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

Linus Torvalds [off-list ref] 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 <redacted>
---
 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