Re: [PATCH] merge: use skip_prefix to parse config key

[Jeff King <peff@peff.net>]

On Fri, Apr 10, 2020 at 05:10:32PM +0200, Martin Ågren wrote:

> Instead of using `starts_with()`, the magic number 7, `strlen()` and a
> fair number of additions to verify the three parts of the config key
> "branch.<branch>.mergeoptions", use `skip_prefix()` to jump through them
> more explicitly.

The conversion looks correct to me and is certainly an improvement.

> We need to introduce a new variable for this (we certainly can't modify
> `k` just because we see "branch."!). With `skip_prefix()` we often use
> quite bland names like `p` or `str`. Let's do the same. If and when this
> function needs to do more prefix-skipping, we'll have a generic variable
> ready for this.

I was about to comment on this in the patch, but your explanation
preempted me. :) The logic here makes sense.

> diff --git a/builtin/merge.c b/builtin/merge.c
> index d127d2225f..bde5f14f05 100644
> --- a/builtin/merge.c
> +++ b/builtin/merge.c
> @@ -597,10 +597,10 @@ static void parse_branch_merge_options(char *bmo)
>  static int git_merge_config(const char *k, const char *v, void *cb)
>  {
>  	int status;
> +	const char *str;
>  
> -	if (branch && starts_with(k, "branch.") &&
> -		starts_with(k + 7, branch) &&
> -		!strcmp(k + 7 + strlen(branch), ".mergeoptions")) {
> +	if (branch && skip_prefix(k, "branch.", &str) &&
> +	    skip_prefix(str, branch, &str) && !strcmp(str, ".mergeoptions")) {
>  		free(branch_mergeoptions);
>  		branch_mergeoptions = xstrdup(v);
>  		return 0;

In general, parsing subsections accurately involves looking from both
the start and the end of the string, pulling out the section and key and
leaving the rest in the middle. But I think we can get away with this
left-to-right parsing because we're only interested in matching a
_specific_ subsection name, and a specific key. So there are no cases it
will handle incorrectly.

The more general form would be:

  const char *subsection, *key;
  int subsection_len;

  if (!parse_config_key("branch", &subsection, &subsection_len, &key) &&
      subsection_len == strlen(branch) && !strncmp(subsection, branch) &&
      !strcmp(key, "mergeoptions"))
         ...

but that's a bit more awkward (it would be less so if we had a helper
function for comparing a NUL-terminated string against a ptr/len pair).

-Peff