fftools/ffmpeg: eliminate the main_return_code global

Properly pass muxing return codes through the call stack instead.
Slightly changes behavior in case of errors:
* the output IO stream is closed even if writing the trailer returns an
  error, which should be more correct
* all files get properly closed with -xerror, even if one of them fails

fftools/ffmpeg.c

@@ -318,7 +318,6 @@ static volatile int received_sigterm = 0;
 static volatile int received_nb_signals = 0;
 static atomic_int transcode_init_done = ATOMIC_VAR_INIT(0);
 static volatile int ffmpeg_exited = 0;
-int main_return_code = 0;
 static int64_t copy_ts_first_pts = AV_NOPTS_VALUE;
 
 static void
@@ -2951,9 +2950,8 @@ static int transcode(void)
 
     /* write the trailer if needed */
     for (i = 0; i < nb_output_files; i++) {
-        ret = of_write_trailer(output_files[i]);
+        int err = of_write_trailer(output_files[i]);
-        if (ret < 0 && exit_on_error)
+        ret = err_merge(ret, err);
-            exit_program(1);
     }
 
     /* dump report by using the first video and audio streams */
@@ -2975,7 +2973,7 @@ static int transcode(void)
         exit_program(1);
     }
 
-    return 0;
+    return ret;
 }
 
 static BenchmarkTimeStamps get_benchmark_time_stamps(void)
@@ -3061,9 +3059,8 @@ int main(int argc, char **argv)
     }
 
     current_time = ti = get_benchmark_time_stamps();
-    if (transcode() < 0)
+    ret = transcode();
-        exit_program(1);
+    if (ret >= 0 && do_benchmark) {
-    if (do_benchmark) {
         int64_t utime, stime, rtime;
         current_time = get_benchmark_time_stamps();
         utime = current_time.user_usec - ti.user_usec;
@@ -3078,6 +3075,7 @@ int main(int argc, char **argv)
     if ((decode_error_stat[0] + decode_error_stat[1]) * max_error_rate < decode_error_stat[1])
         exit_program(69);
 
-    exit_program(received_nb_signals ? 255 : main_return_code);
+    ret = received_nb_signals ? 255 : ret;
-    return main_return_code;
+    exit_program(ret);
+    return ret;
 }

fftools/ffmpeg.h

@@ -756,7 +756,6 @@ extern const OptionDef options[];
 extern HWDevice *filter_hw_device;
 
 extern unsigned nb_output_dumped;
-extern int main_return_code;
 
 extern int ignore_unknown_streams;
 extern int copy_unknown_streams;
@@ -887,6 +886,17 @@ void close_output_stream(OutputStream *ost);
 int trigger_fix_sub_duration_heartbeat(OutputStream *ost, const AVPacket *pkt);
 void update_benchmark(const char *fmt, ...);
 
+/**
+ * Merge two return codes - return one of the error codes if at least one of
+ * them was negative, 0 otherwise.
+ * Currently just picks the first one, eventually we might want to do something
+ * more sophisticated, like sorting them by priority.
+ */
+static inline int err_merge(int err0, int err1)
+{
+    return (err0 < 0) ? err0 : FFMIN(err1, 0);
+}
+
 #define SPECIFIER_OPT_FMT_str  "%s"
 #define SPECIFIER_OPT_FMT_i    "%i"
 #define SPECIFIER_OPT_FMT_i64  "%"PRId64

fftools/ffmpeg_mux.c

@@ -602,7 +602,7 @@ int of_write_trailer(OutputFile *of)
 {
     Muxer *mux = mux_from_of(of);
     AVFormatContext *fc = mux->fc;
-    int ret;
+    int ret, mux_result = 0;
 
     if (!mux->tq) {
         av_log(mux, AV_LOG_ERROR,
@@ -611,14 +611,12 @@ int of_write_trailer(OutputFile *of)
         return AVERROR(EINVAL);
     }
 
-    ret = thread_stop(mux);
+    mux_result = thread_stop(mux);
-    if (ret < 0)
-        main_return_code = ret;
 
     ret = av_write_trailer(fc);
     if (ret < 0) {
         av_log(mux, AV_LOG_ERROR, "Error writing trailer: %s\n", av_err2str(ret));
-        return ret;
+        mux_result = err_merge(mux_result, ret);
     }
 
     mux->last_filesize = filesize(fc->pb);
@@ -627,11 +625,11 @@ int of_write_trailer(OutputFile *of)
         ret = avio_closep(&fc->pb);
         if (ret < 0) {
             av_log(mux, AV_LOG_ERROR, "Error closing file: %s\n", av_err2str(ret));
-            return ret;
+            mux_result = err_merge(mux_result, ret);
         }
     }
 
-    return 0;
+    return mux_result;
 }
 
 static void ost_free(OutputStream **post)