Unify thread ID allocation between Wasm Workers and pthreads. NFC (#26670)

Followup to #26660 and #26472

Author: sbc100

src/lib/libsigs.js

@@ -313,7 +313,8 @@ sigs = {
   _embind_register_value_object__sig: 'vpppppp',
   _embind_register_value_object_field__sig: 'vpppppppppp',
   _embind_register_void__sig: 'vpp',
-  _emscripten_create_wasm_worker__sig: 'ipi',
+  _emscripten_create_audio_worklet__sig: 'viipipp',
+  _emscripten_create_wasm_worker__sig: 'iipi',
   _emscripten_dlopen_js__sig: 'vpppp',
   _emscripten_dlsync_threads__sig: 'v',
   _emscripten_fetch_get_response_headers__sig: 'pipp',
@@ -808,7 +809,6 @@ sigs = {
   emscripten_stack_snapshot__sig: 'p',
   emscripten_stack_unwind_buffer__sig: 'ippi',
   emscripten_start_fetch__sig: 'vp',
-  emscripten_start_wasm_audio_worklet_thread_async__sig: 'vipipp',
   emscripten_supports_offscreencanvas__sig: 'i',
   emscripten_terminate_all_wasm_workers__sig: 'v',
   emscripten_terminate_wasm_worker__sig: 'vi',

src/lib/libwasm_worker.js

@@ -50,7 +50,7 @@
   // This is the way that we signal to the Web Worker that it is hosting
   // a Wasm Worker.
 #if ASSERTIONS
-  'name': 'em-ww-' + _wasmWorkersID,
+  'name': 'em-ww-' + wwID,
 #else
   'name': 'em-ww',
 #endif
@@ -62,16 +62,6 @@
 
 addToLibrary({
   $_wasmWorkers: {},
-#if PTHREADS
-  // When the build contains both pthreads and Wasm Workers, offset the
-  // Wasm Worker ID space to avoid collisions with pthread TIDs (which start
-  // at 42). We use `1 << 21` since it's ~1/2 way through `pid_t` space,
-  // essentially giving pthreads the first 1/2 of the range and wasm workers the
-  // second half.
-  $_wasmWorkersID: {{{ 1 << 21 }}},
-#else
-  $_wasmWorkersID: 1,
-#endif
 
   // Starting up a Wasm Worker is an asynchronous operation, hence if the parent
   // thread performs any postMessage()-based wasm function calls to the
@@ -175,7 +165,7 @@ addToLibrary({
   },
 
   _emscripten_create_wasm_worker__deps: [
-    '$_wasmWorkers', '$_wasmWorkersID',
+    '$_wasmWorkers',
     '$_wasmWorkerAppendToQueue', '$_wasmWorkerRunPostMessage',
 #if ASSERTIONS
     'emscripten_has_threading_support',
@@ -191,11 +181,11 @@ if (ENVIRONMENT_IS_WASM_WORKER
   _wasmWorkers[0] = globalThis;
   addEventListener("message", _wasmWorkerAppendToQueue);
 }`,
-  _emscripten_create_wasm_worker: (stackLowestAddress, stackSize) => {
+  _emscripten_create_wasm_worker: (wwID, stackLowestAddress, stackSize) => {
 #if ASSERTIONS
     if (!_emscripten_has_threading_support()) {
       err('create_wasm_worker: environment does not support SharedArrayBuffer, wasm workers are not available');
-      return 0;
+      return false;
     }
 #endif
     let worker;
@@ -205,15 +195,15 @@ if (ENVIRONMENT_IS_WASM_WORKER
       var p = trustedTypes.createPolicy(
           'emscripten#workerPolicy1', { createScriptURL: (ignored) => {{{ wasmWorkerJs }}}}
       );
-      worker = _wasmWorkers[_wasmWorkersID] = new Worker(p.createScriptURL('ignored'), {{{ wasmWorkerOptions }}});
+      worker = _wasmWorkers[wwID] = new Worker(p.createScriptURL('ignored'), {{{ wasmWorkerOptions }}});
     } else
 #endif
-    worker = _wasmWorkers[_wasmWorkersID] = new Worker({{{ wasmWorkerJs }}}, {{{ wasmWorkerOptions }}});
+    worker = _wasmWorkers[wwID] = new Worker({{{ wasmWorkerJs }}}, {{{ wasmWorkerOptions }}});
     // Craft the Module object for the Wasm Worker scope:
     worker.postMessage({
       // Signal with a non-zero value that this Worker will be a Wasm Worker,
       // and not the main browser thread.
-      wwID: _wasmWorkersID,
+      wwID,
       wasm: wasmModule,
       wasmMemory,
       stackLowestAddress, // sb = stack bottom (lowest stack address, SP points at this when stack is full)
@@ -237,9 +227,9 @@ if (ENVIRONMENT_IS_WASM_WORKER
     }
 #endif
 #if RUNTIME_DEBUG
-    dbg("done _emscripten_create_wasm_worker", _wasmWorkersID)
+    dbg("done _emscripten_create_wasm_worker", wwID)
 #endif
-    return _wasmWorkersID++;
+    return true;
   },
 
