throws "Cannot
resume an already running generator" when trying to resume the generator to
handle the provided exception.

However, when calling Generator::throw() on a generator with a non-Generator
delegate, we release the delegate regardless. If a Fiber was suspended in
the delegate, this causes use after frees when the Fiber is resumed.

Fix this by throwing "Cannot resume an already running generator" earlier.

Fixes GH-19326
Closes GH-19327

NEWS

@@ -15,6 +15,8 @@ PHP                                                                        NEWS
     causes assertion failure). (nielsdos)
   . Fixed bug GH-19306 (Generator can be resumed while fetching next value from
     delegated Generator). (Arnaud)
+  . Fixed bug GH-19326 (Calling Generator::throw() on a running generator with
+    a non-Generator delegate crashes). (Arnaud)
 
 - FTP:
   . Fix theoretical issues with hrtime() not being available. (nielsdos)

Zend/tests/gh19326.phpt

@@ -0,0 +1,36 @@
+--TEST--
+GH-19326: Calling Generator::throw() on a running generator with a non-Generator delegate crashes
+--FILE--
+<?php
+
+class It implements IteratorAggregate {
+    public function getIterator(): Generator {
+        yield "";
+        Fiber::suspend();
+    }
+}
+
+function g() {
+    yield from new It();
+}
+
+$b = g();
+$b->rewind();
+
+$fiber = new Fiber(function () use ($b) {
+    $b->next();
+});
+
+$fiber->start();
+
+try {
+    $b->throw(new Exception('test'));
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+$fiber->resume();
+
+?>
+--EXPECT--
+Cannot resume an already running generator

Zend/zend_generators.c

@@ -492,8 +492,14 @@ ZEND_API zend_execute_data *zend_generator_check_placeholder_frame(zend_execute_
 	return ptr;
 }
 
-static void zend_generator_throw_exception(zend_generator *generator, zval *exception)
+static zend_result zend_generator_throw_exception(zend_generator *generator, zval *exception)
 {
+	if (generator->flags & ZEND_GENERATOR_CURRENTLY_RUNNING) {
+		zval_ptr_dtor(exception);
+		zend_throw_error(NULL, "Cannot resume an already running generator");
+		return FAILURE;
+	}
+
 	zend_execute_data *original_execute_data = EG(current_execute_data);
 
 	/* Throw the exception in the context of the generator. Decrementing the opline
@@ -519,6 +525,8 @@ static void zend_generator_throw_exception(zend_generator *generator, zval *exce
 	}
 
 	EG(current_execute_data) = original_execute_data;
+
+	return SUCCESS;
 }
 
 static void zend_generator_add_child(zend_generator *generator, zend_generator *child)
@@ -1026,7 +1034,9 @@ ZEND_METHOD(Generator, throw)
 	if (generator->execute_data) {
 		zend_generator *root = zend_generator_get_current(generator);
 
-		zend_generator_throw_exception(root, exception);
+		if (zend_generator_throw_exception(root, exception) == FAILURE) {
+			return;
+		}
 
 		zend_generator_resume(generator);