Support fiber API with JSPI

ChangeLog.md

@@ -29,6 +29,9 @@ See docs/process.md for more on how version tagging works.
   performed when the linker inputs carry the wasm-bindgen Emscripten marker
   section, so `-sWASM_BINDGEN` can safely be passed to non-wasm-bindgen builds.
   (#27208)
+- The fiber API (`emscripten/fiber.h`) is now supported under JSPI (`-sJSPI`).
+  When compiling with JSPI, the `asyncify_stack` argument to `emscripten_fiber_init`
+  and `emscripten_fiber_init_from_current_context` is optional and can be `NULL`.
 
 6.0.9 - 09/01/26
 ----------------

site/source/docs/api_reference/fiber.h.rst

@@ -8,8 +8,8 @@ fiber.h
 co-operative threads of execution. The `fiber.h
 <https://github.com/emscripten-core/emscripten/blob/main/system/include/emscripten/fiber.h>`_
 header defines a low-level API for manipulating Fibers in Emscripten. Fibers are
-implemented with :ref:`asyncify section`, so you must link your program with
-:ref:`ASYNCIFY` if you intend to use them.
+implemented with :ref:`asyncify section` or JSPI, so you must link your program with
+:ref:`ASYNCIFY` or ``-sJSPI`` if you intend to use them.
 
 Fibers are intended as a building block for asynchronous control flow
 constructs, such as coroutines. They supersede the legacy coroutine API that was
@@ -53,16 +53,16 @@ Types
   .. c:member:: em_arg_callback_func entry
 
     Entry point. If not NULL, this function will be called when the fiber is
-    switched into. Otherwise, :c:member:`emscripten_fiber_t.asyncify_data` is
-    used to rewind the call stack.
+    switched into. Otherwise, :c:member:`emscripten_fiber_t.asyncify_data` (under
+    Asyncify) or native stack switching (under JSPI) is used to resume the call stack.
 
   .. c:member:: void *user_data
 
     Opaque pointer, passed as-is to :c:member:`emscripten_fiber_t.entry`.
 
   .. c:member:: asyncify_data_t asyncify_data
 
-    Asyncify data structure. Used to unwind and rewind the call stack when switching fibers.
+    Asyncify data structure. Used to unwind and rewind the call stack when switching fibers under Asyncify (under JSPI, only rewind_id is used).
 
 .. c:type:: asyncify_data_t
 
@@ -98,8 +98,8 @@ Functions
   :param void* entry_func_arg: Opaque pointer passed to `entry_func`.
   :param void* c_stack: Pointer to memory region to use for the C stack. Must be at least 16-byte aligned. This points to the lower bound of the stack, regardless of growth direction.
   :param size_t c_stack_size: Size of the C stack memory region, in bytes.
-  :param void* asyncify_stack: Pointer to memory region to use for the Asyncify stack. No special alignment requirements.
-  :param size_t asyncify_stack_size: Size of the Asyncify stack memory region, in bytes.
+  :param void* asyncify_stack: Pointer to memory region to use for the Asyncify stack. No special alignment requirements. Under JSPI, this parameter may be `NULL`.
+  :param size_t asyncify_stack_size: Size of the Asyncify stack memory region, in bytes. Under JSPI, this parameter may be `0`.
 
   .. note:: If `entry_func` returns, the entire program will end, as if `main` had returned. To avoid this, you can use :c:func:`emscripten_fiber_swap` to jump to another fiber.
 
@@ -122,8 +122,10 @@ Functions
 
   :param emscripten_fiber_t* fiber: Pointer to the fiber structure.
   :param void* asyncify_stack: Pointer to memory region to use for the Asyncify
-                               stack. No special alignment requirements.
+                               stack. No special alignment requirements. Under JSPI,
+                               this parameter may be `NULL`.
   :param size_t asyncify_stack_size: Size of the Asyncify stack memory region, in bytes.
+                                     Under JSPI, this parameter may be `0`.
 
