Fix GH-17246: Nested shm protections cause segfault

ext/opcache/ZendAccelerator.c

@@ -2153,7 +2153,10 @@ zend_op_array *persistent_compile_file(zend_file_handle *file_handle, int type)
 		 */
 		from_shared_memory = false;
 		if (persistent_script) {
+			/* See GH-17246: we disable GC so that user code cannot be executed during the optimizer run. */
+			bool orig_gc_state = gc_enable(false);
 			persistent_script = cache_script_in_shared_memory(persistent_script, key, &from_shared_memory);
+			gc_enable(orig_gc_state);
 		}
 
 		/* Caching is disabled, returning op_array;

ext/opcache/tests/jit/gh17246.inc

@@ -0,0 +1,8 @@
+<?php
+
+// Need to cause a trace exit, so extend non existent class
+class MyXSLTProcessor extends NonExistentClass {
+    public function registerCycle() {
+        [[$this]]; // Non trivial array
+    }
+}

ext/opcache/tests/jit/gh17246.phpt

@@ -0,0 +1,39 @@
+--TEST--
+GH-17246 (Nested shm protections cause segfault)
+--EXTENSIONS--
+opcache
+--INI--
+opcache.protect_memory=1
+opcache.jit_buffer_size=32M
+opcache.jit=1254
+--FILE--
+<?php
+
+class Test
+{
+    private $field;
+
+    public function __construct()
+    {
+        $this->field = function() {};
+    }
+
+    public function __destruct()
+    {
+        // Necessary because we need to invoke tracing JIT during destruction
+    }
+}
+
+for ($i = 0; $i < 10000; ++$i) {
+    $obj = new Test();
+}
+
+require __DIR__.'/gh17246.inc';
+
+?>
+--EXPECTF--
+Fatal error: Uncaught Error: Class "NonExistentClass" not found in %s:%d
+Stack trace:
+#0 %s(%d): require()
+#1 {main}
+  thrown in %s on line %d