Re: [PATCH] describe: confirm that blobs actually exist

[Jeff King <peff@peff.net>]

On Mon, Feb 12, 2018 at 12:23:06PM -0500, Jeff King wrote:

> We can fix this by replacing the lookup_blob() call with a
> check of the true type via sha1_object_info(). This is not
> quite as efficient as we could possibly make this check. We
> know in most cases that the object was already parsed in the
> earlier commit lookup, so we could call lookup_object(),
> which does auto-create, and check the resulting struct's
> type (or NULL).  However it's not worth the fragility nor
> code complexity to save a single object lookup.

By the way, I did notice one other inefficiency here: we always call
lookup_commit_reference_gently() first, which will call parse_object().
So if you were to "git describe" an enormous blob, we'd load the whole
thing into memory for no purpose. We could structure this as:

  type = sha1_object_info(oid.hash, NULL);
  if (type == OBJ_BLOB)
          describe_blob(&oid);
  else if (lookup_commit_reference_gently(&oid, 1))
          describe_commit(&oid);
  else
          describe("neither commit nor blob");

That incurs an extra object lookup for the commit case, but potentially
saves reading the blob. We could have our cake and eat it, too, if
sha1_file.c had a function like "parse this object unless it's a blob,
in which case just fill in the type info".

Arguably that should be the default when parse_object() is called on a
blob, but I suspect some older code may rely on parse_object() to check
that the object is present and consistent.

Anyway, I don't know that it's really worth caring about too much, but
just something I noticed.

Maybe a #leftoverbits if somebody cares.

-Peff