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mtmd: video: fix moov atom at the end of file (#27596)

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* mtmd: video: fix moov at the end of file

Co-authored-by: rkfg <rkfg@rkfg.me>

* fix SIGPIPE

* windows: handle broken pipe case

---------

Co-authored-by: rkfg <rkfg@rkfg.me>
This commit is contained in:
Xuan-Son Nguyen
2026-08-24 09:59:04 +02:00
committed by GitHub
co-authored by rkfg
parent 985b14912b
commit 160c6b0bdd
1 changed files with 42 additions and 9 deletions
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tools/mtmd/mtmd-helper.cpp
+42 -9
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@@ -42,6 +42,11 @@
#ifdef MTMD_VIDEO
#include "sheredom/subprocess.h"
#include <thread>
#ifndef _WIN32
#include <csignal>
#include <fcntl.h>
#include <pthread.h>
#endif
#endif
//
@@ -522,7 +527,8 @@ struct mtmd_helper_video {
// RAII wrapper for managing subprocess
struct subprocess_handle {
struct subprocess_s proc = {};
bool alive = false;
bool created = false; // process exists and must be cleaned up
bool alive = false; // process can still give us data
std::thread feeder;
subprocess_handle() = default;
@@ -531,18 +537,27 @@ struct mtmd_helper_video {
~subprocess_handle() { stop(); }
void stop() {
if (alive) {
subprocess_terminate(&proc);
// note: alive becomes false on stdout EOF, but the process still needs cleanup
if (!created) {
return;
}
subprocess_terminate(&proc);
#ifdef _WIN32
// no SIGPIPE on windows: a blocked feeder only gets a broken pipe once we close our read end of the child stdin
if (proc.hStdInput) {
CloseHandle(proc.hStdInput);
proc.hStdInput = nullptr;
}
#endif
// join before destroy: feeder holds a FILE* from subprocess_stdin;
// subprocess_destroy closes it, so the thread must finish first
if (feeder.joinable()) {
feeder.join();
}
if (alive) {
subprocess_destroy(&proc);
alive = false;
}
subprocess_join(&proc, nullptr); // reap the child, or else it stays a zombie
subprocess_destroy(&proc);
created = false;
alive = false;
}
FILE * stdout_pipe() {
@@ -552,10 +567,21 @@ struct mtmd_helper_video {
// buf is tied to lifetime of mtmd_helper_video, so it's guaranteed to outlive the feeder thread
void start_feeder(const std::vector<uint8_t> & buf) {
feeder = std::thread([this, &buf]() {
#ifndef _WIN32
// ffmpeg can exit before it reads all the input, for example when ffprobe already got the metadata.
// the write below must then fail with EPIPE, instead of killing the process with SIGPIPE
sigset_t sigpipe_set;
sigemptyset(&sigpipe_set);
sigaddset(&sigpipe_set, SIGPIPE);
pthread_sigmask(SIG_BLOCK, &sigpipe_set, nullptr); // linux sends the signal to the writing thread
#endif
FILE * f = subprocess_stdin(&proc);
if (!f) {
return;
}
#ifdef F_SETNOSIGPIPE
fcntl(fileno(f), F_SETNOSIGPIPE, 1); // macos/bsd send it to the process, so turn it off per fd
#endif
fwrite(buf.data(), 1, buf.size(), f);
fclose(f);
proc.stdin_file = nullptr; // prevent double-close in subprocess_destroy
@@ -601,7 +627,8 @@ struct mtmd_helper_video {
LOG_ERR("%s: failed to launch ffprobe\n", __func__);
return false;
}
probe_sp.alive = true;
probe_sp.created = true;
probe_sp.alive = true;
if (is_buf_input()) {
probe_sp.start_feeder(input_buf);
@@ -673,6 +700,11 @@ struct mtmd_helper_video {
}
cmd.push_back("-nostdin");
if (is_buf_input()) {
// remove the 64KB read-ahead limit of cache:, or else ffmpeg cannot reach a moov atom at end of file
cmd.push_back("-read_ahead_limit");
cmd.push_back("-1");
}
cmd.push_back("-i");
// cache:pipe:0 wraps stdin with a seekable in-memory cache, letting ffmpeg seek
// backwards for container headers (e.g. MP4 moov atom at end of file)
@@ -711,7 +743,8 @@ struct mtmd_helper_video {
subprocess_option_search_user_path | subprocess_option_inherit_environment,
&sp.proc);
sp.alive = (ret == 0);
sp.created = (ret == 0);
sp.alive = (ret == 0);
LOG_DBG("%s: subprocess_create ret=%d proc_alive=%d\n", __func__, ret, (int)sp.alive);
if (sp.alive && is_buf_input()) {