Add PA_WEBAUDIO_ASYNCIFY and experimental synchronous mode

This synchronous mode uses `emscripten_thread_sleep` instead of
`emscripten_sleep` and is intended to run from a separate thread where
blocking is allowed.

In its current state, this seems to depend on

    WebAudio/web-audio-api#2423

The alternative would be to proxy audio context-related calls to the
main thread.

CMakeLists.txt

@@ -412,12 +412,19 @@ elseif(UNIX)
         target_compile_definitions(PortAudio PUBLIC PA_USE_WEBAUDIO=1)
         target_compile_options(PortAudio PUBLIC -pthread)
         target_link_options(PortAudio PUBLIC
-          -sASYNCIFY
-          -sASYNCIFY_IMPORTS=emscripten_asm_const_int
           -sAUDIO_WORKLET
           -sWASM_WORKERS
           -sEXPORTED_RUNTIME_METHODS=emscriptenGetAudioObject
         )
+
+        option(PA_WEBAUDIO_ASYNCIFY "Build with -sASYNCIFY and use async operations. Recommended when intending to call PortAudio from the main thread to avoid blocking." ON)
+        if(PA_WEBAUDIO_ASYNCIFY)
+          target_compile_definitions(PortAudio PUBLIC PA_WEBAUDIO_ASYNCIFY=1)
+          target_link_options(PortAudio PUBLIC
+            -sASYNCIFY
+            -sASYNCIFY_IMPORTS=emscripten_asm_const_int
+          )
+        endif()
         set(PKGCONFIG_CFLAGS "${PKGCONFIG_CFLAGS} -DPA_USE_WEBAUDIO=1")
       endif()
 

src/hostapi/webaudio/README.md

@@ -14,6 +14,9 @@ emcmake cmake -B build -DPA_BUILD_EXAMPLES=ON
 cmake --build build
 ```
 
+> [!TIP]
+> By default PortAudio will be built with `-sASYNCIFY`. This makes it safe to be called from the main thread, but also introduces overhead and potentially other issues when the application already uses other asynchronous operations. PortAudio can also be built without Asyncify (i.e. using synchronous/blocking operations) by setting `-DPA_WEBAUDIO_ASYNCIFY=OFF`, in that case it is recommended to call PortAudio from a worker thread to avoid blocking the main thread (which may result in locking up the UI or in worse cases deadlock the application, e.g. if PortAudio waits for user interaction, as it does during `Pa_StartStream`). This can be done automatically using `-sPROXY_TO_PTHREAD`.
+
 > [!TIP]
 > For debug logging, set `-DPA_ENABLE_DEBUG_OUTPUT=ON`
 

src/hostapi/webaudio/pa_webaudio.c

@@ -718,12 +718,24 @@ static EM_BOOL WebAudioHostProcessingLoop( int numInputs, const AudioSampleFrame
 
 void WebAudioSuspendContextSync( EMSCRIPTEN_WEBAUDIO_T context )
 {
+#ifdef PA_WEBAUDIO_ASYNCIFY
     EM_ASM({
         const context = emscriptenGetAudioObject($0);
         Asyncify.handleAsync(async () => {
             await context.suspend();
         });
     }, context);
+#else
+    EM_ASM({
+        const context = emscriptenGetAudioObject($0);
+        context.suspend();
+    }, context);
+
+    while ( emscripten_audio_context_state( context ) != AUDIO_CONTEXT_STATE_SUSPENDED )
+    {
+        Pa_Sleep ( 100 );
+    }
+#endif
 }
 
 
@@ -772,7 +784,7 @@ static PaError StartStream( PaStream *s )
     while ( !IsStreamActive(stream) )
     {
         PA_DEBUG(("Audio context is not running yet, waiting for user interaction...\n"));
-        emscripten_sleep( 500 );
+        Pa_Sleep( 500 );
         emscripten_resume_audio_context_sync( stream->context );
     }
 

src/os/unix/pa_unix_util.c

@@ -53,6 +53,7 @@
 
 #ifdef __EMSCRIPTEN__
 #include <emscripten.h>
+#include <emscripten/threading.h>
 #endif
 
 #if defined(__APPLE__) && !defined(HAVE_MACH_ABSOLUTE_TIME)
@@ -116,7 +117,11 @@ int PaUtil_CountCurrentlyAllocatedBlocks( void )
 void Pa_Sleep( long msec )
 {
 #ifdef __EMSCRIPTEN__
+#ifdef PA_WEBAUDIO_ASYNCIFY
     emscripten_sleep(msec);
+#else
+    emscripten_thread_sleep(msec);
+#endif
 #elif defined(HAVE_NANOSLEEP)
     struct timespec req = {0}, rem = {0};
     PaTime time = msec / 1.e3;