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cli: fork+exec subdaemons, don't just fork

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this resolves problems with aio roots becoming invalid after fork (which
so far forced us to run the daemon loop in an aio-rootless thread), does
not require restarting the signal handler thread in the subdaemon (since
we no longer lose it), and is a step towards solving #18 (with transient
daemons doing the store manipulation started transparently when needed).

Change-Id: Iad0149cbc807e31964407c9a83d12314702c8122
This commit is contained in:
eldritch horrors
2025-07-04 14:01:11 +02:00
parent dffb8e9865
commit 7b37d5ea6a
1 changed files with 123 additions and 53 deletions
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lix/nix/daemon.cc
+123 -53
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@@ -6,6 +6,9 @@
#include "lix/libstore/remote-store.hh"
#include "lix/libstore/remote-store-connection.hh"
#include "lix/libstore/store-api.hh"
#include "lix/libutil/current-process.hh"
#include "lix/libutil/error.hh"
#include "lix/libutil/processes.hh"
#include "lix/libutil/serialise.hh"
#include "lix/libutil/archive.hh"
#include "lix/libstore/globals.hh"
@@ -14,13 +17,16 @@
#include "lix/libcmd/legacy.hh"
#include "lix/libutil/signals.hh"
#include "lix/libstore/daemon.hh"
#include "lix/libutil/strings.hh"
#include "lix/libutil/unix-domain-socket.hh"
#include "daemon.hh"
#include <algorithm>
#include <climits>
#include <cstdint>
#include <cstring>
#include <string>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
@@ -41,6 +47,9 @@
#include <membership.h>
#endif
static constexpr int SUBDAEMON_CONNECTION_FD = 0;
static constexpr int SUBDAEMON_SETTINGS_FD = 3;
namespace nix {
using namespace nix::daemon;
@@ -303,6 +312,14 @@ static void daemonLoopImpl(std::optional<TrustedFlag> forceTrustClientOpt)
// Get rid of children automatically; don't let them become zombies.
setSigChldAction(true);
const auto self = [] {
auto tmp = getSelfExe();
if (!tmp) {
throw Error("can't locate the daemon binary!");
}
return *tmp;
}();
// Loop accepting connections.
while (1) {
@@ -321,60 +338,44 @@ static void daemonLoopImpl(std::optional<TrustedFlag> forceTrustClientOpt)
closeOnExec(remote.get());
PeerInfo peer { .pidKnown = false };
TrustedFlag trusted;
std::string user;
PeerInfo peer = getPeerInfo(remote.get());
printInfo(
"accepted connection from %1%",
peer.pidKnown ? fmt("pid %1%", peer.pid) : "unknown peer"
);
if (forceTrustClientOpt)
trusted = *forceTrustClientOpt;
else {
peer = getPeerInfo(remote.get());
auto [_trusted, _user] = authPeer(peer);
trusted = _trusted;
user = _user;
Pipe settings;
settings.create();
// Fork a child to handle the connection. make sure it's called with
// argv0 `nix-daemon` so we don't try to run `nix --for` when called
// from more modern scripts that assume nix-command being available.
RunOptions options{
.program = self,
.argv0 = "nix-daemon",
.args =
{
"--for",
peer.pidKnown ? fmt("%1%", peer.pid) : "unknown",
"--log-level",
fmt("%1%", int(verbosity)),
},
.redirections =
{
{.dup = SUBDAEMON_CONNECTION_FD, .from = remote.get()},
{.dup = SUBDAEMON_SETTINGS_FD, .from = settings.readSide.get()},
}
};
runProgram2(options).release();
printInfo((std::string) "accepted connection from pid %1%, user %2%" + (trusted ? " (trusted)" : ""),
peer.pidKnown ? std::to_string(peer.pid) : "<unknown>",
peer.uidKnown ? user : "<unknown>");
// Fork a child to handle the connection.
ProcessOptions options;
options.errorPrefix = "unexpected Nix daemon error: ";
options.dieWithParent = false;
options.runExitHandlers = true;
startProcess([&]() {
fdSocket.reset();
// Background the daemon.
if (setsid() == -1)
throw SysError("creating a new session");
AsyncIoRoot aio;
// Restart the signal handler thread since it met its untimely
// demise at fork time.
startSignalHandlerThread(DoSignalSave::DontSaveBecauseAdvancedProcess);
// Restore normal handling of SIGCHLD.
setSigChldAction(false);
// For debugging, stuff the pid into argv[1].
if (peer.pidKnown && savedArgv[1]) {
auto processName = std::to_string(peer.pid);
strncpy(savedArgv[1], processName.c_str(), strlen(savedArgv[1]));
}
// Handle the connection.
