Re: [PATCH v2 5/5] index-pack: make pointer-alias fallbacks safer

> The final() function accepts a NULL value for certain
> parameters, and falls back to writing into a reusable "name"
> buffer, and then either:
> 
>   1. For "keep_name", requiring all uses to do "keep_name ?
>      keep_name : name.buf". This is awkward, and it's easy
>      to accidentally look at the maybe-NULL keep_name.
> 
>   2. For "final_index_name" and "final_pack_name", aliasing
>      those pointers to the "name" buffer. This is easier to
>      use, but the aliased pointers become invalid after the
>      buffer is reused (this isn't a bug now, but it's a
>      potential pitfall).
> 
> One way to make this safer would be to introduce an extra
> pointer to do the aliasing, and have its lifetime match the
> validity of the "name" buffer. But it's still easy to
> accidentally use the wrong name (i.e., to use
> "final_pack_name" instead of the aliased pointer).
> 
> Instead, let's use three separate buffers that will remain
> valid through the function. That makes it safe to alias the
> pointers and use them consistently. The extra allocations
> shouldn't matter, as this function is not performance
> sensitive.
> 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  builtin/index-pack.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/builtin/index-pack.c b/builtin/index-pack.c
> index dcb346ab7..88d205f85 100644
> --- a/builtin/index-pack.c
> +++ b/builtin/index-pack.c
> @@ -1386,7 +1386,9 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
>  		  unsigned char *sha1)
>  {
>  	const char *report = "pack";
> -	struct strbuf name = STRBUF_INIT;
> +	struct strbuf pack_name = STRBUF_INIT;
> +	struct strbuf index_name = STRBUF_INIT;
> +	struct strbuf keep_name_buf = STRBUF_INIT;
>  	int err;
>  
>  	if (!from_stdin) {
> @@ -1402,13 +1404,13 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
>  		int keep_fd, keep_msg_len = strlen(keep_msg);
>  
>  		if (!keep_name)
> -			odb_pack_name(&name, sha1, "keep");
> +			keep_name = odb_pack_name(&keep_name_buf, sha1, "keep");
>  
> -		keep_fd = odb_pack_keep(keep_name ? keep_name : name.buf);
> +		keep_fd = odb_pack_keep(keep_name);
>  		if (keep_fd < 0) {
>  			if (errno != EEXIST)
>  				die_errno(_("cannot write keep file '%s'"),
> -					  keep_name ? keep_name : name.buf);
> +					  keep_name);
>  		} else {
>  			if (keep_msg_len > 0) {
>  				write_or_die(keep_fd, keep_msg, keep_msg_len);
> @@ -1416,14 +1418,14 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
>  			}
>  			if (close(keep_fd) != 0)
>  				die_errno(_("cannot close written keep file '%s'"),
> -					  keep_name ? keep_name : name.buf);
> +					  keep_name);
>  			report = "keep";
>  		}
>  	}
>  
>  	if (final_pack_name != curr_pack_name) {
>  		if (!final_pack_name)
> -			final_pack_name = odb_pack_name(&name, sha1, "pack");
> +			final_pack_name = odb_pack_name(&pack_name, sha1, "pack");
>  		if (finalize_object_file(curr_pack_name, final_pack_name))
>  			die(_("cannot store pack file"));
>  	} else if (from_stdin)
> @@ -1431,7 +1433,7 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
>  
>  	if (final_index_name != curr_index_name) {
>  		if (!final_index_name)
> -			final_index_name = odb_pack_name(&name, sha1, "idx");
> +			final_index_name = odb_pack_name(&index_name, sha1, "idx");
>  		if (finalize_object_file(curr_index_name, final_index_name))
>  			die(_("cannot store index file"));
>  	} else
> @@ -1458,7 +1460,9 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
>  		}
>  	}
>  
> -	strbuf_release(&name);
> +	strbuf_release(&index_name);
> +	strbuf_release(&pack_name);
> +	strbuf_release(&keep_name_buf);
>  }
>  
>  static int git_index_pack_config(const char *k, const char *v, void *cb)
>