[FFmpeg-devel] [PATCH] libavformat/spdifdec: Fix SPDIF detection score for EAC3 (PR #21019)

[Shulyaka via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21019 opened by Shulyaka
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21019
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21019.patch

When EAC3 spdif decoding support was added in d3538dd293125e0a8d135ffe229c8b441345d833, the `SPDIF_MAX_OFFSET` was not increased.
Also, the spdif is based on blocks with fixed length, each block starts with a sync word. It matters how many sync words we find during probe and whether we find them in expected place. The `SPDIF_MAX_OFFSET` constant is the maximum supported block size. When we find a sync word, we shift the upper boundary of the search area (`probe_end`) by `SPDIF_MAX_OFFSET`, however __this boundary is exclusive (the condition in the loop is "strictly less")__, and when the actual block size matches the max supported block size, then the second sync word appears to be outside of the search area by 1 byte. As a result, we find only one sync word and return an incorrect score (`AVPROBE_SCORE_EXTENSION / 4` instead of `AVPROBE_SCORE_MAX`).
The fix increases the `SPDIF_MAX_OFFSET` and gives 1 more byte to the search area to allow finding the subsequent sync word.
The existing `fate-spdif-eac3-remux` test passes successfully.


>From 9f2e5c7cf8e33f42a33dbaa6943beab3d2514e46 Mon Sep 17 00:00:00 2001
From: Denis Shulyaka <denis.shulyaka@techno.holdings>
Date: Tue, 25 Nov 2025 22:32:24 +0300
Subject: [PATCH] avformat/spdifdec: Increase max probe offset

---
 libavformat/spdifdec.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/libavformat/spdifdec.c b/libavformat/spdifdec.c
index dcfe471f45..8287c19ad3 100644
--- a/libavformat/spdifdec.c
+++ b/libavformat/spdifdec.c
@@ -109,9 +109,8 @@ static int spdif_get_offset_and_codec(AVFormatContext *s,
     return 0;
 }
 
-/* Largest offset between bursts we currently handle, i.e. AAC with
-   samples = 4096 */
-#define SPDIF_MAX_OFFSET 16384
+/* Largest offset between bursts we currently handle, i.e. E-AC-3 */
+#define SPDIF_MAX_OFFSET 24576
 
 static int spdif_probe(const AVProbeData *p)
 {
@@ -122,7 +121,7 @@ static int spdif_probe(const AVProbeData *p)
 int ff_spdif_probe(const uint8_t *p_buf, int buf_size, enum AVCodecID *codec)
 {
     const uint8_t *buf = p_buf;
-    const uint8_t *probe_end = p_buf + FFMIN(2 * SPDIF_MAX_OFFSET, buf_size - 1);
+    const uint8_t *probe_end = p_buf + FFMIN(2 * SPDIF_MAX_OFFSET + 1, buf_size - 1);
     const uint8_t *expected_code = buf + 7;
     uint32_t state = 0;
     int sync_codes = 0;
@@ -146,7 +145,7 @@ int ff_spdif_probe(const uint8_t *p_buf, int buf_size, enum AVCodecID *codec)
                 break;
 
             /* continue probing to find more sync codes */
-            probe_end = FFMIN(buf + SPDIF_MAX_OFFSET, p_buf + buf_size - 1);
+            probe_end = FFMIN(buf + SPDIF_MAX_OFFSET + 1, p_buf + buf_size - 1);
 
             /* skip directly to the next sync code */
             if (!spdif_get_offset_and_codec(NULL, (buf[2] << 8) | buf[1],
@@ -234,10 +233,7 @@ int ff_spdif_read_packet(AVFormatContext *s, AVPacket *pkt)
         }
         st->codecpar->codec_type = AVMEDIA_TYPE_AUDIO;
         st->codecpar->codec_id = codec_id;
-        if (codec_id == AV_CODEC_ID_EAC3)
-            ffstream(st)->need_parsing = AVSTREAM_PARSE_FULL;
-        else
-            ffstream(st)->need_parsing = AVSTREAM_PARSE_HEADERS;
+        ffstream(st)->need_parsing = AVSTREAM_PARSE_HEADERS;
     } else if (codec_id != s->streams[0]->codecpar->codec_id) {
         avpriv_report_missing_feature(s, "Codec change in IEC 61937");
         return AVERROR_PATCHWELCOME;
-- 
2.49.1

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