Re: [PATCH 6/6] update-ref: add support for 'symref-update' command

> From: Karthik Nayak <karthik.188@gmail.com>
> 
> Add 'symref-update' command to the '--stdin' mode of 'git-update-ref' to
> allow updates of symbolic refs. The 'symref-update' command takes in a
> <new-target>, which the <ref> will be updated to. If the <ref> doesn't
> exist it will be created.
> 
> It also optionally takes either an `ref <old-target>` or `oid
> <old-oid>`. If the <old-target> is provided, it checks to see if the
> <ref> targets the <old-target> before the update. If <old-oid> is provided
> it checks <ref> to ensure that it is a regular ref and <old-oid> is the
> OID before the update. This by extension also means that this when a
> zero <old-oid> is provided, it ensures that the ref didn't exist before.

> The command allows users to perform symbolic ref updates within a
> transaction. This provides atomicity and allows users to perform a set
> of operations together.
> 
> This command will also support deref mode, to ensure that we can update
> dereferenced regular refs to symrefs.

> diff --git a/builtin/update-ref.c b/builtin/update-ref.c
> index 16d184603b..389136dc2f 100644
> --- a/builtin/update-ref.c
> +++ b/builtin/update-ref.c
> @@ -98,6 +98,41 @@ static char *parse_next_refname(const char **next)
>  	return parse_refname(next);
>  }
>  
> +/*
> + * Wrapper around parse_arg which skips the next delimiter.
> + */
> +static char *parse_next_arg(const char **next)
> +{
> +	struct strbuf arg = STRBUF_INIT;
> +
> +	if (line_termination) {
> +		/* Without -z, consume SP and use next argument */
> +		if (!**next || **next == line_termination)
> +			return NULL;
> +		if (**next != ' ')
> +			die("expected SP but got: %s", *next);
> +	} else {
> +		/* With -z, read the next NUL-terminated line */
> +		if (**next)
> +			return NULL;
> +	}
> +	/* Skip the delimiter */
> +	(*next)++;
> +
> +	if (line_termination) {
> +		/* Without -z, use the next argument */
> +		*next = parse_arg(*next, &arg);
> +	} else {
> +		/* With -z, use everything up to the next NUL */
> +		strbuf_addstr(&arg, *next);
> +		*next += arg.len;
> +	}
> +
> +	if (arg.len)
> +		return strbuf_detach(&arg, NULL);
> +	return NULL;
> +}
> +
>  

>  /*
>   * The value being parsed is <old-oid> (as opposed to <new-oid>; the
> @@ -237,6 +272,55 @@ static void parse_cmd_update(struct ref_transaction *transaction,
>  	strbuf_release(&err);
>  }
>  
> +static void parse_cmd_symref_update(struct ref_transaction *transaction,
> +				    const char *next, const char *end)
> +{
> +	char *refname, *new_target, *old_arg;
> +	char *old_target = NULL;
> +	struct strbuf err = STRBUF_INIT;
> +	struct object_id old_oid;
> +	int have_old = 0;
> +
> +	refname = parse_refname(&next);
> +	if (!refname)
> +		die("symref-update: missing <ref>");
> +
> +	new_target = parse_next_refname(&next);
> +	if (!new_target)
> +		die("symref-update %s: missing <new-target>", refname);
> +
> +	old_arg = parse_next_arg(&next);
> +	if (old_arg) {
> +		old_target = parse_next_refname(&next);
> +		if (!old_target)
> +			die("symref-update %s: expected old value", refname);
> +
> +		if (!strcmp(old_arg, "oid") &&
> +		    !repo_get_oid(the_repository, old_target, &old_oid)) {
> +			old_target = NULL;
> +			have_old = 1;

> +	}
> +
> +	if (*next != line_termination)
> +		die("symref-update %s: extra input: %s", refname, next);
> +
> +	if (ref_transaction_update(transaction, refname, NULL,
> +				   have_old ? &old_oid : NULL,
> +				   new_target, old_target,
> +				   update_flags |= create_reflog_flag,