From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 04 Mar 2026 20:51:00 GMT
Subject: Re: [PATCH v8 08/12] run-command: add close_fd_above_stderr option
Message-ID: <xmqq8qc771zf.fsf@gitster.g>
In-Reply-To: <29a6461915ce9d2abedb29e475d589bb8d24934a.1772648125.git.gitgitgadget@gmail.com>

"Paul Tarjan via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Paul Tarjan <github@paulisageek.com>
>
> Add a new option to struct child_process that closes file descriptors
> 3 and above in the child after forking but before exec.  Without this,
> long-running child processes inherit pipe endpoints and other
> descriptors from the parent environment.
>
> The upper bound for the fd scan comes from sysconf(_SC_OPEN_MAX),
> capped at 4096 to avoid excessive iteration when the limit is set
> very high.
>
> Signed-off-by: Paul Tarjan <github@paulisageek.com>
> ---
>  run-command.c | 11 +++++++++++
>  run-command.h |  9 +++++++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/run-command.c b/run-command.c
> index e3e02475cc..cbadcf5ff8 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -832,6 +832,17 @@ fail_pipe:
>  			child_close(cmd->out);
>  		}
>  
> +		if (cmd->close_fd_above_stderr) {
> +			long max_fd = sysconf(_SC_OPEN_MAX);
> +			int fd;
> +			if (max_fd < 0 || max_fd > 4096)
> +				max_fd = 4096;
> +			for (fd = 3; fd < max_fd; fd++) {
> +				if (fd != child_notifier)
> +					close(fd);
> +			}
> +		}
> +
>  		if (cmd->dir && chdir(cmd->dir))
>  			child_die(CHILD_ERR_CHDIR);

The need for this particular "close file descriptors other than the
standard ones" may be common enough that I do not mind to have it
inside "run-command.c", but I wonder if a generic callback function
to call here in the child between fork and exec that the caller can
supply would be a good thing to have.  Then, any caller who may want
to set close_fd_above_stderr could instead prepare a callback that
does the body of the above if statement themselves.

> diff --git a/run-command.h b/run-command.h
> index 0df25e445f..a1aa1b1069 100644
> --- a/run-command.h
> +++ b/run-command.h
> @@ -141,6 +141,15 @@ struct child_process {
>  	unsigned stdout_to_stderr:1;
>  	unsigned clean_on_exit:1;
>  	unsigned wait_after_clean:1;
> +
> +	/**
> +	 * Close file descriptors 3 and above in the child after forking
> +	 * but before exec.  This prevents the long-running child from
> +	 * inheriting pipe endpoints or other descriptors from the parent
> +	 * environment (e.g., the test harness).
> +	 */
> +	unsigned close_fd_above_stderr:1;
> +
>  	void (*clean_on_exit_handler)(struct child_process *process);
>  };