Re: [RFC PATCH] parallel-checkout: send the new object_id algo field to the workers

On 2021-05-14 at 14:36:00, Matheus Tavares wrote:

An object_id storing a SHA-1 name has some unused bytes at the end of
the hash array. Since these bytes are not used, they are usually not
initialized to any value either. However, at
parallel_checkout.c:send_one_item() the object_id of a cache entry is
copied into a buffer which is later sent to a checkout worker through a
pipe write(). This makes Valgrind complain about passing uninitialized
bytes to a syscall. The worker won't use these uninitialized bytes
either, but the warning could confuse someone trying to debug this code;
So instead of using oidcpy(), send_one_item() uses hashcpy() to only
copy the used/initialized bytes of the object_id, and leave the
remaining part with zeros.

However, since cf0983213c ("hash: add an algo member to struct
object_id", 2021-04-26), using hashcpy() is no longer sufficient here as
it won't copy the new algo field from the object_id. Let's add and use a
new function which meets both our requirements of copying all the
important object_id data while still avoiding the uninitialized bytes,
by padding the end of the hash array in the destination object_id. With
this change, we also no longer need the destination buffer from
send_one_item() to be initialized with zeros, so let's switch from
xcalloc() to xmalloc() to make this clear.

Signed-off-by: Matheus Tavares <redacted>
---

Hi, brian

I've read the hash transition plan, but I'm not confident to say that I
fully understand it yet, so maybe this patch is not exactly what we need
here. Mainly, I'm not sure I understand in which cases we will have an
object_id.algo that is not the_hash_algo. Is it for the early transition
phases, where we have a SHA-256 repo that accepts user input as SHA-1? 

Yes, that's correct, as well as for interoperability with remotes using
a different hash algorithm.

Also, the object_id's copied here at send_one_item() always come from a
`struct cache_entry`. In this case, can they still have different
`algo`s or do we expect them to be the_hash_algo?

No, things in the index should always use the same algorithm..

The patch looks fine to me.
-- 
brian m. carlson (he/him or they/them)
Houston, Texas, US