coroutine: consistently return the context that resumed transfer. (#18814)

Merged-By: ioquatix <samuel@codeotaku.com>

coroutine/asyncify/Context.h

@@ -62,9 +62,6 @@ static inline void coroutine_initialize(struct coroutine_context *context, corou
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
     if (ASYNCIFY_CORO_DEBUG) fprintf(stderr, "[%s] entry (current = %p, target = %p)\n", __func__, current, target);
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    struct coroutine_context * previous = target->from;
-#endif
 
     target->from = current;
     if (ASYNCIFY_CORO_DEBUG) fprintf(stderr, "[%s] current->current_sp = %p -> %p\n", __func__, current->current_sp, rb_wasm_get_stack_pointer());
@@ -79,13 +76,8 @@ static inline struct coroutine_context * coroutine_transfer(struct coroutine_con
 
     rb_wasm_set_stack_pointer(current->current_sp);
 
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    /* from is read only by coroutine_trampoline, when target starts, which has
-     * happened before we get here. */
-    target->from = previous;
-#endif
-
-    return target;
+    /* current may have been resumed by a context other than target. */
+    return current->from;
 }
 
 static inline void coroutine_destroy(struct coroutine_context * context)

coroutine/emscripten/Context.h

@@ -60,19 +60,11 @@ static inline void coroutine_initialize(
 
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    struct coroutine_context * previous = target->from;
-#endif
-
     target->from = current;
     emscripten_fiber_swap(&current->state, &target->state);
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    /* from is read only by coroutine_trampoline, when target starts, which has
-     * happened before we get here. */
-    target->from = previous;
-#endif
 
-    return target;
+    /* current may have been resumed by a context other than target. */
+    return current->from;
 }
 
 static inline void coroutine_destroy(struct coroutine_context * context)

coroutine/pthread/Context.c

@@ -229,13 +229,8 @@ struct coroutine_context * coroutine_transfer(struct coroutine_context * current
     pthread_testcancel();
 #endif
 
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    /* from is read only by coroutine_trampoline, when target starts, which has
-     * happened before we get here. */
-    target->from = previous;
-#endif
-
-    return target;
+    /* current may have been resumed by a context other than target. */
+    return current->from;
 }
 
 static

coroutine/ucontext/Context.h

@@ -60,19 +60,16 @@ static inline void coroutine_initialize(
 
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
     struct coroutine_context * previous = target->from;
-#endif
 
     target->from = current;
-    swapcontext(&current->state, &target->state);
-#ifndef COROUTINE_TARGET_MAY_BE_FREED
-    /* from is read only by coroutine_trampoline, when target starts, which has
-     * happened before we get here. */
-    target->from = previous;
-#endif
+    if (swapcontext(&current->state, &target->state) == -1) {
+        target->from = previous;
+        return NULL;
+    }
 
-    return target;
+    /* current may have been resumed by a context other than target. */
+    return current->from;
 }
 
 static inline void coroutine_destroy(struct coroutine_context * context)

thread_pthread.c

@@ -67,10 +67,6 @@ static const void *const condattr_monotonic = NULL;
 // otherwise the caller has to restate the deadline in the condvar's clock.
 #define RB_NATIVE_COND_HRTIME_DEADLINE_P() (condattr_monotonic != NULL)
 
-/* A retiring shared native thread frees its own context while the threads it
- * parked are still suspended with that context as their target. */
-#define COROUTINE_TARGET_MAY_BE_FREED 1
-
 #include COROUTINE_H
 
 #ifndef HAVE_SYS_EVENT_H