Subject: [PATCH v2 02/17] chunk-format: create chunk format write API

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Wed, 27 Jan 2021 15:01:41 +0000

From: Derrick Stolee <[email-masked]>

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 <[email-masked]>
Signed-off-by: Derrick Stolee <[email-masked]>
---
 Makefile       |  1 +
 chunk-format.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++
 chunk-format.h | 20 +++++++++++
 3 files changed, 112 insertions(+)
 create mode 100644 chunk-format.c
 create mode 100644 chunk-format.h

diff --git a/Makefile b/Makefile
index 7b64106930a..50a7663841e 100644
--- 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
index 00000000000..ab914c55856
--- /dev/null
+++ b/chunk-format.c
@@ -0,0 +1,91 @@
+#include "cache.h"
+#include "chunk-format.h"
+#include "csum-file.h"
+#define CHUNK_LOOKUP_WIDTH 12
+
+/*
+ * 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,
+	       uint64_t id,
+	       chunk_write_fn fn,
+	       size_t size)
+{
+	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;
+	size_t cur_offset = cf->f->offset + cf->f->total;
+
+	/* Add the table of contents to the current offset */
+	cur_offset += (cf->chunks_nr + 1) * CHUNK_LOOKUP_WIDTH;
+
+	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++) {
+		uint64_t start_offset = cf->f->total + cf->f->offset;
+		int result = cf->chunks[i].write_fn(cf->f, data);
+
+		if (result)
+			return result;
+
+		if (cf->f->total + cf->f->offset - 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,
+			    cf->f->total + cf->f->offset - start_offset);
+	}
+
+	return 0;
+}
diff --git a/chunk-format.h b/chunk-format.h
new file mode 100644
index 00000000000..bfaed672813
--- /dev/null
+++ b/chunk-format.h
@@ -0,0 +1,20 @@
+#ifndef CHUNK_FORMAT_H
+#define CHUNK_FORMAT_H
+
+#include "git-compat-util.h"
+
+struct hashfile;
+struct chunkfile;
+
+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,
+	       uint64_t id,
+	       chunk_write_fn fn,
+	       size_t size);
+int write_chunkfile(struct chunkfile *cf, void *data);
+
+#endif
-- 
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