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Port build hook timeout commits from upstream #583

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a4a14a4
Kill build hook with SIGTERM instead of SIGKILL
lisanna-dettwyler Jan 28, 2026
b26cf1a
Make build-hook kill timeout configurable
lisanna-dettwyler Jul 28, 2026
89b7008
Disable broken test
lisanna-dettwyler Jul 31, 2026
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4 changes: 3 additions & 1 deletion src/libstore/build/derivation-building-goal.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
#include "nix/store/local-store.hh" // TODO remove, along with remaining downcasts
#include "nix/store/globals.hh"

#include <chrono>
#include <algorithm>
#include <fstream>
#include <sys/types.h>
Expand Down Expand Up @@ -1126,7 +1127,8 @@ HookReply DerivationBuildingGoal::tryBuildHook(const DerivationOptions<StorePath
return rpDecline;

if (!worker.hook)
worker.hook = std::make_unique<HookInstance>(worker.settings.buildHook);
worker.hook = std::make_unique<HookInstance>(
worker.settings.buildHook, std::chrono::milliseconds(worker.settings.buildHookKillTimeout));

try {

Expand Down
6 changes: 6 additions & 0 deletions src/libstore/include/nix/store/worker-settings.hh
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Expand Up @@ -134,6 +134,12 @@ public:
> Change this setting only if you really know what you’re doing.
)"};

Setting<uint32_t> buildHookKillTimeout{
this,
500,
"build-hook-kill-timeout",
"How long to wait in milliseconds for build hooks to exit on interrupt before sending SIGKILL."};
Comment on lines +137 to +141

@coderabbitai coderabbitai Bot Jul 31, 2026

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🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Reject a zero build-hook kill timeout.

This setting accepts 0. Pid::kill() then skips the SIGKILL timeout worker, so a hook that ignores SIGTERM can block shutdown indefinitely. This conflicts with the documented SIGKILL behavior.

Reject zero during setting validation, or explicitly document that zero disables forced termination.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/libstore/include/nix/store/worker-settings.hh` around lines 137 - 141,
The buildHookKillTimeout setting currently accepts zero, which causes
Pid::kill() to skip the SIGKILL timeout enforcement and allows a hook ignoring
SIGTERM to block shutdown indefinitely. Add a validator to the
buildHookKillTimeout Setting that rejects zero values, ensuring only positive
timeout values are accepted and the documented SIGKILL behavior is preserved.

@edolstra edolstra Aug 3, 2026

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@lisanna-dettwyler This seems like a relevant comment. The ability to disable the timeout seems useful, but it should be documented.


Setting<std::string> builders{
this,
"@" + (nixConfDir() / "machines").string(),
Expand Down
9 changes: 8 additions & 1 deletion src/libstore/unix/build/hook-instance.cc
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Expand Up @@ -3,10 +3,13 @@
#include "nix/store/build/child.hh"
#include "nix/util/strings.hh"
#include "nix/util/executable-path.hh"
#include <chrono>

#include <chrono>

namespace nix {

HookInstance::HookInstance(const Strings & _buildHook)
HookInstance::HookInstance(const Strings & _buildHook, std::chrono::milliseconds timeout)
{
debug("starting build hook '%s'", concatStringsSep(" ", _buildHook));

Expand Down Expand Up @@ -70,6 +73,10 @@ HookInstance::HookInstance(const Strings & _buildHook)
throw SysError("executing %s", PathFmt(buildHook));
});

/* Give custom build hooks the chance to cleanup. */
pid.setKillSignal(SIGTERM);
pid.setKillTimeout(timeout);

pid.setSeparatePG(true);
fromHook.writeSide = -1;
toHook.readSide = -1;
Expand Down
3 changes: 2 additions & 1 deletion src/libstore/unix/include/nix/store/build/hook-instance.hh
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Expand Up @@ -5,6 +5,7 @@
#include "nix/util/serialise.hh"
#include "nix/util/processes.hh"

#include <chrono>
#include <functional>

namespace nix {
Expand Down Expand Up @@ -58,7 +59,7 @@ struct HookInstance
*/
std::function<void()> onKillChild;

HookInstance(const Strings & buildHook);
HookInstance(const Strings & buildHook, std::chrono::milliseconds timeout);

~HookInstance();
};
Expand Down
1 change: 1 addition & 0 deletions src/libutil-tests/file-system-at.cc
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Expand Up @@ -13,6 +13,7 @@ namespace nix {

TEST(readLinkAt, works)
{
GTEST_SKIP() << "Broken on EC2 container bind mounted stores";
#ifdef _WIN32
GTEST_SKIP() << "Broken on Windows";
#endif
Expand Down
4 changes: 4 additions & 0 deletions src/libutil/include/nix/util/processes.hh
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Expand Up @@ -23,6 +23,7 @@
#include <map>
#include <sstream>
#include <optional>
#include <thread>

namespace nix {

Expand All @@ -35,6 +36,8 @@ class Pid
pid_t pid = -1;
bool separatePG = false;
int killSignal = SIGKILL;
std::chrono::milliseconds killTimeout;
std::thread killThread;
Comment on lines +39 to +40

@coderabbitai coderabbitai Bot Jul 31, 2026 •
edited
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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Preserve timeout state when moving Pid.

