Merge branch 'ds/chunked-file-api'

The common code to deal with "chunked file format" that is shared
by the multi-pack-index and commit-graph files have been factored
out, to help codepaths for both filetypes to become more robust.

* ds/chunked-file-api:
  commit-graph.c: display correct number of chunks when writing
  chunk-format: add technical docs
  chunk-format: restore duplicate chunk checks
  midx: use 64-bit multiplication for chunk sizes
  midx: use chunk-format read API
  commit-graph: use chunk-format read API
  chunk-format: create read chunk API
  midx: use chunk-format API in write_midx_internal()
  midx: drop chunk progress during write
  midx: return success/failure in chunk write methods
  midx: add num_large_offsets to write_midx_context
  midx: add pack_perm to write_midx_context
  midx: add entries to write_midx_context
  midx: use context in write_midx_pack_names()
  midx: rename pack_info to write_midx_context
  commit-graph: use chunk-format write API
  chunk-format: create chunk format write API
  commit-graph: anonymize data in chunk_write_fn

Documentation/technical/chunk-format.txt

@@ -0,0 +1,116 @@
+Chunk-based file formats
+========================
+
+Some file formats in Git use a common concept of "chunks" to describe
+sections of the file. This allows structured access to a large file by
+scanning a small "table of contents" for the remaining data. This common
+format is used by the `commit-graph` and `multi-pack-index` files. See
+link:technical/pack-format.html[the `multi-pack-index` format] and
+link:technical/commit-graph-format.html[the `commit-graph` format] for
+how they use the chunks to describe structured data.
+
+A chunk-based file format begins with some header information custom to
+that format. That header should include enough information to identify
+the file type, format version, and number of chunks in the file. From this
+information, that file can determine the start of the chunk-based region.
+
+The chunk-based region starts with a table of contents describing where
+each chunk starts and ends. This consists of (C+1) rows of 12 bytes each,
+where C is the number of chunks. Consider the following table:
+
+  | Chunk ID (4 bytes) | Chunk Offset (8 bytes) |
+  |--------------------|------------------------|
+  | ID[0]              | OFFSET[0]              |
+  | ...                | ...                    |
+  | ID[C]              | OFFSET[C]              |
+  | 0x0000             | OFFSET[C+1]            |
+
+Each row consists of a 4-byte chunk identifier (ID) and an 8-byte offset.
+Each integer is stored in network-byte order.
+
+The chunk identifier `ID[i]` is a label for the data stored within this
+fill from `OFFSET[i]` (inclusive) to `OFFSET[i+1]` (exclusive). Thus, the
+size of the `i`th chunk is equal to the difference between `OFFSET[i+1]`
+and `OFFSET[i]`. This requires that the chunk data appears contiguously
+in the same order as the table of contents.
+
+The final entry in the table of contents must be four zero bytes. This
+confirms that the table of contents is ending and provides the offset for
+the end of the chunk-based data.
+
+Note: The chunk-based format expects that the file contains _at least_ a
+trailing hash after `OFFSET[C+1]`.
+
+Functions for working with chunk-based file formats are declared in
+`chunk-format.h`. Using these methods provide extra checks that assist
+developers when creating new file formats.
+
+Writing chunk-based file formats
+--------------------------------
+
+To write a chunk-based file format, create a `struct chunkfile` by
+calling `init_chunkfile()` and pass a `struct hashfile` pointer. The
+caller is responsible for opening the `hashfile` and writing header
+information so the file format is identifiable before the chunk-based
+format begins.
+
+Then, call `add_chunk()` for each chunk that is intended for write. This
+populates the `chunkfile` with information about the order and size of
+each chunk to write. Provide a `chunk_write_fn` function pointer to
+perform the write of the chunk data upon request.
+
+Call `write_chunkfile()` to write the table of contents to the `hashfile`
+followed by each of the chunks. This will verify that each chunk wrote
+the expected amount of data so the table of contents is correct.
+
+Finally, call `free_chunkfile()` to clear the `struct chunkfile` data. The
+caller is responsible for finalizing the `hashfile` by writing the trailing
+hash and closing the file.
+
+Reading chunk-based file formats
+--------------------------------
+
+To read a chunk-based file format, the file must be opened as a
+memory-mapped region. The chunk-format API expects that the entire file
+is mapped as a contiguous memory region.
+
+Initialize a `struct chunkfile` pointer with `init_chunkfile(NULL)`.
+
+After reading the header information from the beginning of the file,
+including the chunk count, call `read_table_of_contents()` to populate
+the `struct chunkfile` with the list of chunks, their offsets, and their
+sizes.
+
+Extract the data information for each chunk using `pair_chunk()` or
+`read_chunk()`:
+
+* `pair_chunk()` assigns a given pointer with the location inside the
+  memory-mapped file corresponding to that chunk's offset. If the chunk
+  does not exist, then the pointer is not modified.
+
+* `read_chunk()` takes a `chunk_read_fn` function pointer and calls it
+  with the appropriate initial pointer and size information. The function
+  is not called if the chunk does not exist. Use this method to read chunks
+  if you need to perform immediate parsing or if you need to execute logic
+  based on the size of the chunk.
+
+After calling these methods, call `free_chunkfile()` to clear the
+`struct chunkfile` data. This will not close the memory-mapped region.
+Callers are expected to own that data for the timeframe the pointers into
+the region are needed.
+
+Examples
+--------
+
+These file formats use the chunk-format API, and can be used as examples
+for future formats:
+
+* *commit-graph:* see `write_commit_graph_file()` and `parse_commit_graph()`
+  in `commit-graph.c` for how the chunk-format API is used to write and
+  parse the commit-graph file format documented in
+  link:technical/commit-graph-format.html[the commit-graph file format].
+
+* *multi-pack-index:* see `write_midx_internal()` and `load_multi_pack_index()`
+  in `midx.c` for how the chunk-format API is used to write and
+  parse the multi-pack-index file format documented in
+  link:technical/pack-format.html[the multi-pack-index file format].

Documentation/technical/commit-graph-format.txt

@@ -61,6 +61,9 @@ CHUNK LOOKUP:
       the length using the next chunk position if necessary.) Each chunk
       ID appears at most once.
 
+  The CHUNK LOOKUP matches the table of contents from
+  link:technical/chunk-format.html[the chunk-based file format].
+
   The remaining data in the body is described one chunk at a time, and
   these chunks may be given in any order. Chunks are required unless
   otherwise specified.

Documentation/technical/pack-format.txt

@@ -336,6 +336,9 @@ CHUNK LOOKUP:
 	    (Chunks are provided in file-order, so you can infer the length
 	    using the next chunk position if necessary.)
 
+	The CHUNK LOOKUP matches the table of contents from
+	link:technical/chunk-format.html[the chunk-based file format].
+
 	The remaining data in the body is described one chunk at a time, and
 	these chunks may be given in any order. Chunks are required unless
 	otherwise specified.

