[RFC PATCH 0/2] Alternate ab/valgrind-fixes fix-up

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Mon, May 16 2022, Derrick Stolee wrote:

> On 5/12/2022 7:39 PM, Junio C Hamano wrote:
> [...]
> This switch statement was recently added to make it clear that
> unpack_loose_header() returns an enum value, not an int. This adds
> complications for future developers if that enum gains new values, since
> that developer would need to add a case statement to this switch for
> little real value.
>
> Instead, we can revert back to an 'if' statement, but make the enum
> explicit by using "!= ULHR_OK" instead of assuming it has the numerical
> value zero.
>
> Co-authored-by: Junio C Hamano <gitster@pobox.com>
> Signed-off-by: Derrick Stolee <derrickstolee@github.com>
> ---
>
>  object-file.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/object-file.c b/object-file.c
> index b5d1d12b68a..52e4ae1b5f0 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -2623,12 +2623,8 @@ int read_loose_object(const char *path,
>  		goto out;
>  	}
>  
> -	switch (unpack_loose_header(&stream, map, mapsize, hdr, sizeof(hdr),
> -				    NULL)) {
> -	case ULHR_OK:
> -		break;
> -	case ULHR_BAD:
> -	case ULHR_TOO_LONG:
> +	if (unpack_loose_header(&stream, map, mapsize, hdr, sizeof(hdr),
> +				NULL) != ULHR_OK) {
>  		error(_("unable to unpack header of %s"), path);
>  		goto out;
>  	}

This whole topic-at-large is a stylistic fix-up for a case where I
obviously got it wrong, so take this with a double grain of salt.

Re the "What's Cooking" mention of
ds/object-file-unpack-loose-header-fix: I don't mind it being merged
down at all. The below is all small potatoes.

I don't think the rationale ("adds complications for future
developers") makes sense in this case.

Let's leave aside the question of whether we exhaustively check enum
arms as in the pre-image, or check "not ok" as in the post-image.

Surely we can agree that whatever pattern is preferred we're better
off consistently picking one or the other?

I think this proposed change would make more sense and be in line with
its commit message if it also proposed this:
	
	diff --git a/streaming.c b/streaming.c
	index fe54665d86e..bb4ed198463 100644
	--- a/streaming.c
	+++ b/streaming.c
	@@ -230,15 +230,10 @@ static int open_istream_loose(struct git_istream *st, struct repository *r,
	 	st->u.loose.mapped = map_loose_object(r, oid, &st->u.loose.mapsize);
	 	if (!st->u.loose.mapped)
	 		return -1;
	-	switch (unpack_loose_header(&st->z, st->u.loose.mapped,
	-				    st->u.loose.mapsize, st->u.loose.hdr,
	-				    sizeof(st->u.loose.hdr), NULL)) {
	-	case ULHR_OK:
	-		break;
	-	case ULHR_BAD:
	-	case ULHR_TOO_LONG:
	+	if (unpack_loose_header(&st->z, st->u.loose.mapped,
	+				st->u.loose.mapsize, st->u.loose.hdr,
	+				sizeof(st->u.loose.hdr), NULL) != ULHR_OK)
	 		goto error;
	-	}
	 	if (parse_loose_header(st->u.loose.hdr, &oi) < 0 || *type < 0)
	 		goto error;

I.e. now we've converted the 2/3 callers of the API that only cared
about "not OK", there's a third one that cares about all the enum arms
currently, so that one remains a "switch".

The reason I think the rationale doesn't make sense is because of this
inconsistency. I.e. if we suppose a developer adds another enum value,
they'll then discover those three callers.

Surely whatever our preference for how to handle those 2/3 callers
it's less complicated if they don't use different patterns for no
obvious reason.

But anyway. Looking a bit deeper at this code again I think these two
patches are where we'd eventually want to head with this API. I.e. I
think the whole business of making this a tri-state return was
premature on my part.

After this RFC unpack_loose_header() is again a function that returns
a negative value on error, and the enum is gone. As noted in 2/2
there's a slight trade-off there, which I think is for the better,
both in terms of API simplicity, and in the new "error" output we'll
omit in these obscure cases. I.e.:

	-				error: header for $bogus_long_sha1 too long, exceeds 32 bytes
	+				error: header too long, exceeds 32 bytes
	+				error: unable to unpack $bogus_long_sha1 header

This whole "switch" complexity was because the old error message
wanted to note the OID in the "header too long" message.

Again, I'm perfectly fine with ds/object-file-unpack-loose-header-fix
advancing to "next", I can rebase this on top, or drop it depending on
the consensus about whether it's worthwile. I did want to un-block
that topic one way or the other, so to the extent that it was waiting
on my feedback...

Ævar Arnfjörð Bjarmason (2):
  object-file API: fix obscure unpack_loose_header() return
  object-file API: have unpack_loose_header() return "int" again

 cache.h             | 25 +++++-------------------
 object-file.c       | 46 +++++++++++++++++----------------------------
 streaming.c         | 11 +++--------
 t/t1006-cat-file.sh |  6 ++++--
 4 files changed, 29 insertions(+), 59 deletions(-)

-- 
2.36.1.957.g2c13267e09b