Subject: Re: [PATCH 2/4] reftable/stack: refactor reloading to use file descriptor

From: Junio C Hamano <[email-masked]>
Date: Wed, 10 Jan 2024 11:57:25 -0800

Patrick Steinhardt <[email-masked]> writes:

> We're about to introduce a stat(3P)-based caching mechanism to reload
> the list of stacks only when it has changed. In order to avoid race
> conditions this requires us to have a file descriptor available that we
> can use to call fstat(3P) on.
>
> Prepare for this by converting the code to use `fd_read_lines()` so that
> we have the file descriptor readily available.
> ---

Missing sign-off.

Other than that, the change is a refactoring that is very faithful
to the original.  Instead of doing the "open - pretend we opened an
empty file if it does not exist - read - close" dance all inside the
read_lines() call, this sort-of open codes the helper in this caller,
so that later steps of this series can look at the file descriptor
open to it.

Looking good.

>  reftable/stack.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/reftable/stack.c b/reftable/stack.c
> index bf869a6772..b1ee247601 100644
> --- a/reftable/stack.c
> +++ b/reftable/stack.c
> @@ -308,6 +308,7 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
>  	struct timeval deadline;
>  	int64_t delay = 0;
>  	int tries = 0, err;
> +	int fd = -1;
>  
>  	err = gettimeofday(&deadline, NULL);
>  	if (err < 0)
> @@ -329,9 +330,19 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
>  		if (tries > 3 && tv_cmp(&now, &deadline) >= 0)
>  			goto out;
>  
> -		err = read_lines(st->list_file, &names);
> -		if (err < 0)
> -			goto out;
> +		fd = open(st->list_file, O_RDONLY);
> +		if (fd < 0) {
> +			if (errno != ENOENT) {
> +				err = REFTABLE_IO_ERROR;
> +				goto out;
> +			}
> +
> +			names = reftable_calloc(sizeof(char *));
> +		} else {
> +			err = fd_read_lines(fd, &names);
> +			if (err < 0)
> +				goto out;
> +		}
>  
>  		err = reftable_stack_reload_once(st, names, reuse_open);
>  		if (!err)
> @@ -356,12 +367,16 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
>  		names = NULL;
>  		free_names(names_after);
>  		names_after = NULL;
> +		close(fd);
> +		fd = -1;
>  
>  		delay = delay + (delay * rand()) / RAND_MAX + 1;
>  		sleep_millisec(delay);
>  	}
>  
>  out:
> +	if (fd >= 0)
> +		close(fd);
>  	free_names(names);
>  	free_names(names_after);
>  	return err;