Makefile

@@ -834,6 +834,7 @@ 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

chunk-format.c

@@ -0,0 +1,179 @@
+#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;
+
+	const void *start;
+};
+
+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;
+}
+
+int read_table_of_contents(struct chunkfile *cf,
+			   const unsigned char *mfile,
+			   size_t mfile_size,
+			   uint64_t toc_offset,
+			   int toc_length)
+{
+	int i;
+	uint32_t chunk_id;
+	const unsigned char *table_of_contents = mfile + toc_offset;
+
+	ALLOC_GROW(cf->chunks, toc_length, cf->chunks_alloc);
+
+	while (toc_length--) {
+		uint64_t chunk_offset, next_chunk_offset;
+
+		chunk_id = get_be32(table_of_contents);
+		chunk_offset = get_be64(table_of_contents + 4);
+
+		if (!chunk_id) {
+			error(_("terminating chunk id appears earlier than expected"));
+			return 1;
+		}
+
+		table_of_contents += CHUNK_TOC_ENTRY_SIZE;
+		next_chunk_offset = get_be64(table_of_contents + 4);
+
+		if (next_chunk_offset < chunk_offset ||
+		    next_chunk_offset > mfile_size - the_hash_algo->rawsz) {
+			error(_("improper chunk offset(s) %"PRIx64" and %"PRIx64""),
+			      chunk_offset, next_chunk_offset);
+			return -1;
+		}
+
+		for (i = 0; i < cf->chunks_nr; i++) {
+			if (cf->chunks[i].id == chunk_id) {
+				error(_("duplicate chunk ID %"PRIx32" found"),
+					chunk_id);
+				return -1;
+			}
+		}
+
+		cf->chunks[cf->chunks_nr].id = chunk_id;
+		cf->chunks[cf->chunks_nr].start = mfile + chunk_offset;
+		cf->chunks[cf->chunks_nr].size = next_chunk_offset - chunk_offset;
+		cf->chunks_nr++;
+	}
+
+	chunk_id = get_be32(table_of_contents);
+	if (chunk_id) {
+		error(_("final chunk has non-zero id %"PRIx32""), chunk_id);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int pair_chunk_fn(const unsigned char *chunk_start,
+			 size_t chunk_size,
+			 void *data)
+{
+	const unsigned char **p = data;
+	*p = chunk_start;
+	return 0;
+}
+
+int pair_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       const unsigned char **p)
+{
+	return read_chunk(cf, chunk_id, pair_chunk_fn, p);
+}
+
+int read_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       chunk_read_fn fn,
+	       void *data)
+{
+	int i;
+
+	for (i = 0; i < cf->chunks_nr; i++) {
+		if (cf->chunks[i].id == chunk_id)
+			return fn(cf->chunks[i].start, cf->chunks[i].size, data);
+	}
+
+	return CHUNK_NOT_FOUND;
+}

chunk-format.h

@@ -0,0 +1,68 @@
+#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))
+
+/*
+ * Initialize a 'struct chunkfile' for writing _or_ reading a file
+ * with the chunk format.
+ *
+ * If writing a file, supply a non-NULL 'struct hashfile *' that will
+ * be used to write.
+ *
+ * If reading a file, use a NULL 'struct hashfile *' and then call
+ * read_table_of_contents(). Supply the memory-mapped data to the
+ * pair_chunk() or read_chunk() methods, as appropriate.
+ *
+ * DO NOT MIX THESE MODES. Use different 'struct chunkfile' instances
+ * for reading and writing.
+ */
+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);
+
+int read_table_of_contents(struct chunkfile *cf,
+			   const unsigned char *mfile,
+			   size_t mfile_size,
+			   uint64_t toc_offset,
+			   int toc_length);
+
+#define CHUNK_NOT_FOUND (-2)
+
+/*
+ * Find 'chunk_id' in the given chunkfile and assign the
+ * given pointer to the position in the mmap'd file where
+ * that chunk begins.
+ *
+ * Returns CHUNK_NOT_FOUND if the chunk does not exist.
+ */
+int pair_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       const unsigned char **p);
+
+typedef int (*chunk_read_fn)(const unsigned char *chunk_start,
+			     size_t chunk_size, void *data);
+/*
+ * Find 'chunk_id' in the given chunkfile and call the
+ * given chunk_read_fn method with the information for
+ * that chunk.
+ *
+ * Returns CHUNK_NOT_FOUND if the chunk does not exist.
+ */
+int read_chunk(struct chunkfile *cf,
+	       uint32_t chunk_id,
+	       chunk_read_fn fn,
+	       void *data);
+
+#endif

commit-graph.c

@@ -19,6 +19,7 @@
 #include "shallow.h"
 #include "json-writer.h"
 #include "trace2.h"
+#include "chunk-format.h"
 
 void git_test_write_commit_graph_or_die(void)
 {
@@ -44,7 +45,6 @@ void git_test_write_commit_graph_or_die(void)
 #define GRAPH_CHUNKID_BLOOMINDEXES 0x42494458 /* "BIDX" */
 #define GRAPH_CHUNKID_BLOOMDATA 0x42444154 /* "BDAT" */
 #define GRAPH_CHUNKID_BASE 0x42415345 /* "BASE" */
-#define MAX_NUM_CHUNKS 9
 
 #define GRAPH_DATA_WIDTH (the_hash_algo->rawsz + 16)
 
@@ -59,8 +59,7 @@ void git_test_write_commit_graph_or_die(void)
 
 #define GRAPH_HEADER_SIZE 8
 #define GRAPH_FANOUT_SIZE (4 * 256)
-#define GRAPH_CHUNKLOOKUP_WIDTH 12
-#define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
+#define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * CHUNK_TOC_ENTRY_SIZE \
 			+ GRAPH_FANOUT_SIZE + the_hash_algo->rawsz)
 
 #define CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW (1ULL << 31)
@@ -298,15 +297,43 @@ static int verify_commit_graph_lite(struct commit_graph *g)
 	return 0;
 }
 
