MPD ignores opus header gain even with ReplayGain enabled

[awused]

Bug report

Describe the bug

Right now MPD only applies the r128_track_gain or r128_album_gain from opus files and ignores the output gain field in the header. The contents of the output gain header are not always consistent between different tools I've used to apply tags. Some use it for track gain and store the album gain as a difference in r128_album_gain with r128_track_gain set to 0 and some set output gain to 0 and only use the r128 tags, but in every case adding the appropriate r128 gain to the output gain gives the correct gain.

Per the Opus Spec (https://tools.ietf.org/html/rfc7845#section-5.1):
Players and media frameworks SHOULD apply it by default. If a player chooses to apply any volume adjustment or gain modification, such as the R128_TRACK_GAIN (see Section 5.2), the adjustment MUST be applied in addition to this output gain in order to achieve playback at the normalized volume.

There's a further complication in that ReplayGain uses a reference volume of 89dBFS but EBU R128 specifies -23 LUFS. ReplayGain scanners typically write tags to match ReplayGain's levels even when using EBU R128 for determining loudness, except in the case of Opus where every tagging program I've used writes gain for -23 LUFS per the spec. Other music players apply a +5dB correction to get playback to the same level.

Expected Behavior

The gain with ReplayGain enabled is r128_track/album_gain + output gain + 5dB. Output volume is consistent with other formats in MPD and other programs, even when using tagging programs (loudness-scanner, deadbeef, foobar, r128gain) with different behaviour.

Actual Behavior

Only r128_track_gain or r128_album_gain is applied. This results in inconsistent playback volume that varies depending on the program used to tag the file.

Version

Music Player Daemon 0.21.25 (0.21.25)
Copyright 2003-2007 Warren Dukes warren.dukes@gmail.com
Copyright 2008-2018 Max Kellermann max.kellermann@gmail.com
This is free software; see the source for copying conditions. There is NO
warranty; not even MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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local smbclient udisks nfs curl

Neighbor plugins:
smbclient upnp udisks

Decoders plugins:
[mad] mp3 mp2
[mpg123] mp3
[vorbis] ogg oga
[oggflac] ogg oga
[flac] flac
[opus] opus ogg oga
[sndfile] wav aiff aif au snd paf iff svx sf voc w64 pvf xi htk caf sd2
[audiofile] wav au aiff aif
[dsdiff] dff
[dsf] dsf
[hybrid_dsd] m4a
[faad] aac
[wavpack] wv
[modplug] 669 amf ams dbm dfm dsm far it med mdl mod mtm mt2 okt s3m stm ult umx xm
[mikmod] amf dsm far gdm imf it med mod mtm s3m stm stx ult uni xm
[sidplay] sid mus str prg P00
[wildmidi] mid
[fluidsynth] mid
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[pcm]

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Encoder plugins:
null vorbis opus lame twolame wave flac

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Log

[MaxKellermann]

You were asked to provide the MPD version number, but you decided not to tell.

[MaxKellermann]

23d5a2b