hashfile: allow skipping the hash function

The hashfile API is useful for generating files that include a trailing
hash of the file's contents up to that point. Using such a hash is
helpful for verifying the file for corruption-at-rest, such as a faulty
drive causing flipped bits.

Git's index file includes this trailing hash, so it uses a 'struct
hashfile' to handle the I/O to the file. This was very convenient to
allow using the hashfile methods during these operations.

However, hashing the file contents during write comes at a performance
penalty. It's slower to hash the bytes on their way to the disk than
without that step. This problem is made worse by the replacement of
hardware-accelerated SHA1 computations with the software-based sha1dc
computation.

This write cost is significant, and the checksum capability is likely
not worth that cost for such a short-lived file. The index is rewritten
frequently and the only time the checksum is checked is during 'git
fsck'. Thus, it would be helpful to allow a user to opt-out of the hash
computation.

We first need to allow Git to opt-out of the hash computation in the
hashfile API. The buffered writes of the API are still helpful, so it
makes sense to make the change here.

Introduce a new 'skip_hash' option to 'struct hashfile'. When set, the
update_fn and final_fn members of the_hash_algo are skipped. When
finalizing the hashfile, the trailing hash is replaced with the null
hash.

This use of a trailing null hash would be desireable in either case,
since we do not want to special case a file format to have a different
length depending on whether it was hashed or not. When the final bytes
of a file are all zero, we can infer that it was written without
hashing, and thus that verification is not available as a check for file
consistency. This also means that we could easily toggle hashing for any
file format we desire.

A version of this patch has existed in the microsoft/git fork since
2017 [1] (the linked commit was rebased in 2018, but the original dates
back to January 2017). Here, the change to make the index use this fast
path is delayed until a later change.

[1] 21fed2d914

Co-authored-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

csum-file.c

@@ -45,7 +45,8 @@ void hashflush(struct hashfile *f)
 	unsigned offset = f->offset;
 
 	if (offset) {
-		the_hash_algo->update_fn(&f->ctx, f->buffer, offset);
+		if (!f->skip_hash)
+			the_hash_algo->update_fn(&f->ctx, f->buffer, offset);
 		flush(f, f->buffer, offset);
 		f->offset = 0;
 	}
@@ -64,7 +65,12 @@ int finalize_hashfile(struct hashfile *f, unsigned char *result,
 	int fd;
 
 	hashflush(f);
-	the_hash_algo->final_fn(f->buffer, &f->ctx);
+
+	if (f->skip_hash)
+		hashclr(f->buffer);
+	else
+		the_hash_algo->final_fn(f->buffer, &f->ctx);
+
 	if (result)
 		hashcpy(result, f->buffer);
 	if (flags & CSUM_HASH_IN_STREAM)
@@ -108,7 +114,8 @@ void hashwrite(struct hashfile *f, const void *buf, unsigned int count)
 			 * the hashfile's buffer. In this block,
 			 * f->offset is necessarily zero.
 			 */
-			the_hash_algo->update_fn(&f->ctx, buf, nr);
+			if (!f->skip_hash)
+				the_hash_algo->update_fn(&f->ctx, buf, nr);
 			flush(f, buf, nr);
 		} else {
 			/*
@@ -153,6 +160,7 @@ static struct hashfile *hashfd_internal(int fd, const char *name,
 	f->tp = tp;
 	f->name = name;
 	f->do_crc = 0;
+	f->skip_hash = 0;
 	the_hash_algo->init_fn(&f->ctx);
 
 	f->buffer_len = buffer_len;

csum-file.h

@@ -20,6 +20,13 @@ struct hashfile {
 	size_t buffer_len;
 	unsigned char *buffer;
 	unsigned char *check_buffer;
+
+	/**
+	 * If non-zero, skip_hash indicates that we should
+	 * not actually compute the hash for this hashfile and
+	 * instead only use it as a buffered write.
+	 */
+	int skip_hash;
 };
 
 /* Checkpoint */