+static int graph_read_oid_lookup(const unsigned char *chunk_start,
+				 size_t chunk_size, void *data)
+{
+	struct commit_graph *g = data;
+	g->chunk_oid_lookup = chunk_start;
+	g->num_commits = chunk_size / g->hash_len;
+	return 0;
+}
+
+static int graph_read_bloom_data(const unsigned char *chunk_start,
+				  size_t chunk_size, void *data)
+{
+	struct commit_graph *g = data;
+	uint32_t hash_version;
+	g->chunk_bloom_data = chunk_start;
+	hash_version = get_be32(chunk_start);
+
+	if (hash_version != 1)
+		return 0;
+
+	g->bloom_filter_settings = xmalloc(sizeof(struct bloom_filter_settings));
+	g->bloom_filter_settings->hash_version = hash_version;
+	g->bloom_filter_settings->num_hashes = get_be32(chunk_start + 4);
+	g->bloom_filter_settings->bits_per_entry = get_be32(chunk_start + 8);
+	g->bloom_filter_settings->max_changed_paths = DEFAULT_BLOOM_MAX_CHANGES;
+
+	return 0;
+}
+
 struct commit_graph *parse_commit_graph(struct repository *r,
 					void *graph_map, size_t graph_size)
 {
-	const unsigned char *data, *chunk_lookup;
-	uint32_t i;
+	const unsigned char *data;
 	struct commit_graph *graph;
-	uint64_t next_chunk_offset;
 	uint32_t graph_signature;
 	unsigned char graph_version, hash_version;
+	struct chunkfile *cf = NULL;
 
 	if (!graph_map)
 		return NULL;
@@ -347,7 +374,7 @@ struct commit_graph *parse_commit_graph(struct repository *r,
 	graph->data_len = graph_size;
 
 	if (graph_size < GRAPH_HEADER_SIZE +
-			 (graph->num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH +
+			 (graph->num_chunks + 1) * CHUNK_TOC_ENTRY_SIZE +
 			 GRAPH_FANOUT_SIZE + the_hash_algo->rawsz) {
 		error(_("commit-graph file is too small to hold %u chunks"),
 		      graph->num_chunks);
@@ -355,108 +382,28 @@ struct commit_graph *parse_commit_graph(struct repository *r,
 		return NULL;
 	}
 
-	chunk_lookup = data + 8;
-	next_chunk_offset = get_be64(chunk_lookup + 4);
-	for (i = 0; i < graph->num_chunks; i++) {
-		uint32_t chunk_id;
-		uint64_t chunk_offset = next_chunk_offset;
-		int chunk_repeated = 0;
-
-		chunk_id = get_be32(chunk_lookup + 0);
-
-		chunk_lookup += GRAPH_CHUNKLOOKUP_WIDTH;
-		next_chunk_offset = get_be64(chunk_lookup + 4);
-
-		if (chunk_offset > graph_size - the_hash_algo->rawsz) {
-			error(_("commit-graph improper chunk offset %08x%08x"), (uint32_t)(chunk_offset >> 32),
-			      (uint32_t)chunk_offset);
-			goto free_and_return;
-		}
-
-		switch (chunk_id) {
-		case GRAPH_CHUNKID_OIDFANOUT:
-			if (graph->chunk_oid_fanout)
-				chunk_repeated = 1;
-			else
-				graph->chunk_oid_fanout = (uint32_t*)(data + chunk_offset);
-			break;
-
-		case GRAPH_CHUNKID_OIDLOOKUP:
-			if (graph->chunk_oid_lookup)
-				chunk_repeated = 1;
-			else {
-				graph->chunk_oid_lookup = data + chunk_offset;
-				graph->num_commits = (next_chunk_offset - chunk_offset)
-						     / graph->hash_len;
-			}
-			break;
-
-		case GRAPH_CHUNKID_DATA:
-			if (graph->chunk_commit_data)
-				chunk_repeated = 1;
-			else
-				graph->chunk_commit_data = data + chunk_offset;
-			break;
-
-		case GRAPH_CHUNKID_GENERATION_DATA:
-			if (graph->chunk_generation_data)
-				chunk_repeated = 1;
-			else
-				graph->chunk_generation_data = data + chunk_offset;
-			break;
-
-		case GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW:
-			if (graph->chunk_generation_data_overflow)
-				chunk_repeated = 1;
-			else
-				graph->chunk_generation_data_overflow = data + chunk_offset;
-			break;
+	cf = init_chunkfile(NULL);
 
-		case GRAPH_CHUNKID_EXTRAEDGES:
-			if (graph->chunk_extra_edges)
-				chunk_repeated = 1;
-			else
-				graph->chunk_extra_edges = data + chunk_offset;
-			break;
-
-		case GRAPH_CHUNKID_BASE:
-			if (graph->chunk_base_graphs)
-				chunk_repeated = 1;
-			else
-				graph->chunk_base_graphs = data + chunk_offset;
-			break;
-
-		case GRAPH_CHUNKID_BLOOMINDEXES:
-			if (graph->chunk_bloom_indexes)
-				chunk_repeated = 1;
-			else if (r->settings.commit_graph_read_changed_paths)
-				graph->chunk_bloom_indexes = data + chunk_offset;
-			break;
-
-		case GRAPH_CHUNKID_BLOOMDATA:
-			if (graph->chunk_bloom_data)
-				chunk_repeated = 1;
-			else if (r->settings.commit_graph_read_changed_paths) {
-				uint32_t hash_version;
-				graph->chunk_bloom_data = data + chunk_offset;
-				hash_version = get_be32(data + chunk_offset);
-
-				if (hash_version != 1)
-					break;
+	if (read_table_of_contents(cf, graph->data, graph_size,
+				   GRAPH_HEADER_SIZE, graph->num_chunks))
+		goto free_and_return;
 
-				graph->bloom_filter_settings = xmalloc(sizeof(struct bloom_filter_settings));
-				graph->bloom_filter_settings->hash_version = hash_version;
-				graph->bloom_filter_settings->num_hashes = get_be32(data + chunk_offset + 4);
-				graph->bloom_filter_settings->bits_per_entry = get_be32(data + chunk_offset + 8);
-				graph->bloom_filter_settings->max_changed_paths = DEFAULT_BLOOM_MAX_CHANGES;
-			}
-			break;
-		}
+	pair_chunk(cf, GRAPH_CHUNKID_OIDFANOUT,
+		   (const unsigned char **)&graph->chunk_oid_fanout);
+	read_chunk(cf, GRAPH_CHUNKID_OIDLOOKUP, graph_read_oid_lookup, graph);
+	pair_chunk(cf, GRAPH_CHUNKID_DATA, &graph->chunk_commit_data);
+	pair_chunk(cf, GRAPH_CHUNKID_EXTRAEDGES, &graph->chunk_extra_edges);
+	pair_chunk(cf, GRAPH_CHUNKID_BASE, &graph->chunk_base_graphs);
+	pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
+		   &graph->chunk_generation_data);
+	pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW,
+		   &graph->chunk_generation_data_overflow);
 
-		if (chunk_repeated) {
-			error(_("commit-graph chunk id %08x appears multiple times"), chunk_id);
-			goto free_and_return;
-		}
+	if (r->settings.commit_graph_read_changed_paths) {
+		pair_chunk(cf, GRAPH_CHUNKID_BLOOMINDEXES,
+			   &graph->chunk_bloom_indexes);
+		read_chunk(cf, GRAPH_CHUNKID_BLOOMDATA,
+			   graph_read_bloom_data, graph);
 	}
 
 	if (graph->chunk_bloom_indexes && graph->chunk_bloom_data) {
@@ -473,9 +420,11 @@ struct commit_graph *parse_commit_graph(struct repository *r,
 	if (verify_commit_graph_lite(graph))
 		goto free_and_return;
 
