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From: Derek Buitenhuis <derek.buitenhuis@gmail.com>
To: ffmpeg-devel@ffmpeg.org
Date: Mon, 24 Mar 2025 15:57:37 +0000
Message-ID: <20250324155737.83118-1-derek.buitenhuis@gmail.com>
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Subject: [FFmpeg-devel] [PATCH] avformat/dhav: fix backward scanning for
get_duration and optimize seeking
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From: Justin Ruggles <justinr@vimeo.com>
The backwards scanning done for incomplete final packets should not
assume a specific alignment at the end of the file. Truncated files
result in hundreds of thousands of seeks if the final packet does not
fall on a specific byte boundary, which can be extremely slow.
For example, with HTTP, each backwards seek results in a separate
HTTP request.
This changes the scanning to check for the end tag 1 byte at a time
and buffers the last 1 MiB using ffio_ensure_seekback to avoid additional
seek operations.
Signed-off-by: Justin Ruggles <justinr@vimeo.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
---
libavformat/dhav.c | 38 +++++++++++++++++++++++++++++---------
1 file changed, 29 insertions(+), 9 deletions(-)
diff --git a/libavformat/dhav.c b/libavformat/dhav.c
index b2ead99609..15a75b5033 100644
--- a/libavformat/dhav.c
+++ b/libavformat/dhav.c
@@ -232,34 +232,54 @@ static void get_timeinfo(unsigned date, struct tm *timeinfo)
timeinfo->tm_sec = sec;
}
+#define MAX_DURATION_BUFFER_SIZE (1024*1024)
+
static int64_t get_duration(AVFormatContext *s)
{
DHAVContext *dhav = s->priv_data;
int64_t start_pos = avio_tell(s->pb);
+ int64_t end_pos = -1;
int64_t start = 0, end = 0;
struct tm timeinfo;
- int max_interations = 100000;
+ uint8_t *end_buffer;
+ int64_t end_buffer_size;
+ int64_t end_buffer_pos;
if (!s->pb->seekable)
return 0;
+ end_buffer_size = FFMIN(MAX_DURATION_BUFFER_SIZE, avio_size(s->pb));
+ end_buffer = av_malloc(end_buffer_size);
+ if (!end_buffer)
+ return 0;
+ end_buffer_pos = avio_size(s->pb) - end_buffer_size;
+ avio_seek(s->pb, end_buffer_pos, SEEK_SET);
+ ffio_ensure_seekback(s->pb, end_buffer_size);
+ avio_read(s->pb, end_buffer, end_buffer_size);
+ av_freep(&end_buffer);
+
avio_seek(s->pb, avio_size(s->pb) - 8, SEEK_SET);
- while (avio_tell(s->pb) > 12 && max_interations--) {
+ while (avio_tell(s->pb) > end_buffer_pos) {
if (avio_rl32(s->pb) == MKTAG('d','h','a','v')) {
int64_t seek_back = avio_rl32(s->pb);
-
- avio_seek(s->pb, -seek_back, SEEK_CUR);
- read_chunk(s);
- get_timeinfo(dhav->date, &timeinfo);
- end = av_timegm(&timeinfo) * 1000LL;
+ end_pos = avio_tell(s->pb) - seek_back;
break;
} else {
- avio_seek(s->pb, -12, SEEK_CUR);
+ avio_seek(s->pb, -5, SEEK_CUR);
}
}
- avio_seek(s->pb, start_pos, SEEK_SET);
+ if (end_pos < 0) {
+ avio_seek(s->pb, start_pos, SEEK_SET);
+ return 0;
+ }
+
+ avio_seek(s->pb, end_pos, SEEK_SET);
+ read_chunk(s);
+ get_timeinfo(dhav->date, &timeinfo);
+ end = av_timegm(&timeinfo) * 1000LL;
+ avio_seek(s->pb, start_pos, SEEK_SET);
read_chunk(s);
get_timeinfo(dhav->date, &timeinfo);
start = av_timegm(&timeinfo) * 1000LL;
--
2.47.1
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