From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 24 Oct 2025 16:21:23 GMT
Subject: Re: [PATCH 01/13] odb: fix subtle logic to check whether an alternate is usable
Message-ID: <xmqqo6pwp8xo.fsf@gitster.g>
In-Reply-To: <20251024-b4-pks-odb-loose-backend-v1-1-1a4202273c38@pks.im>

Patrick Steinhardt <ps@pks.im> writes:

> +static int alt_odb_usable(struct object_database *o, const char *path,
> +			  const char *normalized_objdir)
>  {
>  	int r;
>  
>  	/* Detect cases where alternate disappeared */
> -	if (!is_directory(path->buf)) {
> +	if (!is_directory(path)) {
>  		error(_("object directory %s does not exist; "
>  			"check .git/objects/info/alternates"),
> -		      path->buf);
> +		      path);
>  		return 0;
>  	}
>  
> @@ -113,11 +112,14 @@ static int alt_odb_usable(struct object_database *o,
>  		assert(r == 1); /* never used */
>  		kh_value(o->source_by_path, p) = o->sources;
>  	}
> -	if (fspatheq(path->buf, normalized_objdir))
> +
> +	if (fspatheq(path, normalized_objdir))
> +		return 0;
> +
> +	if (kh_get_odb_path_map(o->source_by_path, path) < kh_end(o->source_by_path))
>  		return 0;
> -	*pos = kh_put_odb_path_map(o->source_by_path, path->buf, &r);
> -	/* r: 0 = exists, 1 = never used, 2 = deleted */
> -	return r == 0 ? 0 : 1;
> +
> +	return 1;
>  }

In other words, when we say "we ask if this is usable", it says "yes
it is usable" or "no it is not", but ...

> @@ -148,6 +150,7 @@ static struct odb_source *link_alt_odb_entry(struct object_database *odb,
>  	struct strbuf pathbuf = STRBUF_INIT;
>  	struct strbuf tmp = STRBUF_INIT;
>  	khiter_t pos;
> +	int ret;
>  
>  	if (!is_absolute_path(dir) && relative_base) {
>  		strbuf_realpath(&pathbuf, relative_base, 1);
> @@ -172,20 +175,21 @@ static struct odb_source *link_alt_odb_entry(struct object_database *odb,
>  	strbuf_reset(&tmp);
>  	strbuf_realpath(&tmp, odb->sources->path, 1);
>  
> -	if (!alt_odb_usable(odb, &pathbuf, tmp.buf, &pos))
> +	if (!alt_odb_usable(odb, pathbuf.buf, tmp.buf))
>  		goto error;
>  
>  	CALLOC_ARRAY(alternate, 1);
>  	alternate->odb = odb;
>  	alternate->local = false;
> -	/* pathbuf.buf is already in r->objects->source_by_path */
>  	alternate->path = strbuf_detach(&pathbuf, NULL);
>  
>  	/* add the alternate entry */
>  	*odb->sources_tail = alternate;
>  	odb->sources_tail = &(alternate->next);
> -	alternate->next = NULL;
> -	assert(odb->source_by_path);
> +
> +	pos = kh_put_odb_path_map(odb->source_by_path, alternate->path, &ret);
> +	if (!ret)
> +		BUG("source must not yet exist");

... responding to the answer and saying "ok, if it is usable, then
let's use it" is a responsibility of the caller.  Which makes sense.

>  	kh_value(odb->source_by_path, pos) = alternate;
>  
>  	/* recursively add alternates */