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

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Nov 23, 2021, 15:26 UTC
Message-ID: <CAPig+cStZp=AOPHW8i7AqwDOV6djYzHC6GmcVeb=4PUj5bjvAw@mail.gmail.com>
In-Reply-To: <patch-v2-1.9-9cc220ce5a3-20211123T115551Z-avarab@gmail.com>

On Tue, Nov 23, 2021 at 7:08 AM Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:

Show 8 quoted lines

> The clearing of "argv" was added in 7f44e3d1de0 (worktree: make setup
> of new HEAD distinct from worktree population, 2015-07-17) when the
> "cp" variable wasn't initialized. It hasn't been needed since
> 542aa25d974 (use CHILD_PROCESS_INIT to initialize automatic variables,
> 2016-08-05).
>
> Let's remove it to make a later change that gets rid of the "argv"
> member from "struct child_process" smaller.

Let me preface with the caveat that (due to lack of time) I haven't dug into this deeply and I haven't been following changes to the run_command() machinery closely, so what I say below may be inaccurate, but...

Although it will be safe to drop these builtin/worktree.c assignments when the series eventually removes the child_process::argv member, I think that the commit message is misleading (or outright wrong), and that this change so early in the series potentially breaks git-worktree, leaving it in a state where it works only "by accident".

The reason that this code repeatedly clears `cp.argv` is that the `cp` structure is reused for multiple run_command() invocations (which, in retrospect, is pretty ugly since worktree.c is too familiar with the internals of run-command). The reason that `cp.argv` needs to be cleared between each invocation is due to this code from start_command():

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

worktree.c re-populates child_process::args between run_command() invocations and needs to clear child_process::argv to ensure that the latter gets re-initialized from child_process::args each time. The use of CHILD_PROCESS_INIT does not change anything in regard to the requirement; child_process::argv still needs to be cleared between run_command() invocations.

As to why this change so early in the series potentially breaks git-worktree: with the removal of these assignments, child_process::argv now _always_ points at the _initial_ value of child_process::args.v even though that vector gets cleared between run_command() invocations. At best, following this change, git-worktree is only working "by accident" if the underlying child_process::args.v doesn't get reallocated between run_command() invocations. Relying upon this "by accident" behavior feels rather unsafe.

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).

Show 24 quoted lines

> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
> diff --git a/builtin/worktree.c b/builtin/worktree.c
> index d22ece93e1a..7264a5b5de0 100644
> --- a/builtin/worktree.c
> +++ b/builtin/worktree.c
> @@ -355,7 +355,6 @@ static int add_worktree(const char *path, const char *refname,
>                 goto done;
>
>         if (opts->checkout) {
> -               cp.argv = NULL;
>                 strvec_clear(&cp.args);
>                 strvec_pushl(&cp.args, "reset", "--hard", "--no-recurse-submodules", NULL);
>                 if (opts->quiet)
> @@ -390,7 +389,6 @@ static int add_worktree(const char *path, const char *refname,
>                         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()),
> --
> 2.34.0.831.gd33babec0d1