   emscripten_terminate_wasm_worker: (id) => {

src/lib/libwebaudio.js

@@ -165,16 +165,15 @@ var LibraryWebAudio = {
   },
 
 #if AUDIO_WORKLET
-  // emscripten_start_wasm_audio_worklet_thread_async() doesn't use stackAlloc,
+  // _emscripten_create_audio_worklet() doesn't use stackAlloc,
   // etc., but the created worklet does.
-  emscripten_start_wasm_audio_worklet_thread_async__deps: [
-    '$_wasmWorkersID',
+  _emscripten_create_audio_worklet__deps: [
     '$_emAudioDispatchProcessorCallback',
     '$stackAlloc', '$stackRestore', '$stackSave'],
-  emscripten_start_wasm_audio_worklet_thread_async: (contextHandle, stackLowestAddress, stackSize, callback, userData) => {
+  _emscripten_create_audio_worklet: (wwID, contextHandle, stackLowestAddress, stackSize, callback, userData) => {
 
 #if ASSERTIONS || WEBAUDIO_DEBUG
-    emAudioExpectContext(contextHandle, 'emscripten_start_wasm_audio_worklet_thread_async');
+    emAudioExpectContext(contextHandle, '_emscripten_create_audio_worklet');
 #endif
 
     var audioContext = emAudio[contextHandle];
@@ -190,12 +189,12 @@ var LibraryWebAudio = {
 #endif
 
 #if WEBAUDIO_DEBUG
-    dbg(`emscripten_start_wasm_audio_worklet_thread_async() adding audioworklet.js...`);
+    dbg(`_emscripten_create_audio_worklet() adding audioworklet.js...`);
 #endif
 
     var audioWorkletCreationFailed = () => {
 #if ASSERTIONS || WEBAUDIO_DEBUG
-      dbg(`emscripten_start_wasm_audio_worklet_thread_async() addModule() failed!`);
+      dbg(`_emscripten_create_audio_worklet() addModule() failed!`);
 #endif
       {{{ makeDynCall('viip', 'callback') }}}(contextHandle, 0/*EM_FALSE*/, userData);
     };
@@ -214,7 +213,7 @@ var LibraryWebAudio = {
 
     audioWorklet.addModule({{{ wasmWorkerJs }}}).then(() => {
 #if WEBAUDIO_DEBUG
-      dbg(`emscripten_start_wasm_audio_worklet_thread_async() addModule() completed`);
+      dbg(`_emscripten_create_audio_worklet() addModule() completed`);
 #endif
 
 #if MIN_FIREFOX_VERSION < 138 || MIN_CHROME_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION != TARGET_NOT_SUPPORTED
@@ -248,7 +247,7 @@ var LibraryWebAudio = {
         // Assign the loaded AudioWorkletGlobalScope a Wasm Worker ID so that
         // it can utilized its own TLS slots, and it is recognized to not be
         // the main browser thread.
-        wwID: _wasmWorkersID++,
+        wwID,
 #if MINIMAL_RUNTIME
         wasm: Module['wasm'],
 #else

system/lib/libc/emscripten_internal.h

@@ -15,6 +15,7 @@
 
 #include <emscripten/em_macros.h>
 #include <emscripten/proxying.h>
+#include <emscripten/webaudio.h>
 #include <emscripten/html5.h>
 #include <emscripten/wasm_worker.h>
 
@@ -134,7 +135,9 @@ void emscripten_fetch_free(unsigned int);
 
