chunk-format: create chunk format write API

In anticipation of combining the logic from the commit-graph and
multi-pack-index file formats, create a new chunk-format API. Use a
'struct chunkfile' pointer to keep track of data that has been
registered for writes. This struct is anonymous outside of
chunk-format.c to ensure no user attempts to interfere with the data.

The next change will use this API in commit-graph.c, but the general
approach is:

 1. initialize the chunkfile with init_chunkfile(f).
 2. add chunks in the intended writing order with add_chunk().
 3. write any header information to the hashfile f.
 4. write the chunkfile data using write_chunkfile().
 5. free the chunkfile struct using free_chunkfile().

Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Makefile b/Makefile
index 7b64106930a615c2e867a061f94cd6d3ea834641..50a7663841e93dc6402bf5666ac7111a08054dfb 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -854,6 +854,7 @@ LIB_OBJS += bundle.o
 LIB_OBJS += cache-tree.o
 LIB_OBJS += chdir-notify.o
 LIB_OBJS += checkout.o
+LIB_OBJS += chunk-format.o
 LIB_OBJS += color.o
 LIB_OBJS += column.o
 LIB_OBJS += combine-diff.o

diff --git a/chunk-format.c b/chunk-format.c
new file mode 100644 (file)
index 0000000..6c9b52b
--- /dev/null
+++ b/chunk-format.c
@@ -0,0 +1,90 @@
+#include "cache.h"
+#include "chunk-format.h"
+#include "csum-file.h"
+
+/*
+ * When writing a chunk-based file format, collect the chunks in
+ * an array of chunk_info structs. The size stores the _expected_
+ * amount of data that will be written by write_fn.
+ */
+struct chunk_info {
+       uint32_t id;
+       uint64_t size;
+       chunk_write_fn write_fn;
+};
+
+struct chunkfile {
+       struct hashfile *f;
+
+       struct chunk_info *chunks;
+       size_t chunks_nr;
+       size_t chunks_alloc;
+};
+
+struct chunkfile *init_chunkfile(struct hashfile *f)
+{
+       struct chunkfile *cf = xcalloc(1, sizeof(*cf));
+       cf->f = f;
+       return cf;
+}
+
+void free_chunkfile(struct chunkfile *cf)
+{
+       if (!cf)
+               return;
+       free(cf->chunks);
+       free(cf);
+}
+
+int get_num_chunks(struct chunkfile *cf)
+{
+       return cf->chunks_nr;
+}
+
+void add_chunk(struct chunkfile *cf,
+              uint32_t id,
+              size_t size,
+              chunk_write_fn fn)
+{
+       ALLOC_GROW(cf->chunks, cf->chunks_nr + 1, cf->chunks_alloc);
+
+       cf->chunks[cf->chunks_nr].id = id;
+       cf->chunks[cf->chunks_nr].write_fn = fn;
+       cf->chunks[cf->chunks_nr].size = size;
+       cf->chunks_nr++;
+}
+
+int write_chunkfile(struct chunkfile *cf, void *data)
+{
+       int i;
+       uint64_t cur_offset = hashfile_total(cf->f);
+
+       /* Add the table of contents to the current offset */
+       cur_offset += (cf->chunks_nr + 1) * CHUNK_TOC_ENTRY_SIZE;
+
+       for (i = 0; i < cf->chunks_nr; i++) {
+               hashwrite_be32(cf->f, cf->chunks[i].id);
+               hashwrite_be64(cf->f, cur_offset);
+
+               cur_offset += cf->chunks[i].size;
+       }
+
+       /* Trailing entry marks the end of the chunks */
+       hashwrite_be32(cf->f, 0);
+       hashwrite_be64(cf->f, cur_offset);
+
+       for (i = 0; i < cf->chunks_nr; i++) {
+               off_t start_offset = hashfile_total(cf->f);
+               int result = cf->chunks[i].write_fn(cf->f, data);
+
+               if (result)
+                       return result;
+
+               if (hashfile_total(cf->f) - start_offset != cf->chunks[i].size)
+                       BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
+                           cf->chunks[i].size, cf->chunks[i].id,
+                           hashfile_total(cf->f) - start_offset);
+       }
+
+       return 0;
+}

diff --git a/chunk-format.h b/chunk-format.h
new file mode 100644 (file)
index 0000000..ce598b6
--- /dev/null
+++ b/chunk-format.h
@@ -0,0 +1,21 @@
+#ifndef CHUNK_FORMAT_H
+#define CHUNK_FORMAT_H
+
+#include "git-compat-util.h"
+
+struct hashfile;
+struct chunkfile;
+
+#define CHUNK_TOC_ENTRY_SIZE (sizeof(uint32_t) + sizeof(uint64_t))
+
+struct chunkfile *init_chunkfile(struct hashfile *f);
+void free_chunkfile(struct chunkfile *cf);
+int get_num_chunks(struct chunkfile *cf);
+typedef int (*chunk_write_fn)(struct hashfile *f, void *data);
+void add_chunk(struct chunkfile *cf,
+              uint32_t id,
+              size_t size,
+              chunk_write_fn fn);
+int write_chunkfile(struct chunkfile *cf, void *data);
+
+#endif