reftable: explicitly handle hash format IDs

The hash format IDs are used for two different things across the reftable codebase: - They are used as a 32 bit unsigned integer when reading and writing the header in order to identify the hash function. - They are used internally to identify which hash function is in use. When one only considers the second usecase one might think that one can easily change the representation of those hash IDs. But because those IDs end up in the reftable header and footer on disk it is important that those never change. Create separate constants `REFTABLE_FORMAT_ID_*` and use them in contexts where we read or write reftable headers. This serves multiple purposes: - It allows us to more easily discern cases where we actually use those constants for the on-disk format. - It detangles us from the same constants that are defined in libgit.a, which is another required step to convert the reftable library to become standalone. - It makes the next step easier where we stop using `GIT_*_FORMAT_ID` constants in favor of a custom enum. Signed-off-by: Patrick Steinhardt <[email protected]> Signed-off-by: Taylor Blau <[email protected]>
git · carlajarchuleta1-a11y · Aug 6, 2024 · Aug 6, 2024 · Aug 6, 2024 · Aug 6, 2024
commit ac90e9f0b171cf91f2c103e9c77de919d8cd42c6
diff --git a/reftable/basics.h b/reftable/basics.h
@@ -150,4 +150,11 @@ int common_prefix_size(struct reftable_buf *a, struct reftable_buf *b);
 
 int hash_size(uint32_t id);
 
+/*
+ * Format IDs that identify the hash function used by a reftable. Note that
+ * these constants end up on disk and thus mustn't change.
+ */
+#define REFTABLE_FORMAT_ID_SHA1   ((uint32_t) 0x73686131)
+#define REFTABLE_FORMAT_ID_SHA256 ((uint32_t) 0x73323536)
+
 #endif
diff --git a/reftable/reader.c b/reftable/reader.c
@@ -109,16 +109,18 @@ static int parse_footer(struct reftable_reader *r, uint8_t *footer,
 	if (r->version == 1) {
 		r->hash_id = GIT_SHA1_FORMAT_ID;
 	} else {
-		r->hash_id = get_be32(f);
-		switch (r->hash_id) {
-		case GIT_SHA1_FORMAT_ID:
+		switch (get_be32(f)) {
+		case REFTABLE_FORMAT_ID_SHA1:
+			r->hash_id = GIT_SHA1_FORMAT_ID;
 			break;
-		case GIT_SHA256_FORMAT_ID:
+		case REFTABLE_FORMAT_ID_SHA256:
+			r->hash_id = GIT_SHA256_FORMAT_ID;
 			break;
 		default:
 			err = REFTABLE_FORMAT_ERROR;
 			goto done;
 		}
+
 		f += 4;
 	}
 

diff --git a/reftable/writer.c b/reftable/writer.c
@@ -103,8 +103,22 @@ static int writer_write_header(struct reftable_writer *w, uint8_t *dest)
 	put_be64(dest + 8, w->min_update_index);
 	put_be64(dest + 16, w->max_update_index);
 	if (writer_version(w) == 2) {
-		put_be32(dest + 24, w->opts.hash_id);
+		uint32_t hash_id;
+
+		switch (w->opts.hash_id) {
+		case GIT_SHA1_FORMAT_ID:
+			hash_id = REFTABLE_FORMAT_ID_SHA1;
+			break;
+		case GIT_SHA256_FORMAT_ID:
+			hash_id = REFTABLE_FORMAT_ID_SHA256;
+			break;
+		default:
+			return -1;
+		}
+
+		put_be32(dest + 24, hash_id);
 	}
+
 	return header_size(writer_version(w));
 }