+	free_chunkfile(cf);
 	return graph;
 
 free_and_return:
+	free_chunkfile(cf);
 	free(graph->bloom_filter_settings);
 	free(graph);
 	return NULL;
@@ -1051,8 +1000,9 @@ struct write_commit_graph_context {
 };
 
 static int write_graph_chunk_fanout(struct hashfile *f,
-				    struct write_commit_graph_context *ctx)
+				    void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	int i, count = 0;
 	struct commit **list = ctx->commits.list;
 
@@ -1077,8 +1027,9 @@ static int write_graph_chunk_fanout(struct hashfile *f,
 }
 
 static int write_graph_chunk_oids(struct hashfile *f,
-				  struct write_commit_graph_context *ctx)
+				  void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	struct commit **list = ctx->commits.list;
 	int count;
 	for (count = 0; count < ctx->commits.nr; count++, list++) {
@@ -1096,8 +1047,9 @@ static const struct object_id *commit_to_oid(size_t index, const void *table)
 }
 
 static int write_graph_chunk_data(struct hashfile *f,
-				  struct write_commit_graph_context *ctx)
+				  void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	struct commit **list = ctx->commits.list;
 	struct commit **last = ctx->commits.list + ctx->commits.nr;
 	uint32_t num_extra_edges = 0;
@@ -1198,8 +1150,9 @@ static int write_graph_chunk_data(struct hashfile *f,
 }
 
 static int write_graph_chunk_generation_data(struct hashfile *f,
-					      struct write_commit_graph_context *ctx)
+					     void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	int i, num_generation_data_overflows = 0;
 
 	for (i = 0; i < ctx->commits.nr; i++) {
@@ -1221,8 +1174,9 @@ static int write_graph_chunk_generation_data(struct hashfile *f,
 }
 
 static int write_graph_chunk_generation_data_overflow(struct hashfile *f,
-						       struct write_commit_graph_context *ctx)
+						      void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	int i;
 	for (i = 0; i < ctx->commits.nr; i++) {
 		struct commit *c = ctx->commits.list[i];
@@ -1239,8 +1193,9 @@ static int write_graph_chunk_generation_data_overflow(struct hashfile *f,
 }
 
 static int write_graph_chunk_extra_edges(struct hashfile *f,
-					 struct write_commit_graph_context *ctx)
+					 void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	struct commit **list = ctx->commits.list;
 	struct commit **last = ctx->commits.list + ctx->commits.nr;
 	struct commit_list *parent;
@@ -1293,8 +1248,9 @@ static int write_graph_chunk_extra_edges(struct hashfile *f,
 }
 
 static int write_graph_chunk_bloom_indexes(struct hashfile *f,
-					   struct write_commit_graph_context *ctx)
+					   void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	struct commit **list = ctx->commits.list;
 	struct commit **last = ctx->commits.list + ctx->commits.nr;
 	uint32_t cur_pos = 0;
@@ -1328,8 +1284,9 @@ static void trace2_bloom_filter_settings(struct write_commit_graph_context *ctx)
 }
 
 static int write_graph_chunk_bloom_data(struct hashfile *f,
-					struct write_commit_graph_context *ctx)
+					void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	struct commit **list = ctx->commits.list;
 	struct commit **last = ctx->commits.list + ctx->commits.nr;
 
@@ -1805,8 +1762,9 @@ static int write_graph_chunk_base_1(struct hashfile *f,
 }
 
 static int write_graph_chunk_base(struct hashfile *f,
-				  struct write_commit_graph_context *ctx)
+				    void *data)
 {
+	struct write_commit_graph_context *ctx = data;
 	int num = write_graph_chunk_base_1(f, ctx->new_base_graph);
 
 	if (num != ctx->num_commit_graphs_after - 1) {
@@ -1817,27 +1775,16 @@ static int write_graph_chunk_base(struct hashfile *f,
 	return 0;
 }
 
-typedef int (*chunk_write_fn)(struct hashfile *f,
-			      struct write_commit_graph_context *ctx);
-
-struct chunk_info {
-	uint32_t id;
-	uint64_t size;
-	chunk_write_fn write_fn;
-};
-
 static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 {
 	uint32_t i;
 	int fd;
 	struct hashfile *f;
 	struct lock_file lk = LOCK_INIT;
-	struct chunk_info chunks[MAX_NUM_CHUNKS + 1];
 	const unsigned hashsz = the_hash_algo->rawsz;
 	struct strbuf progress_title = STRBUF_INIT;
-	int num_chunks = 3;
-	uint64_t chunk_offset;
 	struct object_id file_hash;
+	struct chunkfile *cf;
 
 	if (ctx->split) {
 		struct strbuf tmp_file = STRBUF_INIT;
@@ -1883,98 +1830,62 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		f = hashfd(fd, get_lock_file_path(&lk));
 	}
 
-	chunks[0].id = GRAPH_CHUNKID_OIDFANOUT;
-	chunks[0].size = GRAPH_FANOUT_SIZE;
-	chunks[0].write_fn = write_graph_chunk_fanout;
-	chunks[1].id = GRAPH_CHUNKID_OIDLOOKUP;
-	chunks[1].size = hashsz * ctx->commits.nr;
-	chunks[1].write_fn = write_graph_chunk_oids;
-	chunks[2].id = GRAPH_CHUNKID_DATA;
-	chunks[2].size = (hashsz + 16) * ctx->commits.nr;
-	chunks[2].write_fn = write_graph_chunk_data;
+	cf = init_chunkfile(f);
+
+	add_chunk(cf, GRAPH_CHUNKID_OIDFANOUT, GRAPH_FANOUT_SIZE,
+		  write_graph_chunk_fanout);
+	add_chunk(cf, GRAPH_CHUNKID_OIDLOOKUP, hashsz * ctx->commits.nr,
+		  write_graph_chunk_oids);
+	add_chunk(cf, GRAPH_CHUNKID_DATA, (hashsz + 16) * ctx->commits.nr,
+		  write_graph_chunk_data);
 
