Re: [PATCH 5/5] packfile: inline custom read_object()

From: Jeff King <peff@peff.net>
Date: Jan 12, 2023, 16:29 UTC
Message-ID: <Y8A1T5kzgqXV5vKr@coredump.intra.peff.net>
In-Reply-To: <230112.86o7r42k13.gmgdl@evledraar.gmail.com>

On Thu, Jan 12, 2023 at 10:01:28AM +0100, Ævar Arnfjörð Bjarmason wrote:

Show 22 quoted lines

> > -				base = read_object(r, &base_oid, &type, &base_size);
> > +
> > +				oi.typep = &type;
> > +				oi.sizep = &base_size;
> > +				oi.contentp = &base;
> > +				if (oid_object_info_extended(r, &base_oid, &oi, 0) < 0)
> > +					base = NULL;
> > +
> >  				external_base = base;
> >  			}
> >  		}
> 
> This isn't introducing a behavior difference, in fact it's diligently
> bending over backwards to preserve existing behavior, but I don't think
> we need to do so, and shouldn't have this "base = NULL" line.
> 
> Here we're within an "if" block where we tested that "base == NULL"
> (which is why we're trying to populate it)
> 
> Before when we had read_object() re-assigning to "base" here was the
> obvious thing to do, but now this seems like undue an incomplete
> paranoia.

I think it's the same paranoia that was in read_object(). There it catches the error and returns NULL, rather than the probably-NULL "content" (though to be fair, it simply did not initialize the pointer, so it would have had to do that to depend on it).

I agree it's probably being overly defensive. But I don't think oid_object_info_extended() makes any promises, and it's not completely clear to me if packed_object_info() could return a non-NULL entry here on an error (e.g., if packed_to_object_type() fails even after we pulled out the content).

So probably yes, we could depend on that (and if not, arguably we should be fixing oid_object_info_extended(), because we are probably leaking a buffer in that case). But we definitely shouldn't be doing it in the middle of another patch.

> If oid_object_info_extended() why can't we trust that it didn't touch
> our "base"? And if we can't trust that, why are we trusting that it left
> "type" and "base_size" untouched?

My assumption is that "base" gated access to "type" and "base_size". So as long as "!base", we do not look at the other two.

Show 22 quoted lines

> I think squashing this in would be much better:
> 	
> 	diff --git a/packfile.c b/packfile.c
> 	index 79e21ab18e7..f45017422a1 100644
> 	--- a/packfile.c
> 	+++ b/packfile.c
> 	@@ -1795,10 +1795,8 @@ void *unpack_entry(struct repository *r, struct packed_git *p, off_t obj_offset,
> 	 				oi.typep = &type;
> 	 				oi.sizep = &base_size;
> 	 				oi.contentp = &base;
> 	-				if (oid_object_info_extended(r, &base_oid, &oi, 0) < 0)
> 	-					base = NULL;
> 	-
> 	-				external_base = base;
> 	+				if (!oid_object_info_extended(r, &base_oid, &oi, 0))
> 	+					external_base = base;
> 	 			}
> 	 		}
> 
> Not only aren't we second-guessing that our "base" was left alone, we're
> using the return value of oid_object_info_extended() to guard that
> assignment to "external_base" instead (it's NULL at this point too).

I don't think we need to guard the assignment (we know it will be NULL if we saw an error). But sure, I don't mind if you want to do that simplification, but it should be on top if at all.

-Peff