Revert "audio: Added SDL_SetAudioIterationCallbacks()."

This reverts commit 608f706a954cb856c8ae2f18d3a6a0339ff7b0ae.

Didn't end up using this in SDL3_mixer, and it's a super-awkward API if we
don't need it.

I _might_ bite the bullet and let people lock a physical audio device, though,
as I could see that being useful but less awkward for the same reasons I
originally wanted it.

include/SDL3/SDL_audio.h

@@ -2035,85 +2035,6 @@ extern SDL_DECLSPEC void SDLCALL SDL_DestroyAudioStream(SDL_AudioStream *stream)
  */
 extern SDL_DECLSPEC SDL_AudioStream * SDLCALL SDL_OpenAudioDeviceStream(SDL_AudioDeviceID devid, const SDL_AudioSpec *spec, SDL_AudioStreamCallback callback, void *userdata);
 
-/**
- * A callback that fires around an audio device's processing work.
- *
- * This callback fires when a logical audio device is about to start accessing
- * its bound audio streams, and fires again when it has finished accessing
- * them. It covers the range of one "iteration" of the audio device.
- *
- * It can be useful to use this callback to update state that must apply to
- * all bound audio streams atomically: to make sure state changes don't happen
- * while half of the streams are already processed for the latest audio
- * buffer.
- *
- * This callback should run as quickly as possible and not block for any
- * significant time, as this callback delays submission of data to the audio
- * device, which can cause audio playback problems. This callback delays all
- * audio processing across a single physical audio device: all its logical
- * devices and all bound audio streams. Use it carefully.
- *
- * \param userdata a pointer provided by the app through
- *                 SDL_SetAudioPostmixCallback, for its own use.
- * \param devid the audio device this callback is running for.
- * \param start true if this is the start of the iteration, false if the end.
- *
- * \threadsafety This will run from a background thread owned by SDL. The
- *               application is responsible for locking resources the callback
- *               touches that need to be protected.
- *
- * \since This datatype is available since SDL 3.4.0.
- *
- * \sa SDL_SetAudioIterationCallbacks
- */
-typedef void (SDLCALL *SDL_AudioIterationCallback)(void *userdata, SDL_AudioDeviceID devid, bool start);
-
-/**
- * Set callbacks that fire around a new iteration of audio device processing.
- *
- * Two callbacks are provided here: one that runs when a device is about to
- * process its bound audio streams, and another that runs when the device has
- * finished processing them.
- *
- * These callbacks can run at any time, and from any thread; if you need to
- * serialize access to your app's data, you should provide and use a mutex or
- * other synchronization device.
- *
- * Generally these callbacks are used to apply state that applies to multiple
- * bound audio streams, with a guarantee that the audio device's thread isn't
- * halfway through processing them. Generally a finer-grained lock through
- * SDL_LockAudioStream() is more appropriate.
- *
- * The callbacks are extremely time-sensitive; the callback should do the
- * least amount of work possible and return as quickly as it can. The longer
- * the callback runs, the higher the risk of audio dropouts or other problems.
- *
- * This function will block until the audio device is in between iterations,
- * so any existing callback that might be running will finish before this
- * function sets the new callback and returns.
- *
- * Physical devices do not accept these callbacks, only logical devices
- * created through SDL_OpenAudioDevice() can be.
- *
- * Setting a NULL callback function disables any previously-set callback.
- * Either callback may be NULL, and the same callback is permitted to be used
- * for both.
- *
- * \param devid the ID of an opened audio device.
- * \param start a callback function to be called at the start of an iteration.
- *              Can be NULL.
- * \param end a callback function to be called at the end of an iteration. Can
- *            be NULL.
- * \param userdata app-controlled pointer passed to callback. Can be NULL.
- * \returns true on success or false on failure; call SDL_GetError() for more
- *          information.
- *
- * \threadsafety It is safe to call this function from any thread.
- *
- * \since This function is available since SDL 3.4.0.
- */
-extern SDL_DECLSPEC bool SDLCALL SDL_SetAudioIterationCallbacks(SDL_AudioDeviceID devid, SDL_AudioIterationCallback start, SDL_AudioIterationCallback end, void *userdata);
-
 /**
  * A callback that fires when data is about to be fed to an audio device.
  *

src/audio/SDL_audio.c

@@ -1164,23 +1164,9 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
 
             // We should have updated this elsewhere if the format changed!
             SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &device->spec, NULL, NULL));
-
             SDL_assert(stream->src_spec.format != SDL_AUDIO_UNKNOWN);
 
-            int br = 0;
-
-            if (!SDL_GetAtomicInt(&logdev->paused)) {
-                if (logdev->iteration_start) {
-                    logdev->iteration_start(logdev->iteration_userdata, logdev->instance_id, true);
-                }
-
-                br = SDL_GetAudioStreamDataAdjustGain(stream, device_buffer, buffer_size, logdev->gain);
-
-                if (logdev->iteration_end) {
-                    logdev->iteration_end(logdev->iteration_userdata, logdev->instance_id, false);
-                }
-            }
-
+            const int br = SDL_GetAtomicInt(&logdev->paused) ? 0 : SDL_GetAudioStreamDataAdjustGain(stream, device_buffer, buffer_size, logdev->gain);
             if (br < 0) {  // Probably OOM. Kill the audio device; the whole thing is likely dying soon anyhow.
                 failed = true;
                 SDL_memset(device_buffer, device->silence_value, buffer_size);  // just supply silence to the device before we die.
@@ -1218,10 +1204,6 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
                     SDL_memset(mix_buffer, '\0', work_buffer_size);  // start with silence.
                 }
 