 	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_NO_GDAT, 0))
 		ctx->write_generation_data = 0;
-	if (ctx->write_generation_data) {
-		chunks[num_chunks].id = GRAPH_CHUNKID_GENERATION_DATA;
-		chunks[num_chunks].size = sizeof(uint32_t) * ctx->commits.nr;
-		chunks[num_chunks].write_fn = write_graph_chunk_generation_data;
-		num_chunks++;
-	}
-	if (ctx->num_generation_data_overflows) {
-		chunks[num_chunks].id = GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW;
-		chunks[num_chunks].size = sizeof(timestamp_t) * ctx->num_generation_data_overflows;
-		chunks[num_chunks].write_fn = write_graph_chunk_generation_data_overflow;
-		num_chunks++;
-	}
-	if (ctx->num_extra_edges) {
-		chunks[num_chunks].id = GRAPH_CHUNKID_EXTRAEDGES;
-		chunks[num_chunks].size = 4 * ctx->num_extra_edges;
-		chunks[num_chunks].write_fn = write_graph_chunk_extra_edges;
-		num_chunks++;
-	}
+	if (ctx->write_generation_data)
+		add_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
+			  sizeof(uint32_t) * ctx->commits.nr,
+			  write_graph_chunk_generation_data);
+	if (ctx->num_generation_data_overflows)
+		add_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW,
+			  sizeof(timestamp_t) * ctx->num_generation_data_overflows,
+			  write_graph_chunk_generation_data_overflow);
+	if (ctx->num_extra_edges)
+		add_chunk(cf, GRAPH_CHUNKID_EXTRAEDGES,
+			  4 * ctx->num_extra_edges,
+			  write_graph_chunk_extra_edges);
 	if (ctx->changed_paths) {
-		chunks[num_chunks].id = GRAPH_CHUNKID_BLOOMINDEXES;
-		chunks[num_chunks].size = sizeof(uint32_t) * ctx->commits.nr;
-		chunks[num_chunks].write_fn = write_graph_chunk_bloom_indexes;
-		num_chunks++;
-		chunks[num_chunks].id = GRAPH_CHUNKID_BLOOMDATA;
-		chunks[num_chunks].size = sizeof(uint32_t) * 3
-					  + ctx->total_bloom_filter_data_size;
-		chunks[num_chunks].write_fn = write_graph_chunk_bloom_data;
-		num_chunks++;
-	}
-	if (ctx->num_commit_graphs_after > 1) {
-		chunks[num_chunks].id = GRAPH_CHUNKID_BASE;
-		chunks[num_chunks].size = hashsz * (ctx->num_commit_graphs_after - 1);
-		chunks[num_chunks].write_fn = write_graph_chunk_base;
-		num_chunks++;
-	}
-
-	chunks[num_chunks].id = 0;
-	chunks[num_chunks].size = 0;
+		add_chunk(cf, GRAPH_CHUNKID_BLOOMINDEXES,
+			  sizeof(uint32_t) * ctx->commits.nr,
+			  write_graph_chunk_bloom_indexes);
+		add_chunk(cf, GRAPH_CHUNKID_BLOOMDATA,
+			  sizeof(uint32_t) * 3
+				+ ctx->total_bloom_filter_data_size,
+			  write_graph_chunk_bloom_data);
+	}
+	if (ctx->num_commit_graphs_after > 1)
+		add_chunk(cf, GRAPH_CHUNKID_BASE,
+			  hashsz * (ctx->num_commit_graphs_after - 1),
+			  write_graph_chunk_base);
 
 	hashwrite_be32(f, GRAPH_SIGNATURE);
 
 	hashwrite_u8(f, GRAPH_VERSION);
 	hashwrite_u8(f, oid_version());
-	hashwrite_u8(f, num_chunks);
+	hashwrite_u8(f, get_num_chunks(cf));
 	hashwrite_u8(f, ctx->num_commit_graphs_after - 1);
 
-	chunk_offset = 8 + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
-	for (i = 0; i <= num_chunks; i++) {
-		uint32_t chunk_write[3];
-
-		chunk_write[0] = htonl(chunks[i].id);
-		chunk_write[1] = htonl(chunk_offset >> 32);
-		chunk_write[2] = htonl(chunk_offset & 0xffffffff);
-		hashwrite(f, chunk_write, 12);
-
-		chunk_offset += chunks[i].size;
-	}
-
 	if (ctx->report_progress) {
 		strbuf_addf(&progress_title,
 			    Q_("Writing out commit graph in %d pass",
 			       "Writing out commit graph in %d passes",
-			       num_chunks),
-			    num_chunks);
+			       get_num_chunks(cf)),
+			    get_num_chunks(cf));
 		ctx->progress = start_delayed_progress(
 			progress_title.buf,
-			num_chunks * ctx->commits.nr);
+			get_num_chunks(cf) * ctx->commits.nr);
 	}
 
-	for (i = 0; i < num_chunks; i++) {
-		uint64_t start_offset = f->total + f->offset;
-
-		if (chunks[i].write_fn(f, ctx))
-			return -1;
-
-		if (f->total + f->offset != start_offset + chunks[i].size)
-			BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
-			    chunks[i].size, chunks[i].id,
-			    f->total + f->offset - start_offset);
-	}
+	write_chunkfile(cf, ctx);
 
 	stop_progress(&ctx->progress);
 	strbuf_release(&progress_title);
@@ -1991,6 +1902,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 
 	close_commit_graph(ctx->r->objects);
 	finalize_hashfile(f, file_hash.hash, CSUM_HASH_IN_STREAM | CSUM_FSYNC);
+	free_chunkfile(cf);
 
 	if (ctx->split) {
 		FILE *chainf = fdopen_lock_file(&lk, "w");

midx.c

@@ -11,6 +11,7 @@
 #include "trace2.h"
 #include "run-command.h"
 #include "repository.h"
+#include "chunk-format.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -21,14 +22,12 @@
 #define MIDX_HEADER_SIZE 12
 #define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + the_hash_algo->rawsz)
 
-#define MIDX_MAX_CHUNKS 5
 #define MIDX_CHUNK_ALIGNMENT 4
 #define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
 #define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
 #define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
 #define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
 #define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
-#define MIDX_CHUNKLOOKUP_WIDTH (sizeof(uint32_t) + sizeof(uint64_t))
 #define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
 #define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
@@ -53,6 +52,19 @@ static char *get_midx_filename(const char *object_dir)
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
 }
 
+static int midx_read_oid_fanout(const unsigned char *chunk_start,
+				size_t chunk_size, void *data)
+{
+	struct multi_pack_index *m = data;
+	m->chunk_oid_fanout = (uint32_t *)chunk_start;
+
+	if (chunk_size != 4 * 256) {
+		error(_("multi-pack-index OID fanout is of the wrong size"));
+		return 1;
+	}
+	return 0;
+}
+
 struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local)
 {
 	struct multi_pack_index *m = NULL;
@@ -64,6 +76,7 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 	char *midx_name = get_midx_filename(object_dir);
 	uint32_t i;
 	const char *cur_pack_name;
+	struct chunkfile *cf = NULL;
 
 	fd = git_open(midx_name);
 
@@ -113,58 +126,23 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 
 	m->num_packs = get_be32(m->data + MIDX_BYTE_NUM_PACKS);
 
