build: allow compilation without any atomics

Not all compilers on all platforms have atomics available (even if they
could, technically speaking).

We don't use atomics that much, only the following things rely on it:
1. the audio pull code, and all audio outputs using it
2. updating global msg levels
3. reading log messages through the client API

Just disable 1. and 3. if atomics are not available. For 2., using fake-
atomics isn't too bad; at worst, message levels won't properly update
under certain situations (but most likely, it will work just fine).

This means if atomics are not available, the client API function
mpv_request_log_messages() will do nothing.

CC: @mpv-player/stable

common/msg.c

@@ -469,6 +469,10 @@ struct mp_log_buffer *mp_msg_log_buffer_new(struct mpv_global *global,
 {
     struct mp_log_root *root = global->log->root;
 
+#if !HAVE_ATOMICS
+    return NULL;
+#endif
+
     pthread_mutex_lock(&mp_msg_lock);
 
     struct mp_log_buffer *buffer = talloc_ptrtype(NULL, buffer);

compat/atomics.h

@@ -22,6 +22,8 @@
 #include <inttypes.h>
 #include "config.h"
 
+#define HAVE_ATOMICS 1
+
 #if HAVE_STDATOMIC
 #include <stdatomic.h>
 #else
@@ -58,8 +60,17 @@ typedef struct { volatile unsigned long long v; } atomic_ullong;
     __sync_fetch_and_add(&(a)->v, b)
 
 #else
-# error "this should have been a configuration error, report a bug please"
-#endif
+
+// This is extremely wrong. The build system actually disables code that has
+// a serious dependency on working atomics, so this is barely ok.
+#define atomic_load(p) ((p)->v)
+#define atomic_store(p, val) ((p)->v = (val))
+#define atomic_fetch_add(a, b) (((a)->v += (b)) - (b))
+
+#undef HAVE_ATOMICS
+#define HAVE_ATOMICS 0
+
+#endif /* no atomics */
 
 #endif /* else HAVE_STDATOMIC */
 

wscript

@@ -138,12 +138,10 @@ main_dependencies = [
                     'test = __sync_add_and_fetch(&test, 1)'),
         'deps_neg': [ 'stdatomic', 'atomic-builtins' ],
     }, {
-        'name': 'thread-synchronization-builtins',
+        'name': 'atomics',
         'desc': 'compiler support for usable thread synchronization built-ins',
         'func': check_true,
         'deps_any': ['stdatomic', 'atomic-builtins', 'sync-builtins'],
-        'req': True,
-        'fmsg': 'your compiler must support either __atomic or __sync built-ins',
     }, {
         'name': 'librt',
         'desc': 'linking with -lrt',
@@ -419,11 +417,13 @@ audio_output_features = [
     {
         'name': '--sdl2',
         'desc': 'SDL2',
+        'deps': ['atomics'],
         'func': check_pkg_config('sdl2'),
         'default': 'disable'
     }, {
         'name': '--sdl1',
         'desc': 'SDL (1.x)',
+        'deps': ['atomics'],
         'deps_neg': [ 'sdl2' ],
         'func': check_pkg_config('sdl'),
         'default': 'disable'
@@ -478,12 +478,13 @@ audio_output_features = [
     }, {
         'name': '--portaudio',
         'desc': 'PortAudio audio output',
-        'deps': [ 'pthreads' ],
+        'deps': [ 'atomics' ],
         'func': check_pkg_config('portaudio-2.0', '>= 19'),
         'default': 'disable',
     }, {
         'name': '--jack',
         'desc': 'JACK audio output',
+        'deps': ['atomics'],
         'func': check_pkg_config('jack'),
     }, {
         'name': '--openal',
@@ -497,6 +498,7 @@ audio_output_features = [
     }, {
         'name': '--coreaudio',
         'desc': 'CoreAudio audio output',
+        'deps': ['atomics'],
         'func': check_cc(
             fragment=load_fragment('coreaudio.c'),
             framework_name=['CoreFoundation', 'CoreAudio', 'AudioUnit', 'AudioToolbox'])
@@ -507,6 +509,7 @@ audio_output_features = [
     }, {
         'name': '--wasapi',
         'desc': 'WASAPI audio output',
+        'deps': ['atomics'],
         'func': check_cc(fragment=load_fragment('wasapi.c'), lib='ole32'),
     }
 ]