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[FFmpeg-devel] libavformat/libgme: avoid infinite reads of looping files

Message ID 20240116181524.2682575-1-bk@ancilla.ca
State New
Headers show
Series [FFmpeg-devel] libavformat/libgme: avoid infinite reads of looping files | expand

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Commit Message

Bex Kelly Jan. 16, 2024, 6:15 p.m. UTC 
From: Rebecca Kelly <bk@ancilla.ca>

This fixes an issue where certain VGM/VGZ files (and possibly other formats
handled by libgme) will report a duration of "N/A" when ffprobed, and will
produce an infinite-length audio file when converted with ffmpeg. The behaviour
here follows the recommendations in the libgme documentation¹: use total
duration if available, otherwise use intro length + two loops, otherwise pretend
the file is 2.5 minutes long.

It uses gme_set_fade() to implement this, which will cause libgme to insert a
clean fade-out and then report end of file at the specified time.

Patch released under the LGPL 2.1 license (or any later version).

———
¹ https://github.com/mcfiredrill/libgme/blob/master/gme.txt

Signed-off-by: Rebecca Kelly <btk@google.com>
---
 libavformat/libgme.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)
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Patch

diff --git a/libavformat/libgme.c b/libavformat/libgme.c
index 4d04537339..b2635fc89f 100644
--- a/libavformat/libgme.c
+++ b/libavformat/libgme.c
@@ -63,7 +63,26 @@  static int load_metadata(AVFormatContext *s, int64_t *duration)
     if (gme_track_info(gme->music_emu, &info, gme->track_index))
         return AVERROR_STREAM_NOT_FOUND;
 
-    *duration = info->length;
+    // Some formats support looped audio. In these cases, length will be -1, and
+    // the track length needs to be determined from intro_length and loop_length
+    // instead. Furthermore, some formats have no length data at all, in which
+    // case all of these fields will be -1.
+    // When reading these formats, GME will return audio frames forever if you
+    // ask for them; it's up to the caller to decide when to stop, either by
+    // closing the file after a certain point or by calling gme_set_fade()
+    // before starting playback, which will cause GME to simulate a fadeout and
+    // end of file at that point.
+    // The GME documentation recommends a default behaviour of playing the intro
+    // followed by two loops, for looped audio, or playing 2.5 minutes and then
+    // fading out, for audio of indeterminate length.
+    if (info->length > 0) {
+        *duration = info->length;
+    } else if (info->intro_length > 0 || info->loop_length > 0) {
+        *duration = info->intro_length + info->loop_length * 2;
+    } else {
+        *duration = 150000; // 2.5 minutes in milliseconds
+    }
+
     add_meta(s, "system",       info->system);
     add_meta(s, "game",         info->game);
     add_meta(s, "song",         info->song);
@@ -138,8 +157,10 @@  static int read_header_gme(AVFormatContext *s)
     if (!st)
         return AVERROR(ENOMEM);
     avpriv_set_pts_info(st, 64, 1, 1000);
-    if (duration > 0)
+    if (duration > 0) {
         st->duration = duration;
+        gme_set_fade(gme->music_emu, duration); // Avoid infinite playback
+    }
     st->codecpar->codec_type  = AVMEDIA_TYPE_AUDIO;
     st->codecpar->codec_id    = AV_NE(AV_CODEC_ID_PCM_S16BE, AV_CODEC_ID_PCM_S16LE);
     st->codecpar->ch_layout.nb_channels = 2;