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

[Matheus Tavares <matheus.bernardino@usp.br>]

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 <matheus.bernardino@usp.br>
---

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? 

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?

Thanks!

 hash.h              | 16 ++++++++++++++++
 parallel-checkout.c | 13 ++++++-------
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/hash.h b/hash.h
index 2986f991c6..9c6df4d952 100644
--- a/hash.h
+++ b/hash.h
@@ -263,6 +263,22 @@ static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 	dst->algo = src->algo;
 }
 
+/* Like oidcpy() but zero-pads the unused bytes in dst's hash array. */
+static inline void oidcpy_with_padding(struct object_id *dst,
+				       struct object_id *src)
+{
+	size_t hashsz;
+
+	if (!src->algo)
+		hashsz = the_hash_algo->rawsz;
+	else
+		hashsz = hash_algos[src->algo].rawsz;
+
+	memcpy(dst->hash, src->hash, hashsz);
+	memset(dst->hash + hashsz, 0, GIT_MAX_RAWSZ - hashsz);
+	dst->algo = src->algo;
+}
+
 static inline struct object_id *oiddup(const struct object_id *src)
 {
 	struct object_id *dst = xmalloc(sizeof(struct object_id));
diff --git a/parallel-checkout.c b/parallel-checkout.c
index 09e8b10a35..684cbb9ab4 100644
--- a/parallel-checkout.c
+++ b/parallel-checkout.c
@@ -393,7 +393,7 @@ static void send_one_item(int fd, struct parallel_checkout_item *pc_item)
 	len_data = sizeof(struct pc_item_fixed_portion) + name_len +
 		   working_tree_encoding_len;
 
-	data = xcalloc(1, len_data);
+	data = xmalloc(len_data);
 
 	fixed_portion = (struct pc_item_fixed_portion *)data;
 	fixed_portion->id = pc_item->id;
@@ -403,13 +403,12 @@ static void send_one_item(int fd, struct parallel_checkout_item *pc_item)
 	fixed_portion->name_len = name_len;
 	fixed_portion->working_tree_encoding_len = working_tree_encoding_len;
 	/*
-	 * We use hashcpy() instead of oidcpy() because the hash[] positions
-	 * after `the_hash_algo->rawsz` might not be initialized. And Valgrind
-	 * would complain about passing uninitialized bytes to a syscall
-	 * (write(2)). There is no real harm in this case, but the warning could
-	 * hinder the detection of actual errors.
+	 * We pad the unused bytes in the hash array because, otherwise,
+	 * Valgrind would complain about passing uninitialized bytes to a
+	 * write() syscall. The warning doesn't represent any real risk here,
+	 * but it could hinder the detection of actual errors.
 	 */
-	hashcpy(fixed_portion->oid.hash, pc_item->ce->oid.hash);
+	oidcpy_with_padding(&fixed_portion->oid, &pc_item->ce->oid);
 
 	variant = data + sizeof(*fixed_portion);
 	if (working_tree_encoding_len) {
-- 
2.30.1