 .. c:function:: void emscripten_fiber_swap(emscripten_fiber_t *old_fiber, emscripten_fiber_t *new_fiber)
 
@@ -137,8 +139,9 @@ Functions
   :param emscripten_fiber_t* new_fiber: Fiber representing the target context.
                                         If the fiber has an entry point, it will
                                         be called in the new context and set
-                                        to `NULL`. Otherwise,
+                                        to `NULL`. Otherwise, the call stack is
+                                        resumed (using
                                         :c:member:`emscripten_fiber_t.asyncify_data`
-                                        is used to rewind the call stack. If the
-                                        fiber is invalid or incomplete, the
-                                        behavior is undefined.
+                                        under Asyncify or native stack switching
+                                        under JSPI). If the fiber is invalid or
+                                        incomplete, the behavior is undefined.

src/lib/libasync.js

@@ -515,8 +515,23 @@ addToLibrary({
     });
   },
 
-  $Fibers__deps: ['$Asyncify', 'emscripten_stack_set_limits', '$stackRestore'],
+  $Fibers__deps: ['emscripten_stack_set_limits', '$stackRestore',
+#if ASYNCIFY == 1
+    '$Asyncify',
+#endif
+  ],
   $Fibers: {
+    restoreStack(fiber) {
+      var stack_base = {{{ makeGetValue('fiber', C_STRUCTS.emscripten_fiber_s.stack_base,  '*') }}};
+      var stack_max =  {{{ makeGetValue('fiber', C_STRUCTS.emscripten_fiber_s.stack_limit, '*') }}};
+      _emscripten_stack_set_limits(stack_base, stack_max);
+#if STACK_OVERFLOW_CHECK >= 2
+      ___set_stack_limits(stack_base, stack_max);
+#endif
+      stackRestore({{{ makeGetValue('fiber', C_STRUCTS.emscripten_fiber_s.stack_ptr, '*') }}});
+    },
+
+#if ASYNCIFY == 1
     nextFiber: 0,
     trampolineRunning: false,
     trampoline() {
@@ -537,15 +552,7 @@ addToLibrary({
      * NOTE: This function is the asynchronous part of emscripten_fiber_swap.
      */
     finishContextSwitch(newFiber) {
-      var stack_base = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.stack_base,  '*') }}};
-      var stack_max =  {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.stack_limit, '*') }}};
-      _emscripten_stack_set_limits(stack_base, stack_max);
-
-#if STACK_OVERFLOW_CHECK >= 2
-      ___set_stack_limits(stack_base, stack_max);
-#endif
-
-      stackRestore({{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.stack_ptr,   '*') }}});
+      Fibers.restoreStack(newFiber);
 
       var entryPoint = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.entry, '*') }}};
 
@@ -562,6 +569,10 @@ addToLibrary({
         var userData = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.user_data, '*') }}};
         {{{ makeDynCall('vp', 'entryPoint') }}}(userData);
       } else {
+#if ASSERTIONS
+        var newAsyncifyStack = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.asyncify_data + C_STRUCTS.asyncify_data_s.stack_ptr, '*') }}};
+        assert(newAsyncifyStack, 'finishContextSwitch: fiber was initialized with a null asyncify_stack, which is only supported under JSPI (-sJSPI)');
+#endif
         var asyncifyData = newFiber + {{{ C_STRUCTS.emscripten_fiber_s.asyncify_data }}};
         Asyncify.currData = asyncifyData;
 
@@ -573,12 +584,73 @@ addToLibrary({
         Asyncify.doRewind(asyncifyData);
       }
     },
+#elif ASYNCIFY == 2
+    fiberResolvers: new Map(),
+    nextFiberId: 0,
+
+    allocateFiberId() {
+      do {
+        // Keep IDs positive and non-zero (fits in signed i32 rewind_id, with 0 reserved).
+        Fibers.nextFiberId = (Fibers.nextFiberId + 1) & 0x7fffffff || 1;
+      } while (Fibers.fiberResolvers.has(Fibers.nextFiberId));
+      return Fibers.nextFiberId;
+    },
+
+    swap(oldFiber, newFiber) {
+      return new Promise((resolve) => {
+        var oldId = Fibers.allocateFiberId();
+        {{{ makeSetValue('oldFiber', C_STRUCTS.emscripten_fiber_s.asyncify_data + C_STRUCTS.asyncify_data_s.rewind_id, 'oldId', 'i32') }}};
+        Fibers.fiberResolvers.set(oldId, resolve);
+        var entryPoint = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.entry, '*') }}};
+        if (entryPoint) {
+          {{{ makeSetValue('newFiber', C_STRUCTS.emscripten_fiber_s.entry, 0, '*') }}};
+          Fibers.restoreStack(newFiber);
+#if STACK_OVERFLOW_CHECK
+          writeStackCookie();
+#endif
+#if ASYNCIFY_DEBUG
+          dbg(`ASYNCIFY/FIBER: entering fiber ${newFiber} for the first time`);
+#endif
+          var userData = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.user_data, '*') }}};
+          // makeDynCall with promising=true wraps entryPoint in WebAssembly.promising,
+          // guaranteeing that start() returns a Promise.
+          var start = {{{ makeDynCall('vp', 'entryPoint', true) }}};
+          start(userData).catch((e) => {
+            abort(String(e));
+          });
+        } else {
+          var newId = {{{ makeGetValue('newFiber', C_STRUCTS.emscripten_fiber_s.asyncify_data + C_STRUCTS.asyncify_data_s.rewind_id, 'i32') }}};
+          var resume = Fibers.fiberResolvers.get(newId);
+#if ASSERTIONS
+          assert(resume, `fiber ${newFiber} (id ${newId}) is not suspended`);
+#endif
+#if ASYNCIFY_DEBUG
+          dbg(`ASYNCIFY/FIBER: resume fiber ${newFiber} (id ${newId})`);
+#endif
+          Fibers.fiberResolvers.delete(newId);
+          {{{ makeSetValue('newFiber', C_STRUCTS.emscripten_fiber_s.asyncify_data + C_STRUCTS.asyncify_data_s.rewind_id, 0, 'i32') }}};
+          resume(newFiber);
+        }
+      });
+    },
+#endif
   },
 
