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media: resolve the audio user once, thread-safely

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SpawnPw() looked the desktop user up with getpwnam() — one static buffer —
from the mic thread and the playout thread at the same instant. When the
race made both lookups fail, uid stayed 0 and both pw helpers were started
with XDG_RUNTIME_DIR=/run/user/0, where no PipeWire session lives: pw-record
hit EOF at once, pw-play never connected, and a call that was up with RTP
flowing both ways was answered into static (tx_mic=0 rx_played=0 qdrop=883,
dev phone 2026-09-08 20:01, first time in years of calls — timing).

AudioUid() resolves AUDIO_USER once under std::call_once with getpwnam_r and
a private buffer, complains when the user does not exist, and logs the
session the helpers are aimed at on every call, so a field journal answers
"where did the audio go" directly.
This commit is contained in:
Jorijn van der Graaf 2026-09-08 20:06:58 +02:00
commit b8d439246a
1 changed files with 23 additions and 3 deletions
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26
implementations/media.cpp
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@ -410,6 +410,28 @@ std::size_t RtpPayloadOffset(std::span<const std::uint8_t> pkt) {
return off;
}
// The desktop user's uid, resolved ONCE and thread-safely. getpwnam() hands
// out one static buffer, and the mic and playout threads spawn their pw
// helpers at the same instant — the race sent both into /run/user/0, where
// no PipeWire session lives, and a call was answered into static (dev phone,
// 2026-09-08 20:01: tx_mic=0 rx_played=0 qdrop=883). Logged per call so a
// journal shows which session the helpers were aimed at.
uid_t AudioUid() {
static std::once_flag once;
static uid_t uid = 0;
std::call_once(once, [] {
std::string user = EnvOr("AUDIO_USER", "user");
passwd pw{};
passwd* res = nullptr;
char buf[4096];
if (getpwnam_r(user.c_str(), &pw, buf, sizeof buf, &res) == 0 && res) uid = res->pw_uid;
else
std::println(std::cerr, "imsd-media: AUDIO_USER '{}' not found; no PipeWire session for uid 0", user);
std::println(std::cerr, "imsd-media: audio session: {} -> /run/user/{}", user, uid);
});
return uid;
}
// Spawn pw-record/pw-play in the desktop user's PipeWire session (sudo -u
// when run as root). AUDIO_USER names the session owner; the default "user"
// is postmarketOS's standard account. `toChild` true = we write the child's
@ -421,10 +443,8 @@ Child SpawnPw(bool play, bool toChild, int rate) {
std::vector<std::string> argv;
if (geteuid() == 0) {
std::string user = EnvOr("AUDIO_USER", "user");
uid_t uid = 0;
if (passwd* pw = getpwnam(user.c_str())) uid = pw->pw_uid;
argv = {"sudo", "-u", user, "env",
std::format("XDG_RUNTIME_DIR=/run/user/{}", uid)};
std::format("XDG_RUNTIME_DIR=/run/user/{}", AudioUid())};
}
const char* tool = play ? "pw-play" : "pw-record";
const char* lat = play ? "40ms" : "20ms";