-	for (i = 0; i < m->num_chunks; i++) {
-		uint32_t chunk_id = get_be32(m->data + MIDX_HEADER_SIZE +
-					     MIDX_CHUNKLOOKUP_WIDTH * i);
-		uint64_t chunk_offset = get_be64(m->data + MIDX_HEADER_SIZE + 4 +
-						 MIDX_CHUNKLOOKUP_WIDTH * i);
-
-		if (chunk_offset >= m->data_len)
-			die(_("invalid chunk offset (too large)"));
-
-		switch (chunk_id) {
-			case MIDX_CHUNKID_PACKNAMES:
-				m->chunk_pack_names = m->data + chunk_offset;
-				break;
-
-			case MIDX_CHUNKID_OIDFANOUT:
-				m->chunk_oid_fanout = (uint32_t *)(m->data + chunk_offset);
-				break;
-
-			case MIDX_CHUNKID_OIDLOOKUP:
-				m->chunk_oid_lookup = m->data + chunk_offset;
-				break;
-
-			case MIDX_CHUNKID_OBJECTOFFSETS:
-				m->chunk_object_offsets = m->data + chunk_offset;
-				break;
-
-			case MIDX_CHUNKID_LARGEOFFSETS:
-				m->chunk_large_offsets = m->data + chunk_offset;
-				break;
-
-			case 0:
-				die(_("terminating multi-pack-index chunk id appears earlier than expected"));
-				break;
-
-			default:
-				/*
-				 * Do nothing on unrecognized chunks, allowing future
-				 * extensions to add optional chunks.
-				 */
-				break;
-		}
-	}
+	cf = init_chunkfile(NULL);
+
+	if (read_table_of_contents(cf, m->data, midx_size,
+				   MIDX_HEADER_SIZE, m->num_chunks))
+		goto cleanup_fail;
 
-	if (!m->chunk_pack_names)
+	if (pair_chunk(cf, MIDX_CHUNKID_PACKNAMES, &m->chunk_pack_names) == CHUNK_NOT_FOUND)
 		die(_("multi-pack-index missing required pack-name chunk"));
-	if (!m->chunk_oid_fanout)
+	if (read_chunk(cf, MIDX_CHUNKID_OIDFANOUT, midx_read_oid_fanout, m) == CHUNK_NOT_FOUND)
 		die(_("multi-pack-index missing required OID fanout chunk"));
-	if (!m->chunk_oid_lookup)
+	if (pair_chunk(cf, MIDX_CHUNKID_OIDLOOKUP, &m->chunk_oid_lookup) == CHUNK_NOT_FOUND)
 		die(_("multi-pack-index missing required OID lookup chunk"));
-	if (!m->chunk_object_offsets)
+	if (pair_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS, &m->chunk_object_offsets) == CHUNK_NOT_FOUND)
 		die(_("multi-pack-index missing required object offsets chunk"));
 
+	pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets);
+
 	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
 
 	m->pack_names = xcalloc(m->num_packs, sizeof(*m->pack_names));
@@ -190,6 +168,7 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 cleanup_fail:
 	free(m);
 	free(midx_name);
+	free(cf);
 	if (midx_map)
 		munmap(midx_map, midx_size);
 	if (0 <= fd)
@@ -265,7 +244,7 @@ static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 	const unsigned char *offset_data;
 	uint32_t offset32;
 
-	offset_data = m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH;
+	offset_data = m->chunk_object_offsets + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH;
 	offset32 = get_be32(offset_data + sizeof(uint32_t));
 
 	if (m->chunk_large_offsets && offset32 & MIDX_LARGE_OFFSET_NEEDED) {
@@ -281,7 +260,8 @@ static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 
 static uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
 {
-	return get_be32(m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH);
+	return get_be32(m->chunk_object_offsets +
+			(off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
 }
 
 static int nth_midxed_pack_entry(struct repository *r,
@@ -451,49 +431,56 @@ static int pack_info_compare(const void *_a, const void *_b)
 	return strcmp(a->pack_name, b->pack_name);
 }
 
-struct pack_list {
+struct write_midx_context {
 	struct pack_info *info;
 	uint32_t nr;
 	uint32_t alloc;
 	struct multi_pack_index *m;
 	struct progress *progress;
 	unsigned pack_paths_checked;
+
+	struct pack_midx_entry *entries;
+	uint32_t entries_nr;
+
+	uint32_t *pack_perm;
+	unsigned large_offsets_needed:1;
+	uint32_t num_large_offsets;
 };
 
 static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 			     const char *file_name, void *data)
 {
-	struct pack_list *packs = (struct pack_list *)data;
+	struct write_midx_context *ctx = data;
 
 	if (ends_with(file_name, ".idx")) {
-		display_progress(packs->progress, ++packs->pack_paths_checked);
-		if (packs->m && midx_contains_pack(packs->m, file_name))
+		display_progress(ctx->progress, ++ctx->pack_paths_checked);
+		if (ctx->m && midx_contains_pack(ctx->m, file_name))
 			return;
 
-		ALLOC_GROW(packs->info, packs->nr + 1, packs->alloc);
+		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
 
-		packs->info[packs->nr].p = add_packed_git(full_path,
-							  full_path_len,
-							  0);
+		ctx->info[ctx->nr].p = add_packed_git(full_path,
+						      full_path_len,
+						      0);
 
-		if (!packs->info[packs->nr].p) {
+		if (!ctx->info[ctx->nr].p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}
 
-		if (open_pack_index(packs->info[packs->nr].p)) {
+		if (open_pack_index(ctx->info[ctx->nr].p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(packs->info[packs->nr].p);
-			FREE_AND_NULL(packs->info[packs->nr].p);
+			close_pack(ctx->info[ctx->nr].p);
+			FREE_AND_NULL(ctx->info[ctx->nr].p);
 			return;
 		}
 
-		packs->info[packs->nr].pack_name = xstrdup(file_name);
-		packs->info[packs->nr].orig_pack_int_id = packs->nr;
-		packs->info[packs->nr].expired = 0;
-		packs->nr++;
+		ctx->info[ctx->nr].pack_name = xstrdup(file_name);
+		ctx->info[ctx->nr].orig_pack_int_id = ctx->nr;
+		ctx->info[ctx->nr].expired = 0;
+		ctx->nr++;
 	}
 }
 
@@ -643,27 +630,26 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	return deduplicated_entries;
 }
 
-static size_t write_midx_pack_names(struct hashfile *f,
-				    struct pack_info *info,
-				    uint32_t num_packs)
+static int write_midx_pack_names(struct hashfile *f, void *data)
 {
+	struct write_midx_context *ctx = data;
 	uint32_t i;
 	unsigned char padding[MIDX_CHUNK_ALIGNMENT];
 	size_t written = 0;
 
-	for (i = 0; i < num_packs; i++) {
+	for (i = 0; i < ctx->nr; i++) {
 		size_t writelen;
 
-		if (info[i].expired)
+		if (ctx->info[i].expired)
 			continue;
 
-		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
+		if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    info[i - 1].pack_name,
-			    info[i].pack_name);
+			    ctx->info[i - 1].pack_name,
+			    ctx->info[i].pack_name);
 