-  emscripten_fiber_swap__deps: ['$Asyncify', '$Fibers', '$stackSave'],
+  emscripten_fiber_swap__deps: ['$Fibers', '$stackSave',
+#if ASYNCIFY == 1
+    '$Asyncify',
+#endif
+  ],
   emscripten_fiber_swap__async: true,
+#if ASYNCIFY == 1
   emscripten_fiber_swap: (oldFiber, newFiber) => {
     if (ABORT) return;
+#if ASSERTIONS
+    assert(oldFiber, 'emscripten_fiber_swap: oldFiber must not be null');
+    assert(newFiber, 'emscripten_fiber_swap: newFiber must not be null');
+    var asyncifyStack = {{{ makeGetValue('oldFiber', C_STRUCTS.emscripten_fiber_s.asyncify_data + C_STRUCTS.asyncify_data_s.stack_ptr, '*') }}};
+    assert(asyncifyStack, 'emscripten_fiber_swap: fiber was initialized with a null asyncify_stack, which is only supported under JSPI (-sJSPI)');
+#endif
 #if ASYNCIFY_DEBUG
     dbg('ASYNCIFY/FIBER: swap', oldFiber, '->', newFiber, 'state:', Asyncify.state);
 #endif
@@ -610,6 +682,26 @@ addToLibrary({
       Asyncify.currData = null;
     }
   },
+#elif ASYNCIFY == 2
+  emscripten_fiber_swap: async (oldFiber, newFiber) => {
+    if (ABORT) return;
+#if ASSERTIONS
+    assert(oldFiber, 'emscripten_fiber_swap: oldFiber must not be null');
+    assert(newFiber, 'emscripten_fiber_swap: newFiber must not be null');
+#endif
+#if ASYNCIFY_DEBUG
+    dbg(`ASYNCIFY/FIBER: swap ${oldFiber} -> ${newFiber}`);
+#endif
+    if (oldFiber === newFiber) return;
+
+    var stackTop = stackSave();
+    {{{ makeSetValue('oldFiber', C_STRUCTS.emscripten_fiber_s.stack_ptr, 'stackTop', '*') }}};
+
+    var resumedFiber = await Fibers.swap(oldFiber, newFiber);
+
+    Fibers.restoreStack(resumedFiber);
+  },
+#endif
 #else // ASYNCIFY
   emscripten_sleep: () => {
     abort('Please compile your program with async support in order to use asynchronous operations like emscripten_sleep');

system/include/emscripten/fiber.h

@@ -26,30 +26,38 @@ typedef struct emscripten_fiber_s {
   void *stack_base;             /** Where the C stack starts (NOTE: grows down). */
   void *stack_limit;            /** Where the C stack ends. */
   void *stack_ptr;              /** Current position in the C stack. */
-  em_arg_callback_func entry;   /** Function to call when resuming this context. If NULL, asyncify_data is used to rewind the call stack. */
+  em_arg_callback_func entry;   /** Function to call when resuming this context. If NULL, asyncify_data (under Asyncify) or native stack switching (under JSPI) is used to resume the call stack. */
   void *user_data;              /** Opaque pointer, passed as-is to the entry function. */
-  asyncify_data_t asyncify_data;
+  asyncify_data_t asyncify_data; /** Asyncify data structure (under JSPI, only rewind_id is used). */
 } emscripten_fiber_t;
 
