reftable/block: create public interface for reading blocks

While users of the reftable library wouldn't generally require access to
individual blocks in a reftable table, there are valid usecases where
one may require low-level access to them. One such upcoming usecase in
the Git codebase is to implement consistency checks for the reftable
library where we want to verify each block individually.

Create a public interface for reading blocks. The interface isn't yet
complete and lacks e.g. a way to read individual records from a block.
Such missing functionality will be backfilled in subsequent commits.

Note that this change also requires us to expose `reftable_buf`, which
is used by the `reftable_block_first_key()` function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 69 insertions, 0 deletions

diff --git a/reftable/reftable-block.h b/reftable/reftable-block.h
new file mode 100644
index 0000000000..13bd68be8c
--- /dev/null
+++ b/reftable/reftable-block.h
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Use of this source code is governed by a BSD-style
+ * license that can be found in the LICENSE file or at
+ * https://developers.google.com/open-source/licenses/bsd
+ */
+
+#ifndef REFTABLE_BLOCK_H
+#define REFTABLE_BLOCK_H
+
+#include <stdint.h>
+
+#include "reftable-basics.h"
+#include "reftable-blocksource.h"
+
+struct z_stream_s;
+
+/*
+ * A block part of a reftable. Contains records as well as some metadata
+ * describing them.
+ */
+struct reftable_block {
+	/*
+	 * Offset of the block header; nonzero for the first block in a
+	 * reftable.
+	 */
+	uint32_t header_off;
+
+	/* The memory block. */
+	struct reftable_block_data block_data;
+	uint32_t hash_size;
+
+	/* Uncompressed data for log entries. */
+	struct z_stream_s *zstream;
+	unsigned char *uncompressed_data;
+	size_t uncompressed_cap;
+
+	/*
+	 * Restart point data. Restart points are located after the block's
+	 * record data.
+	 */
+	uint16_t restart_count;
+	uint32_t restart_off;
+
+	/*
+	 * Size of the data in the file. For log blocks, this is the compressed
+	 * size.
+	 */
+	uint32_t full_block_size;
+	uint8_t block_type;
+};
+
+/* Initialize a reftable block from the given block source. */
+int reftable_block_init(struct reftable_block *b,
+			struct reftable_block_source *source,
+			uint32_t offset, uint32_t header_size,
+			uint32_t table_block_size, uint32_t hash_size);
+
+/* Release resources allocated by the block. */
+void reftable_block_release(struct reftable_block *b);
+
+/* Returns the block type (eg. 'r' for refs). */
+uint8_t reftable_block_type(const struct reftable_block *b);
+
+/* Decodes the first key in the block. */
+int reftable_block_first_key(const struct reftable_block *b, struct reftable_buf *key);
+
+#endif /* REFTABLE_BLOCK_H */