FdSource from(remote.get());
FdSink to(remote.get());
processConnection(
aio, aio.blockOn(openUncachedStore(AllowDaemon::Disallow)), from, to, trusted
);
exit(0);
}, options).release();
FdSink sink(settings.writeSide.get());
std::map<std::string, Config::SettingInfo> overriddenSettings;
globalConfig.getSettings(overriddenSettings, true);
for (auto & setting : overriddenSettings) {
sink << 1 << setting.first << setting.second.value;
}
sink << 0;
sink.flush();
} catch (Interrupted & e) {
return;
} catch (Error & error) {
@@ -400,6 +401,57 @@ static void daemonLoop(AsyncIoRoot & aio, std::optional<TrustedFlag> forceTrustC
}).get();
}
static void daemonInstance(AsyncIoRoot & aio, std::optional<TrustedFlag> forceTrustClientOpt)
{
PeerInfo peer = getPeerInfo(SUBDAEMON_CONNECTION_FD);
TrustedFlag trusted;
std::string user;
if (forceTrustClientOpt) {
trusted = *forceTrustClientOpt;
} else {
std::tie(trusted, user) = authPeer(peer);
}
printInfo(
"%1% is %2% (%3%%4%)",
peer.pidKnown ? fmt("remote pid %s", peer.pid) : "remote with unknown pid",
peer.uidKnown ? fmt("user %s", user) : "unknown user",
trusted ? "trusted" : "untrusted",
forceTrustClientOpt ? " by override" : ""
);
{
FdSource source(SUBDAEMON_SETTINGS_FD);
/* Read the parent's settings. */
while (readInt(source)) {
auto name = readString(source);
auto value = readString(source);
settings.set(name, value);
}
if (close(SUBDAEMON_SETTINGS_FD) < 0) {
throw SysError("preparing subdaemon connection");
}
}
// Background the daemon.
if (setsid() == -1) {
throw SysError("creating a new session");
}
// Restore normal handling of SIGCHLD.
setSigChldAction(false);
// Handle the connection.
FdSource from(SUBDAEMON_CONNECTION_FD);
FdSink to(SUBDAEMON_CONNECTION_FD);
processConnection(
aio, aio.blockOn(openUncachedStore(AllowDaemon::Disallow)), from, to, trusted
);
}
/**
* Forward a standard IO connection to the given remote store.
*
@@ -487,6 +539,8 @@ static int main_nix_daemon(AsyncIoRoot & aio, std::string programName, Strings a
{
auto stdio = false;
std::optional<TrustedFlag> isTrustedOpt = std::nullopt;
bool isInstance = false;
Verbosity subdaemonLogLevel = lvlInfo;
LegacyArgs(aio, programName, [&](Strings::iterator & arg, const Strings::iterator & end) {
if (*arg == "--daemon")
@@ -506,11 +560,27 @@ static int main_nix_daemon(AsyncIoRoot & aio, std::string programName, Strings a
} else if (*arg == "--default-trust") {
experimentalFeatureSettings.require(Xp::DaemonTrustOverride);
isTrustedOpt = std::nullopt;
} else return false;
} else if (*arg == "--for") {
isInstance = true;
getArg(*arg, arg, end);
} else if (*arg == "--log-level") {
if (auto level = string2Int<int>(getArg(*arg, arg, end)); level) {
subdaemonLogLevel = static_cast<Verbosity>(std::min<int>(lvlVomit, *level));
} else {
throw UsageError("--log-level expects an integer in the range [0..7]");
}
} else {
return false;
}
return true;
}).parseCmdline(argv);
runDaemon(aio, stdio, isTrustedOpt);
if (isInstance) {
verbosity = Verbosity(std::min<uint64_t>(subdaemonLogLevel, lvlVomit));
daemonInstance(aio, isTrustedOpt);
} else {
runDaemon(aio, stdio, isTrustedOpt);
}
return 0;
}