fix: add rwlock for implicit opcache restarts (OOM/hash overflow)

PR #2073 drains all threads for explicit opcache_reset() calls, but
WordPress triggers implicit restarts via opcache_invalidate() filling
opcache memory → zend_accel_schedule_restart() → restart_pending → the
actual reset runs during the next php_request_startup(). That path
bypasses the opcache_reset() override and races on shared memory.

Add a pthread_rwlock around the request lifecycle:
- Normal requests: read lock (concurrent, single atomic CAS)
- When zend_accel_schedule_restart_hook fires: set an atomic flag
- Next php_request_startup(): sees flag → write lock (exclusive),
  blocks until all current requests complete, resets safely
- After startup: unlock, all threads resume

Combined with PR #2073, this covers both explicit opcache_reset()
(thread drain) and implicit OOM/hash-overflow restarts (rwlock).

Testing: without this patch, crashes every ~10 min on a WordPress
multisite. With PR #2073 alone, same crash frequency. With the rwlock
from our PR #2349 alone, crashes reduced to every 6-12 hours (worker
threads not covered). Combined fix expected to eliminate both paths.

Author: superdav42

frankenphp.c

@@ -1,5 +1,6 @@
 #include "frankenphp.h"
 #include <SAPI.h>
+#include <Zend/zend.h>
 #include <Zend/zend_alloc.h>
 #include <Zend/zend_exceptions.h>
 #include <Zend/zend_interfaces.h>
@@ -83,6 +84,33 @@ bool original_user_abort_setting = 0;
 frankenphp_interned_strings_t frankenphp_strings = {0};
 HashTable *main_thread_env = NULL;
 
+/* Implicit opcache restart safety — prevents zend_mm_heap corrupted
+ * crashes from the implicit restart path (OOM, hash overflow).
+ *
+ * PR #2073 handles explicit opcache_reset() calls by draining all
+ * threads through the Go state machine. However, WordPress triggers
+ * implicit restarts via opcache_invalidate() filling opcache memory,
+ * which fires zend_accel_schedule_restart() → restart_pending → the
+ * actual reset during the next php_request_startup(). That path is
+ * not covered by the opcache_reset() override.
+ *
+ * Fix: pthread_rwlock around the request lifecycle. Normal requests
+ * take a read lock (concurrent). When the restart hook fires, it sets
+ * a flag; the next php_request_startup() acquires a write lock,
+ * blocking until all other threads' requests complete, then performs
+ * the reset exclusively. */
+static pthread_rwlock_t frankenphp_opcache_rwlock =
+    PTHREAD_RWLOCK_INITIALIZER;
+static volatile int frankenphp_opcache_restart_pending = 0;
+
+#if defined(ZTS) && PHP_VERSION_ID >= 80400
+static void frankenphp_opcache_restart_hook(int reason) {
+  (void)reason;
+  __atomic_store_n(&frankenphp_opcache_restart_pending, 1,
+                   __ATOMIC_RELEASE);
+}
+#endif
+
 __thread uintptr_t thread_index;
 __thread bool is_worker_thread = false;
 __thread HashTable *sandboxed_env = NULL;
@@ -1111,7 +1139,20 @@ static void *php_thread(void *arg) {
 
       frankenphp_update_request_context();
 
+      /* Implicit opcache restart: if scheduled, take exclusive access
+       * so the reset in php_request_startup() runs while no other
+       * thread touches shared memory. Otherwise read lock. */
+      if (__atomic_load_n(&frankenphp_opcache_restart_pending,
+                          __ATOMIC_ACQUIRE)) {
+        pthread_rwlock_wrlock(&frankenphp_opcache_rwlock);
+        __atomic_store_n(&frankenphp_opcache_restart_pending, 0,
+                         __ATOMIC_RELEASE);
+      } else {
+        pthread_rwlock_rdlock(&frankenphp_opcache_rwlock);
+      }
+
       if (UNEXPECTED(php_request_startup() == FAILURE)) {
+        pthread_rwlock_unlock(&frankenphp_opcache_rwlock);
         /* Request startup failed, bail out to zend_catch */
         frankenphp_log_message("Request startup failed, thread is unhealthy",
                                LOG_ERR);
@@ -1143,6 +1184,7 @@ static void *php_thread(void *arg) {
 
       /* shutdown the request, potential bailout to zend_catch */
       php_request_shutdown((void *)0);
+      pthread_rwlock_unlock(&frankenphp_opcache_rwlock);
       frankenphp_free_request_context();
       go_frankenphp_after_script_execution(thread_index, EG(exit_status));
     }
@@ -1158,6 +1200,7 @@ static void *php_thread(void *arg) {
       zend_catch {}
       zend_end_try();
     }
+    pthread_rwlock_unlock(&frankenphp_opcache_rwlock);
 
     /* Log the last error message, it must be cleared to prevent a crash when
      * freeing execution globals */
@@ -1277,6 +1320,13 @@ static void *php_main(void *arg) {
 
   frankenphp_sapi_module.startup(&frankenphp_sapi_module);
 
+#if defined(ZTS) && PHP_VERSION_ID >= 80400
+  /* Hook implicit opcache restarts (OOM, hash overflow). The hook
+   * sets a flag; the next php_request_startup() acquires exclusive
+   * access via the rwlock so the reset runs safely. */
+  zend_accel_schedule_restart_hook = frankenphp_opcache_restart_hook;
+#endif
+
   /* check if a default filter is set in php.ini and only filter if
    * it is, this is deprecated and will be removed in PHP 9 */
   char *default_filter;