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[PATCH 05/23] midx: write header information to lockfile




As we begin writing the multi-pack-index format to disk, start with
the basics: the 12-byte header and the 20-byte checksum footer. Start
with these basics so we can add the rest of the format in small
increments.

As we implement the format, we will use a technique to check that our
computed offsets within the multi-pack-index file match what we are
actually writing. Each method that writes to the hashfile will return
the number of bytes written, and we will track that those values match
our expectations.

Currently, write_midx_header() returns 12, but is not checked. We will
check the return value in a later commit.

Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
---
 midx.c          | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 t/t5319-midx.sh |  5 +++--
 2 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/midx.c b/midx.c
index 616af66b13..3e55422a21 100644
--- a/midx.c
+++ b/midx.c
@@ -1,9 +1,62 @@
 #include "git-compat-util.h"
 #include "cache.h"
 #include "dir.h"
+#include "csum-file.h"
+#include "lockfile.h"
 #include "midx.h"
 
+#define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
+#define MIDX_VERSION 1
+#define MIDX_HASH_VERSION 1 /* SHA-1 */
+#define MIDX_HEADER_SIZE 12
+
+static char *get_midx_filename(const char *object_dir)
+{
+	struct strbuf midx_name = STRBUF_INIT;
+	strbuf_addstr(&midx_name, object_dir);
+	strbuf_addstr(&midx_name, "/pack/multi-pack-index");
+	return strbuf_detach(&midx_name, NULL);
+}
+
+static size_t write_midx_header(struct hashfile *f,
+				unsigned char num_chunks,
+				uint32_t num_packs)
+{
+	char byte_values[4];
+	hashwrite_be32(f, MIDX_SIGNATURE);
+	byte_values[0] = MIDX_VERSION;
+	byte_values[1] = MIDX_HASH_VERSION;
+	byte_values[2] = num_chunks;
+	byte_values[3] = 0; /* unused */
+	hashwrite(f, byte_values, sizeof(byte_values));
+	hashwrite_be32(f, num_packs);
+
+	return MIDX_HEADER_SIZE;
+}
+
 int write_midx_file(const char *object_dir)
 {
+	unsigned char num_chunks = 0;
+	uint32_t num_packs = 0;
+	char *midx_name;
+	struct hashfile *f;
+	struct lock_file lk;
+
+	midx_name = get_midx_filename(object_dir);
+	if (safe_create_leading_directories(midx_name)) {
+		UNLEAK(midx_name);
+		die_errno(_("unable to create leading directories of %s"),
+			  midx_name);
+	}
+
+	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
+	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
+	FREE_AND_NULL(midx_name);
+
+	write_midx_header(f, num_chunks, num_packs);
+
+	finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	commit_lock_file(&lk);
+
 	return 0;
 }
diff --git a/t/t5319-midx.sh b/t/t5319-midx.sh
index a590137af7..80f9389837 100755
--- a/t/t5319-midx.sh
+++ b/t/t5319-midx.sh
@@ -3,8 +3,9 @@
 test_description='multi-pack-indexes'
 . ./test-lib.sh
 
-test_expect_success 'write midx with no pakcs' '
-	git midx --object-dir=. write
+test_expect_success 'write midx with no packs' '
+	git midx --object-dir=. write &&
+	test_path_is_file pack/multi-pack-index
 '
 
 test_done
-- 
2.18.0.rc1