+/**
+ * Initializes a fiber context.
+ * Under JSPI (-sJSPI), asyncify_stack and asyncify_stack_size are ignored.
+ */
 void emscripten_fiber_init(
   emscripten_fiber_t * _Nonnull fiber,
   em_arg_callback_func entry_func,
   void *entry_func_arg,
   void * _Nonnull c_stack,
   size_t c_stack_size,
-  void * _Nonnull asyncify_stack,
+  void *asyncify_stack,
   size_t asyncify_stack_size
 );
 
+/**
+ * Partially initializes a fiber based on the currently active context.
+ * Under JSPI (-sJSPI), asyncify_stack and asyncify_stack_size are ignored.
+ */
 void emscripten_fiber_init_from_current_context(
   emscripten_fiber_t * _Nonnull fiber,
-  void * _Nonnull asyncify_stack,
+  void *asyncify_stack,
   size_t asyncify_stack_size
 );
 
 void emscripten_fiber_swap(
   emscripten_fiber_t * _Nonnull old_fiber,
-  emscripten_fiber_t * _Nonnull new_fibe
+  emscripten_fiber_t * _Nonnull new_fiber
 );
 
 #ifdef __cplusplus

system/lib/libc/emscripten_fiber.c

@@ -22,7 +22,8 @@ void emscripten_fiber_init(
     fiber->entry = entry_func;
     fiber->user_data = entry_func_arg;
     fiber->asyncify_data.stack_ptr = asyncify_stack;
-    fiber->asyncify_data.stack_limit = (char*)asyncify_stack + asyncify_stack_size;
+    fiber->asyncify_data.stack_limit = asyncify_stack ? (char*)asyncify_stack + asyncify_stack_size : NULL;
+    fiber->asyncify_data.rewind_id = 0;
 }
 
 void emscripten_fiber_init_from_current_context(
@@ -34,5 +35,6 @@ void emscripten_fiber_init_from_current_context(
     fiber->stack_limit = (void*)emscripten_stack_get_end();
     fiber->entry = NULL;
     fiber->asyncify_data.stack_ptr = asyncify_stack;
-    fiber->asyncify_data.stack_limit = (char*)asyncify_stack + asyncify_stack_size;
+    fiber->asyncify_data.stack_limit = asyncify_stack ? (char*)asyncify_stack + asyncify_stack_size : NULL;
+    fiber->asyncify_data.rewind_id = 0;
 }

test/test_core.py

@@ -8450,11 +8450,33 @@ def test_async_ccall_promise(self, exit_runtime):
     self.cflags += ['--pre-js', 'pre.js', '-sINCOMING_MODULE_JS_API=onRuntimeInitialized']
     self.do_runf('main.c', 'stringf: first\nsecond\n6.4')
 
-  @no_esm_integration('WASM_ESM_INTEGRATION is not compatible with ASYNCIFY=1')
-  def test_fibers_asyncify(self):
+  @with_asyncify_and_jspi
+  def test_fibers(self):
+    self.maybe_closure()
+    if self.get_setting('JSPI'):
+      self.cflags += ['-DJSPI']
+    self.do_runf('test_fibers.cpp', '*leaf-0-100-1-101-1-102-2-103-3-104-5-105-8-106-13-107-21-108-34-109-direct-1035-*\nmove-342-*\n')
+
+  def test_fibers_asyncify_null_stack(self):
     self.set_setting('ASYNCIFY')
+    self.set_setting('ASSERTIONS')
     self.maybe_closure()
-    self.do_runf('test_fibers.cpp', '*leaf-0-100-1-101-1-102-2-103-3-104-5-105-8-106-13-107-21-108-34-109-*')
+    self.do_run('''
+#include <stdio.h>
+#include <emscripten/fiber.h>
+
+static emscripten_fiber_t main_fiber;
+
+int main() {
+  emscripten_fiber_init_from_current_context(&main_fiber, NULL, 0);
+  emscripten_fiber_t child;
+  alignas(16) char c_stack[4096];
+  emscripten_fiber_init(&child, NULL, NULL, c_stack, sizeof(c_stack), NULL, 0);
+  emscripten_fiber_swap(&main_fiber, &child);
+  return 0;
+}
+''', 'Assertion failed: emscripten_fiber_swap: fiber was initialized with a null asyncify_stack, which is only supported under JSPI (-sJSPI)',
+                assert_returncode=NON_ZERO)
 
   @with_asyncify_and_jspi
   def test_asyncify_unused(self):