ffmpeg: block output == input for files

Fixes #4655

fftools/ffmpeg_opt.c

@@ -900,13 +900,14 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
 
 static void assert_file_overwrite(const char *filename)
 {
+    const char *proto_name = avio_find_protocol_name(filename);
+
     if (file_overwrite && no_file_overwrite) {
         fprintf(stderr, "Error, both -y and -n supplied. Exiting.\n");
         exit_program(1);
     }
 
     if (!file_overwrite) {
-        const char *proto_name = avio_find_protocol_name(filename);
         if (proto_name && !strcmp(proto_name, "file") && avio_check(filename, 0) == 0) {
             if (stdin_interaction && !no_file_overwrite) {
                 fprintf(stderr,"File '%s' already exists. Overwrite ? [y/N] ", filename);
@@ -925,6 +926,19 @@ static void assert_file_overwrite(const char *filename)
             }
         }
     }
+
+    if (proto_name && !strcmp(proto_name, "file")) {
+        for (int i = 0; i < nb_input_files; i++) {
+             InputFile *file = input_files[i];
+             if (file->ctx->iformat->flags & AVFMT_NOFILE)
+                 continue;
+             if (!strcmp(filename, file->ctx->url)) {
+                 av_log(NULL, AV_LOG_FATAL, "Output %s same as Input #%d - exiting\n", filename, i);
+                 av_log(NULL, AV_LOG_WARNING, "FFmpeg cannot edit existing files in-place.\n");
+                 exit_program(1);
+             }
+        }
+    }
 }
 
 static void dump_attachment(AVStream *st, const char *filename)