sdl_mixer (3): if mono requested, capture stereo

Workaround for SDL3 mixer, that doesn't respect selected ch_count=1 now.

Reduced the channel count later in audio stack (this will work for other
acaps as well).

src/audio/audio.cpp

@@ -1141,6 +1141,11 @@ static void *audio_sender_thread(void *arg)
                                 continue;
 
                         audio_frame2 bf_n(buffer);
+                        if (audio_capture_channels != 0 &&
+                            (int) audio_capture_channels !=
+                                bf_n.get_channel_count()) {
+                                bf_n.change_ch_count((int) audio_capture_channels);
+                        }
 
                         // RESAMPLE
                         int resample_to = s->resample_to;

src/audio/capture/sdl_mixer.c

@@ -37,7 +37,7 @@
 /**
  * @file
  * @todo errata (SDL3 vs SDL2)
- * 1. 1 channel capture (-a ch=1) seem no longer work
+ * 1. 1 channel capture (-a ch=1) seem no longer work but there is a workaround
  * 2. unsufficient performance (generates overflow even in default config)
  */
 
@@ -205,6 +205,24 @@ static void try_open_soundfont() {
         }
 }
 
+/// handle SDL 3.0.0 mixer not being able to capture mono
+static void
+adjust_ch_count(struct state_sdl_mixer_capture *s)
+{
+        int             frequency = 0;
+        SDL_AudioFormat format = { 0 };
+        int             channels = 0;
+        Mix_QuerySpec(&frequency, &format, &channels);
+        if (audio_capture_channels > 0 &&
+            channels != (int) audio_capture_channels) {
+                MSG(INFO,
+                    "%d channel capture seem to be broken with SDL3 "
+                    "mixer - capture %d and reduce drop second later\n",
+                    s->audio.ch_count, channels);
+                s->audio.ch_count = channels;
+        }
+}
+
 static void * audio_cap_sdl_mixer_init(struct module *parent, const char *cfg)
 {
         UNUSED(parent);
@@ -234,10 +252,6 @@ static void * audio_cap_sdl_mixer_init(struct module *parent, const char *cfg)
         }
 
 #ifdef HAVE_SDL3
-        if (s->audio.ch_count == 1) {
-                MSG(WARNING,
-                    "! channel capture seem to be broken with SDL3 mixer...\n");
-        }
         SDL_AudioSpec spec = {
                 .format   = audio_format,
                 .channels = s->audio.ch_count,
@@ -251,6 +265,7 @@ static void * audio_cap_sdl_mixer_init(struct module *parent, const char *cfg)
                 log_msg(LOG_LEVEL_ERROR, MOD_NAME "error initalizing sound: %s\n", Mix_GetError());
                 goto error;
         }
+        adjust_ch_count(s);
         const char *filename = s->req_filename;
         if (!filename) {
                 filename = load_song1();

src/audio/types.cpp

@@ -332,6 +332,32 @@ void  audio_frame2::change_bps(int new_bps)
         channels = std::move(new_channels);
 }
 
+/**
+ * simple ch_count changer - excess channels dropped, missing copied from the
+ * last one
+ */
+void audio_frame2::change_ch_count(int ch_count)
+{
+        if ((unsigned) ch_count == channels.size()) {
+                return;
+        }
+
+        if ((unsigned) ch_count < channels.size()) {
+                channels.resize(ch_count);
+                return;
+        }
+
+        const channel &last_ch = channels.at(channels.size() - 1);
+        for (unsigned i = channels.size(); i < (unsigned) ch_count; ++i) {
+                std::unique_ptr<char[]> data =
+                    std::unique_ptr<char[]>(new char[last_ch.len]);
+                memcpy(data.get(), last_ch.data.get(), last_ch.len);
+                channel ch = { std::move(data), last_ch.len, last_ch.len,
+                               last_ch.fec_params };
+                channels.push_back(std::move(ch));
+        }
+}
+
 void audio_frame2::set_timestamp(int64_t ts)
 {
         assert(ts >= -1);

src/audio/types.h

@@ -178,6 +178,7 @@ public:
         void set_fec_params(int channel, fec_desc const &);
         [[nodiscard]] static audio_frame2 copy_with_bps_change(audio_frame2 const &frame, int new_bps);
         void change_bps(int new_bps);
+        void change_ch_count(int new_ch_count);
         /**
          * @note
          * bps of the frame needs to be 16 bits!