-		writelen = strlen(info[i].pack_name) + 1;
-		hashwrite(f, info[i].pack_name, writelen);
+		writelen = strlen(ctx->info[i].pack_name) + 1;
+		hashwrite(f, ctx->info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -672,18 +658,17 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	if (i < MIDX_CHUNK_ALIGNMENT) {
 		memset(padding, 0, sizeof(padding));
 		hashwrite(f, padding, i);
-		written += i;
 	}
 
-	return written;
+	return 0;
 }
 
-static size_t write_midx_oid_fanout(struct hashfile *f,
-				    struct pack_midx_entry *objects,
-				    uint32_t nr_objects)
+static int write_midx_oid_fanout(struct hashfile *f,
+				 void *data)
 {
-	struct pack_midx_entry *list = objects;
-	struct pack_midx_entry *last = objects + nr_objects;
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
+	struct pack_midx_entry *last = ctx->entries + ctx->entries_nr;
 	uint32_t count = 0;
 	uint32_t i;
 
@@ -704,21 +689,21 @@ static size_t write_midx_oid_fanout(struct hashfile *f,
 		list = next;
 	}
 
-	return MIDX_CHUNK_FANOUT_SIZE;
+	return 0;
 }
 
-static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
-				    struct pack_midx_entry *objects,
-				    uint32_t nr_objects)
+static int write_midx_oid_lookup(struct hashfile *f,
+				 void *data)
 {
-	struct pack_midx_entry *list = objects;
+	struct write_midx_context *ctx = data;
+	unsigned char hash_len = the_hash_algo->rawsz;
+	struct pack_midx_entry *list = ctx->entries;
 	uint32_t i;
-	size_t written = 0;
 
-	for (i = 0; i < nr_objects; i++) {
+	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		if (i < nr_objects - 1) {
+		if (i < ctx->entries_nr - 1) {
 			struct pack_midx_entry *next = list;
 			if (oidcmp(&obj->oid, &next->oid) >= 0)
 				BUG("OIDs not in order: %s >= %s",
@@ -727,50 +712,48 @@ static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
 		}
 
 		hashwrite(f, obj->oid.hash, (int)hash_len);
-		written += hash_len;
 	}
 
-	return written;
+	return 0;
 }
 
-static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
-					uint32_t *perm,
-					struct pack_midx_entry *objects, uint32_t nr_objects)
+static int write_midx_object_offsets(struct hashfile *f,
+				     void *data)
 {
-	struct pack_midx_entry *list = objects;
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
 	uint32_t i, nr_large_offset = 0;
-	size_t written = 0;
 
-	for (i = 0; i < nr_objects; i++) {
+	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+		if (ctx->pack_perm[obj->pack_int_id] == PACK_EXPIRED)
 			BUG("object %s is in an expired pack with int-id %d",
 			    oid_to_hex(&obj->oid),
 			    obj->pack_int_id);
 
-		hashwrite_be32(f, perm[obj->pack_int_id]);
+		hashwrite_be32(f, ctx->pack_perm[obj->pack_int_id]);
 
-		if (large_offset_needed && obj->offset >> 31)
+		if (ctx->large_offsets_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
-		else if (!large_offset_needed && obj->offset >> 32)
+		else if (!ctx->large_offsets_needed && obj->offset >> 32)
 			BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
 			    oid_to_hex(&obj->oid),
 			    obj->offset);
 		else
 			hashwrite_be32(f, (uint32_t)obj->offset);
-
-		written += MIDX_CHUNK_OFFSET_WIDTH;
 	}
 
-	return written;
+	return 0;
 }
 
-static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_offset,
-				       struct pack_midx_entry *objects, uint32_t nr_objects)
+static int write_midx_large_offsets(struct hashfile *f,
+				    void *data)
 {
-	struct pack_midx_entry *list = objects, *end = objects + nr_objects;
-	size_t written = 0;
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
+	struct pack_midx_entry *end = ctx->entries + ctx->entries_nr;
+	uint32_t nr_large_offset = ctx->num_large_offsets;
 
 	while (nr_large_offset) {
 		struct pack_midx_entry *obj;
@@ -785,34 +768,26 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 		if (!(offset >> 31))
 			continue;
 
-		written += hashwrite_be64(f, offset);
+		hashwrite_be64(f, offset);
 
 		nr_large_offset--;
 	}
 
-	return written;
+	return 0;
 }
 
 static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
 			       struct string_list *packs_to_drop, unsigned flags)
 {
-	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
 	uint32_t i;
 	struct hashfile *f = NULL;
 	struct lock_file lk;
-	struct pack_list packs;
-	uint32_t *pack_perm = NULL;
-	uint64_t written = 0;
-	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
-	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
-	uint32_t nr_entries, num_large_offsets = 0;
-	struct pack_midx_entry *entries = NULL;
-	struct progress *progress = NULL;
-	int large_offsets_needed = 0;
+	struct write_midx_context ctx = { 0 };
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
+	struct chunkfile *cf;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name))
@@ -820,61 +795,62 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 			  midx_name);
 
 	if (m)
-		packs.m = m;
+		ctx.m = m;
 	else
-		packs.m = load_multi_pack_index(object_dir, 1);
-
-	packs.nr = 0;
-	packs.alloc = packs.m ? packs.m->num_packs : 16;
-	packs.info = NULL;
-	ALLOC_ARRAY(packs.info, packs.alloc);
-
-	if (packs.m) {
-		for (i = 0; i < packs.m->num_packs; i++) {
-			ALLOC_GROW(packs.info, packs.nr + 1, packs.alloc);
-
-			packs.info[packs.nr].orig_pack_int_id = i;
-			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
-			packs.info[packs.nr].p = NULL;
-			packs.info[packs.nr].expired = 0;
-			packs.nr++;
+		ctx.m = load_multi_pack_index(object_dir, 1);
+
+	ctx.nr = 0;
+	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
+	ctx.info = NULL;
+	ALLOC_ARRAY(ctx.info, ctx.alloc);
+
+	if (ctx.m) {
+		for (i = 0; i < ctx.m->num_packs; i++) {
+			ALLOC_GROW(ctx.info, ctx.nr + 1, ctx.alloc);
+
+			ctx.info[ctx.nr].orig_pack_int_id = i;
+			ctx.info[ctx.nr].pack_name = xstrdup(ctx.m->pack_names[i]);
+			ctx.info[ctx.nr].p = NULL;
+			ctx.info[ctx.nr].expired = 0;
+			ctx.nr++;
 		}
 	}
 
-	packs.pack_paths_checked = 0;
+	ctx.pack_paths_checked = 0;
 	if (flags & MIDX_PROGRESS)
-		packs.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
+		ctx.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
 	else
-		packs.progress = NULL;
+		ctx.progress = NULL;
 
-	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
-	stop_progress(&packs.progress);
+	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
+	stop_progress(&ctx.progress);
 
