libutil: remove vfork

vfork confers a large performance advantage over fork, measured locally
at 16µs per vfork agains 90µs per fork. however nix *almost always*
follows a vfork up with an execve-family call, melting the performance
advantage from 6x to only 15%. in most of those cases it's doing things
that are undefined behavior (like manipulating the heap, or even
throwing exceptions and trashing the parent process stack).

most notably the one place that could benefit from the vfork performance
improvement is linux derivation sandbox setup—which doesn't use vfork.

Change-Id: I2037b7384d5a4ca24da219a569e1b1f39531410e
This commit is contained in:
eldritch horrors 2024-03-18 12:38:49 +01:00
parent 2890840b96
commit 1f8b85786e
3 changed files with 3 additions and 17 deletions

View file

@ -1095,16 +1095,9 @@ void killUser(uid_t uid)
//////////////////////////////////////////////////////////////////////
/* Wrapper around vfork to prevent the child process from clobbering
the caller's stack frame in the parent. */
static pid_t doFork(bool allowVfork, std::function<void()> fun) __attribute__((noinline));
static pid_t doFork(bool allowVfork, std::function<void()> fun)
static pid_t doFork(std::function<void()> fun)
{
#ifdef __linux__
pid_t pid = allowVfork ? vfork() : fork();
#else
pid_t pid = fork();
#endif
if (pid != 0) return pid;
fun();
abort();
@ -1124,7 +1117,6 @@ static int childEntry(void * arg)
pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
{
std::function<void()> wrapper = [&]() {
if (!options.allowVfork)
logger = makeSimpleLogger();
try {
#if __linux__
@ -1162,7 +1154,7 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
throw Error("clone flags are only supported on Linux");
#endif
} else
pid = doFork(options.allowVfork, wrapper);
pid = doFork(wrapper);
if (pid == -1) throw SysError("unable to fork");
@ -1226,10 +1218,6 @@ void runProgram2(const RunOptions & options)
if (source) in.create();
ProcessOptions processOptions;
// vfork implies that the environment of the main process and the fork will
// be shared (technically this is undefined, but in practice that's the
// case), so we can't use it if we alter the environment
processOptions.allowVfork = !options.environment;
std::optional<Finally<std::function<void()>>> resumeLoggerDefer;
if (options.isInteractive) {

View file

@ -413,7 +413,6 @@ struct ProcessOptions
std::string errorPrefix = "";
bool dieWithParent = true;
bool runExitHandlers = false;
bool allowVfork = false;
/**
* use clone() with the specified flags (Linux only)
*/

View file

@ -347,7 +347,6 @@ static void daemonLoop(std::optional<TrustedFlag> forceTrustClientOpt)
options.errorPrefix = "unexpected Nix daemon error: ";
options.dieWithParent = false;
options.runExitHandlers = true;
options.allowVfork = false;
startProcess([&]() {
fdSocket = -1;