reftable: add blocksource, an abstraction for random access reads

The reftable format is usually used with files for storage. However, we abstract
away this using the blocksource data structure. This has two advantages:

* log blocks are zlib compressed, and handling them is simplified if we can
  discard byte segments from within the block layer.

* for unittests, it is useful to read and write in-memory. The blocksource
  allows us to abstract the data away from on-disk files.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 49 insertions, 0 deletions

diff --git a/reftable/reftable-blocksource.h b/reftable/reftable-blocksource.h
new file mode 100644
index 0000000000..5aa3990a57
--- /dev/null
+++ b/reftable/reftable-blocksource.h
@@ -0,0 +1,49 @@
+/*
+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_BLOCKSOURCE_H
+#define REFTABLE_BLOCKSOURCE_H
+
+#include <stdint.h>
+
+/* block_source is a generic wrapper for a seekable readable file.
+ */
+struct reftable_block_source {
+	struct reftable_block_source_vtable *ops;
+	void *arg;
+};
+
+/* a contiguous segment of bytes. It keeps track of its generating block_source
+ * so it can return itself into the pool. */
+struct reftable_block {
+	uint8_t *data;
+	int len;
+	struct reftable_block_source source;
+};
+
+/* block_source_vtable are the operations that make up block_source */
+struct reftable_block_source_vtable {
+	/* returns the size of a block source */
+	uint64_t (*size)(void *source);
+
+	/* reads a segment from the block source. It is an error to read
+	   beyond the end of the block */
+	int (*read_block)(void *source, struct reftable_block *dest,
+			  uint64_t off, uint32_t size);
+	/* mark the block as read; may return the data back to malloc */
+	void (*return_block)(void *source, struct reftable_block *blockp);
+
+	/* release all resources associated with the block source */
+	void (*close)(void *source);
+};
+
+/* opens a file on the file system as a block_source */
+int reftable_block_source_from_file(struct reftable_block_source *block_src,
+				    const char *name);
+
+#endif