Re: [PATCH 3/6] introduce pack metadata cache files

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> +static void write_meta_header(struct metapack_writer *mw, const char *id,
> +			      uint32_t version)
> +{
> +	version = htonl(version);
> +
> +	sha1write(mw->out, "META", 4);
> +	sha1write(mw->out, "\0\0\0\1", 4);
> +	sha1write(mw->out, mw->pack->sha1, 20);
> +	sha1write(mw->out, id, 4);
> +	sha1write(mw->out, &version, 4);
> +}

It seems that you are very close to actually having a plumbing that
could also do the pack .idx files.  Until/unless that can be done, I
am not sure how much benefit we would be getting from a file format
that records a subtype "id" and a generic "META" type, instead of
just a single "id" as the type ehader.  But it is OK to use 8 extra
bytes if we can potentially gain something later.

Shouldn't id be validated with at least something like

	if (strlen(id) < 3)
		die("Bad id: %s", id);

to catch a call

	write_meta_header(&mw, "me", 47);

that will stuff 'm', 'e', NUL and the garbage the compiler/linker
combo has placed after that constant string in the 4-byte id field?

> +void metapack_writer_init(struct metapack_writer *mw,
> +			  const char *pack_idx,
> +			  const char *name,
> +			  int version)
> +{
> +	struct strbuf path = STRBUF_INIT;
> +
> +	memset(mw, 0, sizeof(*mw));
> +
> +	mw->pack = add_packed_git(pack_idx, strlen(pack_idx), 1);
> +	if (!mw->pack || open_pack_index(mw->pack))
> +		die("unable to open packfile '%s'", pack_idx);
> +
> +	strbuf_addstr(&path, pack_idx);
> +	strbuf_chompstr(&path, ".idx");
> +	strbuf_addch(&path, '.');
> +	strbuf_addstr(&path, name);

Your chompstr() does not even validate if the given name ends with
".idx", so this sounds like a glorified way to say

	strbuf_splice(&path, path->len - strlen("idx"), strlen("idx"),
			 name, strlen(name));

to me.

> +	mw->path = strbuf_detach(&path, NULL);
> +
> +	mw->out = create_meta_tmp();
> +	write_meta_header(mw, name, version);
> +}
> +
> +void metapack_writer_finish(struct metapack_writer *mw)
> +{
> +	const char *tmp = mw->out->name;
> +
> +	sha1close(mw->out, NULL, CSUM_FSYNC);
> +	if (rename(tmp, mw->path))
> +		die_errno("unable to rename temporary metapack file");

Who is responsible for running adjust_shared_perm()?  The caller, or
this function?