-                if (logdev->iteration_start) {
-                    logdev->iteration_start(logdev->iteration_userdata, logdev->instance_id, true);
-                }
-
                 for (SDL_AudioStream *stream = logdev->bound_streams; stream; stream = stream->next_binding) {
                     // We should have updated this elsewhere if the format changed!
                     SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &outspec, NULL, NULL));
@@ -1246,10 +1228,6 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
                     }
                 }
 
-                if (logdev->iteration_end) {
-                    logdev->iteration_end(logdev->iteration_userdata, logdev->instance_id, false);
-                }
-
                 if (postmix) {
                     SDL_assert(mix_buffer == device->postmix_buffer);
                     postmix(logdev->postmix_userdata, &outspec, mix_buffer, work_buffer_size);
@@ -1991,21 +1969,6 @@ bool SDL_SetAudioPostmixCallback(SDL_AudioDeviceID devid, SDL_AudioPostmixCallba
     return result;
 }
 
-bool SDL_SetAudioIterationCallbacks(SDL_AudioDeviceID devid, SDL_AudioIterationCallback iter_start, SDL_AudioIterationCallback iter_end, void *userdata)
-{
-    SDL_AudioDevice *device = NULL;
-    SDL_LogicalAudioDevice *logdev = ObtainLogicalAudioDevice(devid, &device);
-    bool result = false;
-    if (logdev) {
-        logdev->iteration_start = iter_start;
-        logdev->iteration_end = iter_end;
-        logdev->iteration_userdata = userdata;
-        result = true;
-    }
-    ReleaseAudioDevice(device);
-    return result;
-}
-
 bool SDL_BindAudioStreams(SDL_AudioDeviceID devid, SDL_AudioStream * const *streams, int num_streams)
 {
     const bool islogical = !(devid & (1<<1));

src/audio/SDL_sysaudio.h

@@ -264,13 +264,6 @@ struct SDL_LogicalAudioDevice
     // true if device was opened with SDL_OpenAudioDeviceStream (so it forbids binding changes, etc).
     bool simplified;
 
-    // If non-NULL, callback into the app that alerts it to start/end of device iteration.
-    SDL_AudioIterationCallback iteration_start;
-    SDL_AudioIterationCallback iteration_end;
-
-    // App-supplied pointer for iteration callbacks.
-    void *iteration_userdata;
-
     // If non-NULL, callback into the app that lets them access the final postmix buffer.
     SDL_AudioPostmixCallback postmix;
 

src/dynapi/SDL_dynapi.sym

@@ -1251,7 +1251,6 @@ SDL3_0.0.0 {
     SDL_GetGPUDeviceProperties;
     SDL_CreateGPURenderer;
     SDL_PutAudioStreamPlanarData;
-    SDL_SetAudioIterationCallbacks;
     SDL_GetEventDescription;
     SDL_PutAudioStreamDataNoCopy;
     SDL_IsTraySupported;

src/dynapi/SDL_dynapi_overrides.h

@@ -1276,7 +1276,6 @@
 #define SDL_GetGPUDeviceProperties SDL_GetGPUDeviceProperties_REAL
 #define SDL_CreateGPURenderer SDL_CreateGPURenderer_REAL
 #define SDL_PutAudioStreamPlanarData SDL_PutAudioStreamPlanarData_REAL
-#define SDL_SetAudioIterationCallbacks SDL_SetAudioIterationCallbacks_REAL
 #define SDL_GetEventDescription SDL_GetEventDescription_REAL
 #define SDL_PutAudioStreamDataNoCopy SDL_PutAudioStreamDataNoCopy_REAL
 #define SDL_IsTraySupported SDL_IsTraySupported_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -1284,7 +1284,6 @@ SDL_DYNAPI_PROC(bool,SDL_GetRenderTextureAddressMode,(SDL_Renderer *a,SDL_Textur
 SDL_DYNAPI_PROC(SDL_PropertiesID,SDL_GetGPUDeviceProperties,(SDL_GPUDevice *a),(a),return)
 SDL_DYNAPI_PROC(SDL_Renderer*,SDL_CreateGPURenderer,(SDL_Window *a,SDL_GPUShaderFormat b,SDL_GPUDevice **c),(a,b,c),return)
 SDL_DYNAPI_PROC(bool,SDL_PutAudioStreamPlanarData,(SDL_AudioStream *a,const void * const*b,int c,int d),(a,b,c,d),return)
-SDL_DYNAPI_PROC(bool,SDL_SetAudioIterationCallbacks,(SDL_AudioDeviceID a,SDL_AudioIterationCallback b,SDL_AudioIterationCallback c,void *d),(a,b,c,d),return)
 SDL_DYNAPI_PROC(int,SDL_GetEventDescription,(const SDL_Event *a,char *b,int c),(a,b,c),return)
 SDL_DYNAPI_PROC(bool,SDL_PutAudioStreamDataNoCopy,(SDL_AudioStream *a,const void *b,int c,SDL_AudioStreamDataCompleteCallback d,void *e),(a,b,c,d,e),return)
 SDL_DYNAPI_PROC(bool,SDL_IsTraySupported,(void),(),return)