diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 13f6524d85..fe3757a309 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -971,6 +971,8 @@ enum AVDurationEstimationMethod {
     AVFMT_DURATION_FROM_BITRATE ///< Duration estimated from bitrate (less accurate)
 };
 
+typedef struct AVFormatInternal AVFormatInternal;
+
 /**
  * Format I/O context.
  * New fields can be added to the end with minor version bumps.
@@ -1331,6 +1333,12 @@ typedef struct AVFormatContext {
      */
     AVRational offset_timebase;
 
+    /**
+     * An opaque field for libavformat internal usage.
+     * Must not be accessed in any way by callers.
+     */
+    AVFormatInternal *internal;
+
     /**
      * IO repositioned flag.
      * This is set by avformat when the underlaying IO context read pointer
diff --git a/libavformat/internal.h b/libavformat/internal.h
index 59370565ca..a840f8af05 100644
--- a/libavformat/internal.h
+++ b/libavformat/internal.h
@@ -46,6 +46,14 @@ typedef struct CodecMime{
     enum AVCodecID id;
 } CodecMime;
 
+struct AVFormatInternal {
+    /**
+     * Number of streams relevant for interleaving.
+     * Muxing only.
+     */
+    int nb_interleaved_streams;
+};
+
 #ifdef __GNUC__
 #define dynarray_add(tab, nb_ptr, elem)\
 do {\
diff --git a/libavformat/mux.c b/libavformat/mux.c
index aebfe2c8f7..71a099f643 100644
--- a/libavformat/mux.c
+++ b/libavformat/mux.c
@@ -320,6 +320,9 @@ static int init_muxer(AVFormatContext *s, AVDictionary **options)
             av_log(s, AV_LOG_WARNING,
                    "Codec for stream %d does not use global headers "
                    "but container format requires global headers\n", i);
+
+        if (codec->codec_type != AVMEDIA_TYPE_ATTACHMENT)
+            s->internal->nb_interleaved_streams++;
     }
 
     if (!s->priv_data && of->priv_data_size > 0) {
@@ -727,7 +730,7 @@ int ff_interleave_packet_per_dts(AVFormatContext *s, AVPacket *out,
         }
     }
 
-    if (s->nb_streams == stream_count) {
+    if (s->internal->nb_interleaved_streams == stream_count) {
         flush = 1;
     } else if (!flush) {
         for (i=0; i < s->nb_streams; i++) {
@@ -742,7 +745,7 @@ int ff_interleave_packet_per_dts(AVFormatContext *s, AVPacket *out,
                 delta_dts_max= FFMAX(delta_dts_max, delta_dts);
             }
         }
-        if (s->nb_streams == stream_count+noninterleaved_count &&
+        if (s->internal->nb_interleaved_streams == stream_count+noninterleaved_count &&
            delta_dts_max > 20*AV_TIME_BASE) {
             av_log(s, AV_LOG_DEBUG, "flushing with %d noninterleaved\n", noninterleaved_count);
             flush = 1;
diff --git a/libavformat/options.c b/libavformat/options.c
index 5218e5b92a..e0d6df6fbd 100644
--- a/libavformat/options.c
+++ b/libavformat/options.c
@@ -19,6 +19,7 @@
  */
 #include "avformat.h"
 #include "avio_internal.h"
+#include "internal.h"
 #include "libavutil/opt.h"
 
 /**
@@ -109,6 +110,13 @@ AVFormatContext *avformat_alloc_context(void)
     ic = av_malloc(sizeof(AVFormatContext));
     if (!ic) return ic;
     avformat_get_context_defaults(ic);
+
+    ic->internal = av_mallocz(sizeof(*ic->internal));
+    if (!ic->internal) {
+        avformat_free_context(ic);
+        return NULL;
+    }
+
     return ic;
 }
 
diff --git a/libavformat/utils.c b/libavformat/utils.c
index 56ce38bbcf..2be4279b45 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -3480,6 +3480,7 @@ void avformat_free_context(AVFormatContext *s)
     av_freep(&s->chapters);
     av_dict_free(&s->metadata);
     av_freep(&s->streams);
+    av_freep(&s->internal);
     av_free(s);
 }