[FFmpeg-devel] [PATCH] avformat/matroskaenc: Regression fix for invalid MKV headers

[Soft Works]

==> Revised patch in plain text (the attachment from my previous reply wasn't recognized by patchwork)

The following three commits created a regression by writing initially
invalid mkv headers:

650e17d88b63b5aca6e0a43483e89e64b0f7d2dd avformat/matroskaenc: write a
CRC32 element on Tags
3bcadf822711720ff0f8d14db71ae47cdf97e652 avformat/matroskaenc: write a
CRC32 element on Info
ee888cfbe777cd2916a3548c750e433ab8f8e6a5 avformat/matroskaenc: postpone
writing the Tracks master

Symptoms:

- You can no longer playback a file that is still processed by ffmpeg,
e.g. VLC fails playback
- You can no longer stream a file to a client while if is still being
processed
- Various diagnosing tools show header errors or incomplete headers
(e.g. ffprobe, mediainfo, mkvalidator)

Note: The symptoms do not apply to completed files or ffmpeg runs that
were interrupted with 'q'

Cause:

The mentioned commits made changes in a way that some header elements
are only partially written in
mkv_write_header, leaving the header in an invalid state. Only in
mkv_write_trailer, these elements
are finished correctly, but that does only occur at the end of the
process.

Regression:

Before these commits were applied, mkv headers have always been valid,
even before completion of ffmpeg.
This has worked reliably over many versions of ffmpeg, to it was an
obvious regression.

Bugtracker:

This issue has been recorded as #5977 which is resolved by this patch

Patch:

The patch adds a new function 'end_ebml_master_crc32_preliminary' that
preliminarily finishes the ebl
element without destroying the buffer. The buffer can be used to update
the ebml element later during
mkv_write_trailer. But most important: mkv_write_header finishes with a
valid mkv header again.
---
 libavformat/avio.h        | 12 ++++++++++++
 libavformat/aviobuf.c     | 17 +++++++++++++++++
 libavformat/matroskaenc.c | 28 +++++++++++++++++++++++++---
 3 files changed, 54 insertions(+), 3 deletions(-)

diff --git a/libavformat/avio.h b/libavformat/avio.h
index b1ce1d1..f2b9a6f 100644
--- a/libavformat/avio.h
+++ b/libavformat/avio.h
@@ -704,6 +704,18 @@ int avio_closep(AVIOContext **s);
 int avio_open_dyn_buf(AVIOContext **s);
 
 /**
+ * Return the written size and a pointer to the buffer. 
+ * The AVIOContext stream is left intact.
+ * The buffer must NOT be freed. 
+ * No padding is added to the buffer.
+ *
+ * @param s IO context
+ * @param pbuffer pointer to a byte buffer
+ * @return the length of the byte buffer
+ */
+int avio_get_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
+
+/**
  * Return the written size and a pointer to the buffer. The buffer
  * must be freed with av_free().
  * Padding of AV_INPUT_BUFFER_PADDING_SIZE is added to the buffer.
diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index 134d627..bf7e5f8 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -1277,6 +1277,23 @@ int ffio_open_dyn_packet_buf(AVIOContext **s, int max_packet_size)
     return url_open_dyn_buf_internal(s, max_packet_size);
 }
 
+int avio_get_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
+{
+    DynBuffer *d;
+
+    if (!s) {
+        *pbuffer = NULL;
+        return 0;
+    }
+
+    avio_flush(s);
+
+    d = s->opaque;
+    *pbuffer = d->buffer;
+
+    return d->size;
+}
+
 int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
 {
     DynBuffer *d;
diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 827d755..27d83a6 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -367,6 +367,28 @@ static void end_ebml_master_crc32(AVIOContext *pb, AVIOContext **dyn_cp, Matrosk
     *dyn_cp = NULL;
 }
 
+/**
+* Complete ebml master whithout destroying the buffer, allowing for later updates
+*/
+static void end_ebml_master_crc32_preliminary(AVIOContext *pb, AVIOContext **dyn_cp, MatroskaMuxContext *mkv,
+    ebml_master master)
+{
+    uint8_t *buf, crc[4];
+    int size, skip = 0;
+
+    if (pb->seekable) {
+
+        size = avio_get_dyn_buf(*dyn_cp, &buf);
+        if (mkv->write_crc && mkv->mode != MODE_WEBM) {
+            skip = 6; /* Skip reserved 6-byte long void element from the dynamic buffer. */
+            AV_WL32(crc, av_crc(av_crc_get_table(AV_CRC_32_IEEE_LE), UINT32_MAX, buf + skip, size - skip) ^ UINT32_MAX);
+            put_ebml_binary(pb, EBML_ID_CRC32, crc, sizeof(crc));
+        }
+        avio_write(pb, buf + skip, size - skip);
+        end_ebml_master(pb, master);
+    }
+}
+
 static void put_xiph_size(AVIOContext *pb, int size)
 {
     ffio_fill(pb, 255, size / 255);
@@ -1309,7 +1331,7 @@ static int mkv_write_tracks(AVFormatContext *s)
     }
 
     if (pb->seekable && !mkv->is_live)
-        put_ebml_void(pb, avio_tell(mkv->tracks_bc));
+        end_ebml_master_crc32_preliminary(pb, &mkv->tracks_bc, mkv, mkv->tracks_master);
     else
         end_ebml_master_crc32(pb, &mkv->tracks_bc, mkv, mkv->tracks_master);
 
@@ -1554,7 +1576,7 @@ static int mkv_write_tags(AVFormatContext *s)
 
     if (mkv->tags.pos) {
         if (s->pb->seekable && !mkv->is_live)
-            put_ebml_void(s->pb, avio_tell(mkv->tags_bc));
+            end_ebml_master_crc32_preliminary(s->pb, &mkv->tags_bc, mkv, mkv->tags);
         else
             end_ebml_master_crc32(s->pb, &mkv->tags_bc, mkv, mkv->tags);
     }
@@ -1811,7 +1833,7 @@ static int mkv_write_header(AVFormatContext *s)
         }
     }
     if (s->pb->seekable && !mkv->is_live)
-        put_ebml_void(s->pb, avio_tell(pb));
+        end_ebml_master_crc32_preliminary(s->pb, &mkv->info_bc, mkv, mkv->info);
     else
         end_ebml_master_crc32(s->pb, &mkv->info_bc, mkv, mkv->info);
     pb = s->pb;
-- 
2.10.2.windows.1

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