avformat/mpjpeg: utilize MIME boundary value to detect start of new frame

This code is disabled by default so not to regress endpoints sending invalid MIME, but can be enabled via AVOption 'strict_mime_boundary'
Signed-off-by: Alex Agranovsky <alex@sighthound.com>

doc/demuxers.texi

@@ -450,6 +450,21 @@ to 1 (-1 means automatic setting, 1 means enabled, 0 means
 disabled). Default value is -1.
 @end table
 
+@section mpjpeg
+
+MJPEG encapsulated in multi-part MIME demuxer.
+
+This demuxer allows reading of MJPEG, where each frame is represented as a part of
+multipart/x-mixed-replace stream.
+@table @option
+
+@item strict_mime_boundary
+Default implementation applies a relaxed standard to multi-part MIME boundary detection,
+to prevent regression with numerous existing endpoints not generating a proper MIME
+MJPEG stream. Turning this option on by setting it to 1 will result in a stricter check
+of the boundary value.
+@end table
+
 @section rawvideo
 
 Raw video demuxer.

libavformat/mpjpegdec.c

@@ -20,6 +20,7 @@
  */
 
 #include "libavutil/avstring.h"
+#include "libavutil/opt.h"
 
 #include "avformat.h"
 #include "internal.h"
@@ -28,9 +29,11 @@
 
 
 typedef struct MPJPEGDemuxContext {
+    const AVClass *class;
     char       *boundary;
     char       *searchstr;
     int         searchstr_len;
+    int         strict_mime_boundary;
 } MPJPEGDemuxContext;
 
 
@@ -242,6 +245,41 @@ static int parse_multipart_header(AVIOContext *pb,
 }
 
 
+static char* mpjpeg_get_boundary(AVIOContext* pb)
+{
+    uint8_t *mime_type = NULL;
+    uint8_t *start;
+    uint8_t *end;
+    uint8_t *res = NULL;
+    int     len;
+
+    /* get MIME type, and skip to the first parameter */
+    av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type);
+    start = mime_type;
+    while (start != NULL && *start != '\0') {
+        start = strchr(start, ';');
+        if (start)
+            start = start+1;
+
+        while (av_isspace(*start))
+            start++;
+
+        if (!av_stristart(start, "boundary=", &start)) {
+            end = strchr(start, ';');
+            if (end)
+                len = end - start - 1;
+            else
+                len = strlen(start);
+            res = av_strndup(start, len);
+            break;
+        }
+    }
+
+    av_freep(&mime_type);
+    return res;
+}
+
+
 static int mpjpeg_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
     int size;
@@ -249,8 +287,17 @@ static int mpjpeg_read_packet(AVFormatContext *s, AVPacket *pkt)
 
     MPJPEGDemuxContext *mpjpeg = s->priv_data;
     if (mpjpeg->boundary == NULL) {
-        mpjpeg->boundary = av_strdup("--");
-        mpjpeg->searchstr = av_strdup("\r\n--");
+        uint8_t* boundary = NULL;
+        if (mpjpeg->strict_mime_boundary) {
+            boundary = mpjpeg_get_boundary(s->pb);
+        }
+        if (boundary != NULL) {
+            mpjpeg->boundary = boundary;
+            mpjpeg->searchstr = av_asprintf( "\r\n%s\r\n", boundary );
+        } else {
+            mpjpeg->boundary = av_strdup("--");
+            mpjpeg->searchstr = av_strdup("\r\n--");
+        }
         if (!mpjpeg->boundary || !mpjpeg->searchstr) {
             av_freep(&mpjpeg->boundary);
             av_freep(&mpjpeg->searchstr);
@@ -309,6 +356,22 @@ static int mpjpeg_read_packet(AVFormatContext *s, AVPacket *pkt)
     return ret;
 }
 
+#define OFFSET(x) offsetof(MPJPEGDemuxContext, x)
+
+#define DEC AV_OPT_FLAG_DECODING_PARAM
+const AVOption mpjpeg_options[] = {
+    { "strict_mime_boundary",  "require MIME boundaries match", OFFSET(strict_mime_boundary), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, DEC },
+    { NULL }
+};
+
+
+static const AVClass mpjpeg_demuxer_class = {
+    .class_name     = "MPJPEG demuxer",
+    .item_name      = av_default_item_name,
+    .option         = mpjpeg_options,
+    .version        = LIBAVUTIL_VERSION_INT,
+};
+
 AVInputFormat ff_mpjpeg_demuxer = {
     .name              = "mpjpeg",
     .long_name         = NULL_IF_CONFIG_SMALL("MIME multipart JPEG"),
@@ -318,5 +381,8 @@ AVInputFormat ff_mpjpeg_demuxer = {
     .read_probe        = mpjpeg_read_probe,
     .read_header       = mpjpeg_read_header,
     .read_packet       = mpjpeg_read_packet,
-    .read_close        = mpjpeg_read_close
+    .read_close        = mpjpeg_read_close,
+    .priv_class        = &mpjpeg_demuxer_class
 };
+
+