fftools/ffmpeg: use a mutex for enc_stats_write()

It may be called concurrently from different threads to write into the
same file.

fftools/ffmpeg.h

@@ -480,6 +480,9 @@ typedef struct EncStats {
     int              nb_components;
 
     AVIOContext        *io;
+
+    pthread_mutex_t     lock;
+    int                 lock_initialized;
 } EncStats;
 
 extern const char *const forced_keyframes_const_names[];

fftools/ffmpeg_enc.c

@@ -491,6 +491,8 @@ void enc_stats_write(OutputStream *ost, EncStats *es,
         ptsi = fd->dec.pts;
     }
 
+    pthread_mutex_lock(&es->lock);
+
     for (size_t i = 0; i < es->nb_components; i++) {
         const EncStatsComponent *c = &es->components[i];
 
@@ -538,6 +540,8 @@ void enc_stats_write(OutputStream *ost, EncStats *es,
     }
     avio_w8(io, '\n');
     avio_flush(io);
+
+    pthread_mutex_unlock(&es->lock);
 }
 
 static inline double psnr(double d)

fftools/ffmpeg_mux.c

@@ -778,6 +778,10 @@ static void enc_stats_uninit(EncStats *es)
     for (int i = 0; i < es->nb_components; i++)
         av_freep(&es->components[i].str);
     av_freep(&es->components);
+
+    if (es->lock_initialized)
+        pthread_mutex_destroy(&es->lock);
+    es->lock_initialized = 0;
 }
 
 static void ost_free(OutputStream **post)

fftools/ffmpeg_mux_init.c

@@ -402,6 +402,11 @@ fail:
             return ret;
     }
 
+    ret = pthread_mutex_init(&es->lock, NULL);
+    if (ret)
+        return AVERROR(ret);
+    es->lock_initialized = 1;
+
     ret = enc_stats_get_file(&es->io, path);
     if (ret < 0)
         return ret;