Free rb_native_thread memory at fork

We never freed any resources of rb_native_thread at fork because it would cause it to hang. This is because it called rb_native_cond_destroy for condition variables. We can't call rb_native_cond_destroy here because according to the specs of pthread_cond_destroy: Attempting to destroy a condition variable upon which other threads are currently blocked results in undefined behavior. Specifically, glibc's pthread_cond_destroy waits on all the other listeners. Since after forking all the threads are dead, the condition variable's listeners will never wake up, so it will hang forever. This commit changes it to only free the memory and none of the condition variables.
ruby · peterzhu2118 · Jun 12, 2025 · Jun 11, 2025 · Jun 11, 2025 · eb0f8b6bfe3176e97cacefe52749f2fbc45a6a8a
commit eb0f8b6bfe3176e97cacefe52749f2fbc45a6a8a
diff --git a/thread.c b/thread.c
@@ -519,12 +519,8 @@ thread_cleanup_func(void *th_ptr, int atfork)
     th->locking_mutex = Qfalse;
     thread_cleanup_func_before_exec(th_ptr);
 
-    /*
-     * Unfortunately, we can't release native threading resource at fork
-     * because libc may have unstable locking state therefore touching
-     * a threading resource may cause a deadlock.
-     */
     if (atfork) {
+        native_thread_destroy_atfork(th->nt);
         th->nt = NULL;
         return;
     }

diff --git a/thread_none.c b/thread_none.c
@@ -137,6 +137,12 @@ ruby_mn_threads_params(void)
 {
 }
 
+static void
+native_thread_destroy_atfork(struct rb_native_thread *nt)
+{
+    /* no-op */
+}
+
 static int
 native_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {

diff --git a/thread_pthread.c b/thread_pthread.c
@@ -1816,6 +1816,27 @@ native_thread_assign(struct rb_native_thread *nt, rb_thread_t *th)
        th->nt = nt;
 }
 
+static void
+native_thread_destroy_atfork(struct rb_native_thread *nt)
+{
+    if (nt) {
+        /* We can't call rb_native_cond_destroy here because according to the
+         * specs of pthread_cond_destroy:
+         *
+         *   Attempting to destroy a condition variable upon which other threads
+         *   are currently blocked results in undefined behavior.
+         *
+         * Specifically, glibc's pthread_cond_destroy waits on all the other
+         * listeners. Since after forking all the threads are dead, the condition
+         * variable's listeners will never wake up, so it will hang forever.
+         */
+
+        RB_ALTSTACK_FREE(nt->altstack);
+        ruby_xfree(nt->nt_context);
+        ruby_xfree(nt);
+    }
+}
+
 static void
 native_thread_destroy(struct rb_native_thread *nt)
 {
@@ -1826,9 +1847,7 @@ native_thread_destroy(struct rb_native_thread *nt)
             rb_native_cond_destroy(&nt->cond.intr);
         }
 
-        RB_ALTSTACK_FREE(nt->altstack);
-        ruby_xfree(nt->nt_context);
-        ruby_xfree(nt);
+        native_thread_destroy_atfork(nt);
     }
 }
 

diff --git a/thread_win32.c b/thread_win32.c
@@ -617,6 +617,12 @@ native_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
     th->ec->machine.stack_maxsize = size - space;
 }
 
+static void
+native_thread_destroy_atfork(struct rb_native_thread *nt)
+{
+    /* no-op */
+}
+
 #ifndef InterlockedExchangePointer
 #define InterlockedExchangePointer(t, v) \
     (void *)InterlockedExchange((long *)(t), (long)(v))