alsa: Make hotplug thread optional.

Even without the thread, it'll do an initial hardware detection at startup,
but there won't be any further hotplug events after that. But for many cases,
that is likely complete sufficient.

In either case, this cleaned up the code to no longer need a semaphore at
startup.

Fixes #4862.

src/audio/alsa/SDL_alsa_audio.c

@@ -26,6 +26,11 @@
 #define SDL_ALSA_NON_BLOCKING 0
 #endif
 
+/* without the thread, you will detect devices on startup, but will not get futher hotplug events. But that might be okay. */
+#ifndef SDL_ALSA_HOTPLUG_THREAD
+#define SDL_ALSA_HOTPLUG_THREAD 1
+#endif
+
 /* Allow access to a raw mixing buffer */
 
 #include <sys/types.h>
@@ -802,24 +807,16 @@ add_device(const int iscapture, const char *name, void *hint, ALSA_Device **pSee
 }
 
 
-static SDL_atomic_t ALSA_hotplug_shutdown;
-static SDL_Thread *ALSA_hotplug_thread;
+static ALSA_Device *hotplug_devices = NULL;
 
-static int SDLCALL
-ALSA_HotplugThread(void *arg)
+static void
+ALSA_HotplugIteration(void)
 {
-    SDL_sem *first_run_semaphore = (SDL_sem *) arg;
-    ALSA_Device *devices = NULL;
-    ALSA_Device *next;
-    ALSA_Device *dev;
-    Uint32 ticks;
-
-    SDL_SetThreadPriority(SDL_THREAD_PRIORITY_LOW);
-
-    while (!SDL_AtomicGet(&ALSA_hotplug_shutdown)) {
     void **hints = NULL;
+    ALSA_Device *dev;
     ALSA_Device *unseen;
     ALSA_Device *seen;
+    ALSA_Device *next;
     ALSA_Device *prev;
 
     if (ALSA_snd_device_name_hint(-1, "pcm", &hints) == 0) {
@@ -832,8 +829,9 @@ ALSA_HotplugThread(void *arg)
             "hw:", "sysdefault:", "default:", NULL
         };
 
-            unseen = devices;
+        unseen = hotplug_devices;
         seen = NULL;
+
         /* Apparently there are several different ways that ALSA lists
            actual hardware. It could be prefixed with "hw:" or "default:"
            or "sysdefault:" and maybe others. Go through the list and see
@@ -924,7 +922,7 @@ ALSA_HotplugThread(void *arg)
 
         ALSA_snd_device_name_free_hint(hints);
 
-            devices = seen;   /* now we have a known-good list of attached devices. */
+        hotplug_devices = seen;   /* now we have a known-good list of attached devices. */
 
         /* report anything still in unseen as removed. */
         for (dev = unseen; dev; dev = next) {
@@ -935,58 +933,64 @@ ALSA_HotplugThread(void *arg)
             SDL_free(dev);
         }
     }
+}
 
-        /* On first run, tell ALSA_DetectDevices() that we have a complete device list so it can return. */
-        if (first_run_semaphore) {
-            SDL_SemPost(first_run_semaphore);
-            first_run_semaphore = NULL;  /* let other thread clean it up. */
-        }
+#if SDL_ALSA_HOTPLUG_THREAD
+static SDL_atomic_t ALSA_hotplug_shutdown;
+static SDL_Thread *ALSA_hotplug_thread;
 
+static int SDLCALL
+ALSA_HotplugThread(void *arg)
+{
+    SDL_SetThreadPriority(SDL_THREAD_PRIORITY_LOW);
+
+    while (!SDL_AtomicGet(&ALSA_hotplug_shutdown)) {
         /* Block awhile before checking again, unless we're told to stop. */
-        ticks = SDL_GetTicks() + 5000;
+        const Uint32 ticks = SDL_GetTicks() + 5000;
         while (!SDL_AtomicGet(&ALSA_hotplug_shutdown) && !SDL_TICKS_PASSED(SDL_GetTicks(), ticks)) {
             SDL_Delay(100);
         }
-    }
 
-    /* Shutting down! Clean up any data we've gathered. */
-    for (dev = devices; dev; dev = next) {
-        /*printf("ALSA: at shutdown, removing %s device '%s'\n", dev->iscapture ? "capture" : "output", dev->name);*/
-        next = dev->next;
-        SDL_free(dev->name);
-        SDL_free(dev);
+        ALSA_HotplugIteration();  /* run the check. */
     }
 
     return 0;
 }
+#endif
 
 static void
 ALSA_DetectDevices(void)
 {
-    /* Start the device detection thread here, wait for an initial iteration to complete. */
-    SDL_sem *semaphore = SDL_CreateSemaphore(0);
-    if (!semaphore) {
-        return;  /* oh well. */
-    }
+    ALSA_HotplugIteration();  /* run once now before a thread continues to check. */
 
+#if SDL_ALSA_HOTPLUG_THREAD
     SDL_AtomicSet(&ALSA_hotplug_shutdown, 0);
-
-    ALSA_hotplug_thread = SDL_CreateThread(ALSA_HotplugThread, "SDLHotplugALSA", semaphore);
-    if (ALSA_hotplug_thread) {
-        SDL_SemWait(semaphore);  /* wait for the first iteration to finish. */
-    }
-
-    SDL_DestroySemaphore(semaphore);
+    ALSA_hotplug_thread = SDL_CreateThread(ALSA_HotplugThread, "SDLHotplugALSA", NULL);
+    /* if the thread doesn't spin, oh well, you just don't get further hotplug events. */
+#endif
 }
 
 static void
 ALSA_Deinitialize(void)
 {
+    ALSA_Device *dev;
+    ALSA_Device *next;
+
+#if SDL_ALSA_HOTPLUG_THREAD
     if (ALSA_hotplug_thread != NULL) {
         SDL_AtomicSet(&ALSA_hotplug_shutdown, 1);
         SDL_WaitThread(ALSA_hotplug_thread, NULL);
         ALSA_hotplug_thread = NULL;
     }
+#endif
+
+    /* Shutting down! Clean up any data we've gathered. */
+    for (dev = hotplug_devices; dev; dev = next) {
+        /*printf("ALSA: at shutdown, removing %s device '%s'\n", dev->iscapture ? "capture" : "output", dev->name);*/
+        next = dev->next;
+        SDL_free(dev->name);
+        SDL_free(dev);
+    }
 
     UnloadALSALibrary();
 }