Subject: [TANGENT] run-command: use vfork instead of fork

From: Eric Wong <[email-masked]>
Date: Tue, 16 May 2017 19:35:57 +0000

Linus Torvalds <[email-masked]> wrote:
> Also, if people really want to optimize the code that executes an
> external program (whether in shell or directly), I think it might be
> worth it to look at replacing the "fork()" with a "vfork()".
> 
> Something like this
> 
> -       cmd->pid = fork();
> +       cmd->pid = (cmd->git_cmd || cmd->env) ? fork() : vfork();
> 
> might work (the native git_cmd case needs a real fork, and if we
> change the environment variables we need it too, but the other cases
> look like they might work with vfork()).
> 
> Using vfork() can be hugely more efficient, because you don't have the
> extra page table copies and teardown, but also avoid a lot of possible
> copy-on-write faults.

Fwiw, most of the vfork preparation was already done by Brandon
and myself a few weeks ago, and cooking in pu.

I think only the patch below would be needed to enable vfork
(along with any build-time detection)

However, I haven't noticed enough forking in git to make a
difference (but maybe others do).  I think it would make a
bigger difference if such changes were made to bash, dash,
make, and perl5.

--------8<------------
Subject: [PATCH] run-command: use vfork instead of fork

To enable vfork, we merely have to avoid modifying memory we
share with the parent, so the guard functions
`child_(error|warn|die)_fn` can now be disabled.

FIXME: still missing autoconf + Makefile portability tweaks.

Signed-off-by: Eric Wong <[email-masked]>
---
 run-command.c | 28 +---------------------------
 1 file changed, 1 insertion(+), 27 deletions(-)

diff --git a/run-command.c b/run-command.c
index 9e36151bf9..0292dd94b6 100644
--- a/run-command.c
+++ b/run-command.c
@@ -324,25 +324,6 @@ static void fake_fatal(const char *err, va_list params)
 	vreportf("fatal: ", err, params);
 }
 
-static void child_error_fn(const char *err, va_list params)
-{
-	const char msg[] = "error() should not be called in child\n";
-	xwrite(2, msg, sizeof(msg) - 1);
-}
-
-static void child_warn_fn(const char *err, va_list params)
-{
-	const char msg[] = "warn() should not be called in child\n";
-	xwrite(2, msg, sizeof(msg) - 1);
-}
-
-static void NORETURN child_die_fn(const char *err, va_list params)
-{
-	const char msg[] = "die() should not be called in child\n";
-	xwrite(2, msg, sizeof(msg) - 1);
-	_exit(2);
-}
-
 /* this runs in the parent process */
 static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
 {
@@ -658,17 +639,10 @@ int start_command(struct child_process *cmd)
 	 * never be released in the child process.  This means only
 	 * Async-Signal-Safe functions are permitted in the child.
 	 */
-	cmd->pid = fork();
+	cmd->pid = vfork();
 	failed_errno = errno;
 	if (!cmd->pid) {
 		int sig;
-		/*
-		 * Ensure the default die/error/warn routines do not get
-		 * called, they can take stdio locks and malloc.
-		 */
-		set_die_routine(child_die_fn);
-		set_error_routine(child_error_fn);
-		set_warn_routine(child_warn_fn);
 
 		close(notify_pipe[0]);
 		set_cloexec(notify_pipe[1]);
-- 
Also fetchable:  git://bogomips.org/git-svn vfork-test
commit 5f88d79182aaabc5ea467d1d29e13e45bd2b99bf