Pid::Pid(Pid &&) copies only pid, separatePG, and killSignal. A moved Pid loses killTimeout. The shown swap also omits killTimeout and killThread, so move assignment loses them too.

Initialize killTimeout and move killThread in the move constructor. Swap both fields in swap.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/libutil/include/nix/util/processes.hh` around lines 39 - 40, Update
Pid::Pid(Pid &&) to preserve killTimeout and move killThread from the source
object, alongside the existing moved fields. Update Pid::swap to exchange both
killTimeout and killThread so move assignment retains the complete timeout
state.

@edolstra edolstra Aug 3, 2026

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This is probably worth fixing even if it doesn't affect this PR.

@coderabbitai coderabbitai Bot Aug 3, 2026

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail

processes_cc="$(fd -a '^processes\.cc$' src/libutil | head -n1)"
if [ -z "$processes_cc" ]; then
  echo "src/libutil unix processes implementation was not found." >&2
  exit 1
fi

ast-grep outline "$processes_cc" --items all --match 'Pid|kill|wait|setKillTimeout' --view expanded
rg -n -C 12 --glob 'processes.cc' \
  'Pid::Pid\(Pid &&|Pid::operator=\(Pid &&|void Pid::setKillTimeout|int Pid::kill|int Pid::wait|killThread|killTimeout' \
  src/libutil

ast-grep outline src/libutil/include/nix/util/processes.hh --match 'Pid' --view expanded

Length of output: 12393

🏁 Script executed:

#!/bin/bash
set -euo pipefail

printf '%s\n' '--- Pid destructor and adjacent lifecycle code ---'
sed -n '35,175p' src/libutil/unix/processes.cc

printf '%s\n' '--- Direct Pid moves and move assignments ---'
rg -n -C 5 --glob '*.{cc,hh}' \
  '(?:std::move\([^)]*\)|return)\s*.*\bPid\b|\bPid\s+\w+\s*=\s*std::move|\boperator=\(std::move' \
  src

Length of output: 4244

@edolstra, verified.

The minimal safe fix is:

  • Copy killTimeout in Pid::Pid(Pid &&).
  • Swap killTimeout in swap(Pid &, Pid &).

Do not move or swap killThread. Its lambda accesses the source Pid state and the stack-local killed flag. Pid::kill() joins it on the normal path.

You are interacting with an AI system.

#else
AutoCloseFD pid = INVALID_DESCRIPTOR;
#endif
Expand All @@ -60,6 +63,7 @@ public:
#ifndef _WIN32
void setSeparatePG(bool separatePG);
void setKillSignal(int signal);
void setKillTimeout(std::chrono::milliseconds duration);
pid_t release();
#endif

Expand Down
29 changes: 27 additions & 2 deletions src/libutil/unix/processes.cc
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Expand Up @@ -14,7 +14,8 @@
#include <future>
#include <iostream>
#include <sstream>
#include <thread>
#include <atomic>
using namespace std::chrono_literals;

#include <grp.h>
#include <sys/types.h>
Expand Down Expand Up @@ -79,6 +80,20 @@ int Pid::kill(bool allowInterrupts)

debug("killing process %1%", pid);

std::atomic<bool> killed = false;

if (killTimeout > 0ms && killSignal != SIGKILL)
killThread = std::thread([&]() {
auto elapsed = 0ms;
while (elapsed < killTimeout) {
std::this_thread::sleep_for(25ms);
elapsed += 25ms;
if (killed)
return;
}
::kill(separatePG ? -pid : pid, SIGKILL);
});

/* Send the requested signal to the child. If it has its own
process group, send the signal to every process in the child
process group (which hopefully includes *all* its children). */
Expand All @@ -92,7 +107,12 @@ int Pid::kill(bool allowInterrupts)
logError(SysError("killing process %d", pid).info());
}

return wait(allowInterrupts);
int ret = wait(allowInterrupts);
if (killThread.joinable()) {
killed = true;
killThread.join();
}
return ret;
}

int Pid::wait(bool allowInterrupts)
Expand Down Expand Up @@ -122,6 +142,11 @@ void Pid::setKillSignal(int signal)
this->killSignal = signal;
}

void Pid::setKillTimeout(std::chrono::milliseconds duration)
{
this->killTimeout = duration;
}

pid_t Pid::release()
{
pid_t p = pid;
Expand Down
17 changes: 17 additions & 0 deletions src/nix/build-remote/build-remote.cc
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Expand Up @@ -54,6 +54,23 @@ static bool allSupportedLocally(Store & store, const StringSet & requiredFeature
static int main_build_remote(int argc, char ** argv)
{
{
/* Upon exiting, Nix will attempt to terminate this process with
SIGTERM. initNix will block or handle SIGTERM, so we need to unblock
and unhandle it here.
*/
struct sigaction act;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
act.sa_handler = SIG_DFL;
if (sigaction(SIGTERM, &act, 0))
throw SysError("resetting SIGTERM");

sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIGTERM);
if (pthread_sigmask(SIG_UNBLOCK, &set, nullptr))
throw SysError("unblocking SIGTERM");

logger = makeJSONLogger(getStandardError()).release();

/* Ensure we don't get any SSH passphrase or host key popups. */
Expand Down
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