reftable/block: avoid decoding keys when searching restart points

When searching over restart points in a block we decode the key of each
of the records, which results in a memory allocation. This is quite
pointless though given that records it restart points will never use
prefix compression and thus store their keys verbatim in the block.

Refactor the code so that we can avoid decoding the keys, which saves us
some allocations.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

reftable/block.c

@@ -287,23 +287,32 @@ static int restart_needle_less(size_t idx, void *_args)
 		.buf = args->reader->block.data + off,
 		.len = args->reader->block_len - off,
 	};
-	struct strbuf kth_restart_key = STRBUF_INIT;
-	uint8_t unused_extra;
-	int result, n;
+	uint64_t prefix_len, suffix_len;
+	uint8_t extra;
+	int n;
 
 	/*
-	 * TODO: The restart key is verbatim in the block, so we can in theory
-	 * avoid decoding the key and thus save some allocations.
+	 * Records at restart points are stored without prefix compression, so
+	 * there is no need to fully decode the record key here. This removes
+	 * the need for allocating memory.
 	 */
-	n = reftable_decode_key(&kth_restart_key, &unused_extra, in);
-	if (n < 0) {
+	n = reftable_decode_keylen(in, &prefix_len, &suffix_len, &extra);
+	if (n < 0 || prefix_len) {
 		args->error = 1;
 		return -1;
 	}
 
-	result = strbuf_cmp(&args->needle, &kth_restart_key);
-	strbuf_release(&kth_restart_key);
-	return result < 0;
+	string_view_consume(&in, n);
+	if (suffix_len > in.len) {
+		args->error = 1;
+		return -1;
+	}
+
+	n = memcmp(args->needle.buf, in.buf,
+		   args->needle.len < suffix_len ? args->needle.len : suffix_len);
+	if (n)
+		return n < 0;
+	return args->needle.len < suffix_len;
 }
 
 void block_iter_copy_from(struct block_iter *dest, struct block_iter *src)