workaround for incorrect demuxer detection in libavformat

Makefile

@@ -5,7 +5,7 @@ LDFLAGS :=
 INSTALL ?= install
 PREFIX  ?= /usr/local
 
-COMMON_OBJECTS     := build/validation.o build/png.o
+COMMON_OBJECTS     := build/validation.o build/png.o build/util.o
 MEDIASTAT_OBJECTS  := build/stat.o
 MEDIATHUMB_OBJECTS := build/thumb.o
 

src/stat.c

@@ -6,6 +6,7 @@
 #include <unistd.h>
 
 #include "validation.h"
+#include "util.h"
 
 static int64_t start_time(AVStream *stream)
 {
@@ -34,7 +35,7 @@ int main(int argc, char *argv[])
         return -1;
     }
 
-    if (avformat_open_input(&format, argv[1], NULL, NULL) != 0) {
+    if (open_input_correct_demuxer(&format, argv[1]) != 0) {
         printf("Couldn't read file\n");
         return -1;
     }

src/thumb.c

@@ -1,6 +1,7 @@
 #include <libavformat/avformat.h>
 #include <stdio.h>
 #include "png.h"
+#include "util.h"
 
 int main(int argc, char *argv[])
 {
@@ -24,7 +25,7 @@ int main(int argc, char *argv[])
     const char *output = argv[3];
     AVRational time = av_d2q(atof(argv[2]), INT_MAX);
 
-    if (avformat_open_input(&format, input, NULL, NULL) != 0) {
+    if (open_input_correct_demuxer(&format, input) != 0) {
         printf("Couldn't read file\n");
         return -1;
     }

src/util.c

@@ -0,0 +1,40 @@
+#include "util.h"
+
+static int valid_demuxer(AVInputFormat *fmt)
+{
+    // apng:     animated PNG
+    // png_pipe: static PNG
+    // image2:   JPEG
+    // gif:      GIF
+    // svg_pipe: SVG (recommended not to use this currently)
+    // matroska: MKV/WebM
+
+    return
+      fmt == av_find_input_format("apng")     ||
+      fmt == av_find_input_format("png_pipe") ||
+      fmt == av_find_input_format("image2")   ||
+      fmt == av_find_input_format("gif")      ||
+      fmt == av_find_input_format("svg_pipe") ||
+      fmt == av_find_input_format("matroska");
+}
+
+static AVInputFormat *image2_demuxer()
+{
+    return av_find_input_format("image2");
+}
+
+int open_input_correct_demuxer(AVFormatContext **ctx, const char *filename)
+{
+    if (avformat_open_input(ctx, filename, NULL, NULL) < 0) {
+        return -1;
+    }
+
+    // Should usually happen
+    if (valid_demuxer((*ctx)->iformat)) {
+        return 0;
+    }
+
+    // Wrong demuxer, force to image2 so we don't error out here
+    avformat_close_input(ctx);
+    return avformat_open_input(ctx, filename, image2_demuxer(), NULL);
+}

src/util.h

@@ -0,0 +1,7 @@
+#ifndef _UTIL_H_DEFINED
+#define _UTIL_H_DEFINED
+
+#include <libavformat/avformat.h>
+int open_input_correct_demuxer(AVFormatContext **ctx, const char *filename);
+
+#endif