common/av_log: use bstr for log callback

- don't use 4KiB stack array
- keep one bstr allocation for lifetime of the log
- avoid dummy strlen

common/av_log.c

@@ -28,6 +28,7 @@
 #include "common/global.h"
 #include "common/msg.h"
 #include "config.h"
+#include "misc/bstr.h"
 #include "osdep/threads.h"
 
 #include <libavutil/avutil.h>
@@ -55,6 +56,7 @@ static mp_static_mutex log_lock = MP_STATIC_MUTEX_INITIALIZER;
 static struct mpv_global *log_mpv_instance;
 static struct mp_log *log_root, *log_decaudio, *log_decvideo, *log_demuxer;
 static bool log_print_prefix = true;
+static bstr log_buffer;
 
 static int av_log_level_to_mp_level(int av_level)
 {
@@ -125,19 +127,20 @@ static void mp_msg_av_log_callback(void *ptr, int level, const char *fmt,
     struct mp_log *log = get_av_log(ptr);
 
     if (mp_msg_test(log, mp_level)) {
-        char buffer[4096] = "";
-        int pos = 0;
+        log_buffer.len = 0;
+        bstr_xappend_vasprintf(log_root, &log_buffer, fmt, vl);
+        if (!log_buffer.len)
+            goto done;
         const char *prefix = avc ? avc->item_name(ptr) : NULL;
-        if (log_print_prefix && prefix)
-            pos = snprintf(buffer, sizeof(buffer), "%s: ", prefix);
-        log_print_prefix = fmt[strlen(fmt) - 1] == '\n';
-
-        pos = MPMIN(MPMAX(pos, 0), sizeof(buffer));
-        vsnprintf(buffer + pos, sizeof(buffer) - pos, fmt, vl);
-
-        mp_msg(log, mp_level, "%s", buffer);
+        if (log_print_prefix && prefix) {
+            mp_msg(log, mp_level, "%s: %.*s", prefix, BSTR_P(log_buffer));
+        } else {
+            mp_msg(log, mp_level, "%.*s", BSTR_P(log_buffer));
+        }
+        log_print_prefix = log_buffer.start[log_buffer.len - 1] == '\n';
     }
 
+done:
     mp_mutex_unlock(&log_lock);
 }
 
@@ -150,6 +153,7 @@ void init_libav(struct mpv_global *global)
         log_decaudio = mp_log_new(log_root, log_root, "audio");
         log_decvideo = mp_log_new(log_root, log_root, "video");
         log_demuxer = mp_log_new(log_root, log_root, "demuxer");
+        log_buffer = (bstr){0};
         av_log_set_callback(mp_msg_av_log_callback);
     }
     mp_mutex_unlock(&log_lock);