 // Internal implementation function in JavaScript side that emscripten_create_wasm_worker() calls to
 // to perform the wasm worker creation.
-emscripten_wasm_worker_t _emscripten_create_wasm_worker(void *stackLowestAddress, uint32_t stackSize);
+bool _emscripten_create_wasm_worker(emscripten_wasm_worker_t wwID, void *stackLowestAddress, uint32_t stackSize);
+
+void _emscripten_create_audio_worklet(emscripten_wasm_worker_t wwID, EMSCRIPTEN_WEBAUDIO_T audioContext, void *stackLowestAddress, uint32_t stackSize, EmscriptenStartWebAudioWorkletCallback callback, void *userData2);
 
 void __resumeException(void* exn);
 void __cxa_call_unexpected(void* exn);

system/lib/pthread/emscripten_get_next_tid.c

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2026 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <unistd.h>
+#include <stdatomic.h>
+
+#include "emscripten_internal.h"
+
+// In case the stub syscall is not linked it
+static int dummy_getpid(void) {
+  return 42;
+}
+weak_alias(dummy_getpid, __syscall_getpid);
+
+static _Atomic pid_t next_tid = 0;
+
+pid_t _emscripten_get_next_tid() {
+  // Create threads with monotonically increasing TID starting with the main
+  // thread which has TID == PID.
+  if (next_tid == 0) {
+    // Use CAS to initialize next_tid so that one thread will end up
+    // initializing it.
+    pid_t expected = 0;
+    atomic_compare_exchange_strong(&next_tid, &expected, getpid() + 1);
+  }
+  return next_tid++;
+}

system/lib/pthread/pthread_create.c

@@ -14,6 +14,7 @@
 #include <string.h>
 #include <threads.h>
 #include <unistd.h>
+
 #include <emscripten/heap.h>
 #include <emscripten/threading.h>
 
@@ -62,14 +63,6 @@ static void init_file_lock(FILE *f) {
   if (f && f->lock<0) f->lock = 0;
 }
 
-static pid_t next_tid = 0;
-
-// In case the stub syscall is not linked it
-static int dummy_getpid(void) {
-  return 42;
-}
-weak_alias(dummy_getpid, __syscall_getpid);
-
 static int tl_lock_count;
 static int tl_lock_waiters;
 
@@ -121,12 +114,6 @@ int __pthread_create(pthread_t* restrict res,
     return EINVAL;
   }
 
-  // Create threads with monotonically increasing TID starting with the main
-  // thread which has TID == PID.
-  if (!next_tid) {
-    next_tid = getpid() + 1;
-  }
-
   if (!libc.threaded) {
     for (FILE *f=*__ofl_lock(); f; f=f->next)
       init_file_lock(f);
@@ -177,7 +164,7 @@ int __pthread_create(pthread_t* restrict res,
   // The pthread struct has a field that points to itself - this is used as a
   // magic ID to detect whether the pthread_t structure is 'alive'.
   new->self = new;
-  new->tid = next_tid++;
+  new->tid = _emscripten_get_next_tid();
 
   // pthread struct robust_list head should point to itself.
   new->robust_list.head = &new->robust_list.head;

system/lib/pthread/threading_internal.h

@@ -104,3 +104,5 @@ void _emscripten_run_js_on_main_thread_done(void* ctx, void* arg, double result)
 // if called from the main browser thread, this function will return zero
 // since blocking is not allowed there).
 int _emscripten_thread_supports_atomics_wait(void);
+
+pid_t _emscripten_get_next_tid();

system/lib/wasm_worker/audio_worklet.c

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2026 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <emscripten/webaudio.h>
+
+#include "emscripten_internal.h"
+#include "threading_internal.h"
+
+// Simple wrapper function around the JS _emscripten_create_audio_worklet
+// function that adds the _emscripten_get_next_tid() as arg0
+void emscripten_start_wasm_audio_worklet_thread_async(EMSCRIPTEN_WEBAUDIO_T audioContext, void *stackLowestAddress, uint32_t stackSize, EmscriptenStartWebAudioWorkletCallback callback, void *userData2) {
+  _emscripten_create_audio_worklet(_emscripten_get_next_tid(), audioContext, stackLowestAddress, stackSize, callback, userData2);
+}

system/lib/wasm_worker/library_wasm_worker.c

@@ -14,6 +14,7 @@
 #include "libc.h"
 #include "stdio_impl.h"
 #include "emscripten_internal.h"
+#include "threading_internal.h"
 
 #include <assert.h>
 #include <emscripten/wasm_worker.h>
@@ -101,7 +102,10 @@ emscripten_wasm_worker_t emscripten_create_wasm_worker(void *stackPlusTLSAddress
   // only going one way here.
   if (!libc.threads_minus_1++) libc.need_locks = 1;
 
-  return _emscripten_create_wasm_worker(stackPlusTLSAddress, stackPlusTLSSize);
+  emscripten_wasm_worker_t wwID = _emscripten_get_next_tid();
+  if (!_emscripten_create_wasm_worker(wwID, stackPlusTLSAddress, stackPlusTLSSize))
+    return 0;
+  return wwID;
 }
 
 emscripten_wasm_worker_t emscripten_malloc_wasm_worker(size_t stackSize) {