Re: [PATCH v2 1/9] worktree: remove redundant NULL-ing of "cp.argv

[Eric Sunshine <sunshine@sunshineco.com>]

On Tue, Nov 23, 2021 at 10:26:56AM -0500, Eric Sunshine wrote:
> I think perhaps the simplest thing to do is merely to squash this
> patch into the patch which ultimately removes the child_process::argv
> member (and the removal of these lines from worktree.c probably
> doesn't even need mention in the commit message -- or maybe just a
> minor mention).

On second thought, squashing this patch into the patch which
ultimately retires child_process::argv is probably not necessary. If
the patch is instead rewritten as below, then it prepares
builtin/worktree.c for eventual retirement of child_process::argv
without breaking git-worktree functionality.

(You could also extend this patch so it prepares for removal of
child_process::env, as well, or keep it minimal as I did here.)

-- >8 --
From: Eric Sunshine <sunshine@sunshineco.com>
Subject: [PATCH] worktree: stop being overly intimate with run_command()
 internals

add_worktree() reuses a `child_process` for three run_command()
invocations, but to do so, it has overly-intimate knowledge of
run-command.c internals. In particular, it knows that it must reset
child_process::argv to NULL for each subsequent invocation[*] in order
for start_command() to latch the newly-populated child_process::args for
each invocation, even though this behavior is not a part of the
documented API. Beyond having overly-intimate knowledge of run-command.c
internals, the reuse of one `child_process` for three run_command()
invocations smells like an unnecessary micro-optimization. Therefore,
stop sharing one `child_process` and instead use a new one for each
run_command() call.

[*] If child_process::argv is not reset to NULL, then subsequent
run_command() invocations will instead incorrectly access a dangling
pointer to freed memory which had been allocated by child_process::args
on the previous run. This is due to the following code in
start_command():

    if (!cmd->argv)
        cmd->argv = cmd->args.v;

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---
 builtin/worktree.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/builtin/worktree.c b/builtin/worktree.c
index d22ece93e1..9edd3e2829 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -355,8 +355,8 @@ static int add_worktree(const char *path, const char *refname,
 		goto done;
 
 	if (opts->checkout) {
-		cp.argv = NULL;
-		strvec_clear(&cp.args);
+		struct child_process cp = CHILD_PROCESS_INIT;
+		cp.git_cmd = 1;
 		strvec_pushl(&cp.args, "reset", "--hard", "--no-recurse-submodules", NULL);
 		if (opts->quiet)
 			strvec_push(&cp.args, "--quiet");
@@ -385,12 +385,11 @@ static int add_worktree(const char *path, const char *refname,
 		const char *hook = find_hook("post-checkout");
 		if (hook) {
 			const char *env[] = { "GIT_DIR", "GIT_WORK_TREE", NULL };
-			cp.git_cmd = 0;
+			struct child_process cp = CHILD_PROCESS_INIT;
 			cp.no_stdin = 1;
 			cp.stdout_to_stderr = 1;
 			cp.dir = path;
 			cp.env = env;
-			cp.argv = NULL;
 			cp.trace2_hook_name = "post-checkout";
 			strvec_pushl(&cp.args, absolute_path(hook),
 				     oid_to_hex(null_oid()),
-- 
2.34.0.399.gdf2c515fd2

-- >8 --