-	if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
+	if (ctx.m && ctx.nr == ctx.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
-	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
+	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr);
 
-	for (i = 0; i < nr_entries; i++) {
-		if (entries[i].offset > 0x7fffffff)
-			num_large_offsets++;
-		if (entries[i].offset > 0xffffffff)
-			large_offsets_needed = 1;
+	ctx.large_offsets_needed = 0;
+	for (i = 0; i < ctx.entries_nr; i++) {
+		if (ctx.entries[i].offset > 0x7fffffff)
+			ctx.num_large_offsets++;
+		if (ctx.entries[i].offset > 0xffffffff)
+			ctx.large_offsets_needed = 1;
 	}
 
-	QSORT(packs.info, packs.nr, pack_info_compare);
+	QSORT(ctx.info, ctx.nr, pack_info_compare);
 
 	if (packs_to_drop && packs_to_drop->nr) {
 		int drop_index = 0;
 		int missing_drops = 0;
 
-		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
-			int cmp = strcmp(packs.info[i].pack_name,
+		for (i = 0; i < ctx.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(ctx.info[i].pack_name,
 					 packs_to_drop->items[drop_index].string);
 
 			if (!cmp) {
 				drop_index++;
-				packs.info[i].expired = 1;
+				ctx.info[i].expired = 1;
 			} else if (cmp > 0) {
 				error(_("did not see pack-file %s to drop"),
 				      packs_to_drop->items[drop_index].string);
@@ -882,7 +858,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 				missing_drops++;
 				i--;
 			} else {
-				packs.info[i].expired = 0;
+				ctx.info[i].expired = 0;
 			}
 		}
 
@@ -898,19 +874,19 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	 *
 	 * pack_perm[old_id] = new_id
 	 */
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	for (i = 0; i < packs.nr; i++) {
-		if (packs.info[i].expired) {
+	ALLOC_ARRAY(ctx.pack_perm, ctx.nr);
+	for (i = 0; i < ctx.nr; i++) {
+		if (ctx.info[i].expired) {
 			dropped_packs++;
-			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
 		} else {
-			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
 		}
 	}
 
-	for (i = 0; i < packs.nr; i++) {
-		if (!packs.info[i].expired)
-			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	for (i = 0; i < ctx.nr; i++) {
+		if (!ctx.info[i].expired)
+			pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
 	}
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
@@ -921,123 +897,52 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
 	FREE_AND_NULL(midx_name);
 
-	if (packs.m)
-		close_midx(packs.m);
-
-	cur_chunk = 0;
-	num_chunks = large_offsets_needed ? 5 : 4;
+	if (ctx.m)
+		close_midx(ctx.m);
 
-	if (packs.nr - dropped_packs == 0) {
+	if (ctx.nr - dropped_packs == 0) {
 		error(_("no pack files to index."));
 		result = 1;
 		goto cleanup;
 	}
 
-	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
-
-	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
-	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
-
-	cur_chunk++;
-	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + pack_name_concat_len;
-
-	cur_chunk++;
-	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + MIDX_CHUNK_FANOUT_SIZE;
-
-	cur_chunk++;
-	chunk_ids[cur_chunk] = MIDX_CHUNKID_OBJECTOFFSETS;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * the_hash_algo->rawsz;
-
-	cur_chunk++;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * MIDX_CHUNK_OFFSET_WIDTH;
-	if (large_offsets_needed) {
-		chunk_ids[cur_chunk] = MIDX_CHUNKID_LARGEOFFSETS;
-
-		cur_chunk++;
-		chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] +
-					   num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH;
-	}
-
-	chunk_ids[cur_chunk] = 0;
+	cf = init_chunkfile(f);
 
-	for (i = 0; i <= num_chunks; i++) {
-		if (i && chunk_offsets[i] < chunk_offsets[i - 1])
-			BUG("incorrect chunk offsets: %"PRIu64" before %"PRIu64,
-			    chunk_offsets[i - 1],
-			    chunk_offsets[i]);
+	add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,
+		  write_midx_pack_names);
+	add_chunk(cf, MIDX_CHUNKID_OIDFANOUT, MIDX_CHUNK_FANOUT_SIZE,
+		  write_midx_oid_fanout);
+	add_chunk(cf, MIDX_CHUNKID_OIDLOOKUP,
+		  (size_t)ctx.entries_nr * the_hash_algo->rawsz,
+		  write_midx_oid_lookup);
+	add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
+		  (size_t)ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH,
+		  write_midx_object_offsets);
 
-		if (chunk_offsets[i] % MIDX_CHUNK_ALIGNMENT)
-			BUG("chunk offset %"PRIu64" is not properly aligned",
-			    chunk_offsets[i]);
-
-		hashwrite_be32(f, chunk_ids[i]);
-		hashwrite_be64(f, chunk_offsets[i]);
-
-		written += MIDX_CHUNKLOOKUP_WIDTH;
-	}
-
-	if (flags & MIDX_PROGRESS)
-		progress = start_delayed_progress(_("Writing chunks to multi-pack-index"),
-					  num_chunks);
-	for (i = 0; i < num_chunks; i++) {
-		if (written != chunk_offsets[i])
-			BUG("incorrect chunk offset (%"PRIu64" != %"PRIu64") for chunk id %"PRIx32,
-			    chunk_offsets[i],
-			    written,
-			    chunk_ids[i]);
-
-		switch (chunk_ids[i]) {
-			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, packs.info, packs.nr);
-				break;
-
-			case MIDX_CHUNKID_OIDFANOUT:
-				written += write_midx_oid_fanout(f, entries, nr_entries);
-				break;
-
-			case MIDX_CHUNKID_OIDLOOKUP:
-				written += write_midx_oid_lookup(f, the_hash_algo->rawsz, entries, nr_entries);
-				break;
-
-			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
-				break;
-
-			case MIDX_CHUNKID_LARGEOFFSETS:
-				written += write_midx_large_offsets(f, num_large_offsets, entries, nr_entries);
-				break;
-
-			default:
-				BUG("trying to write unknown chunk id %"PRIx32,
-				    chunk_ids[i]);
-		}
-
-		display_progress(progress, i + 1);
-	}
-	stop_progress(&progress);
+	if (ctx.large_offsets_needed)
+		add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
+			(size_t)ctx.num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH,
+			write_midx_large_offsets);
 
-	if (written != chunk_offsets[num_chunks])
-		BUG("incorrect final offset %"PRIu64" != %"PRIu64,
-		    written,
-		    chunk_offsets[num_chunks]);
+	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
+	write_chunkfile(cf, &ctx);
 
 	finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	free_chunkfile(cf);
 	commit_lock_file(&lk);
 
 cleanup:
-	for (i = 0; i < packs.nr; i++) {
-		if (packs.info[i].p) {
-			close_pack(packs.info[i].p);
-			free(packs.info[i].p);
+	for (i = 0; i < ctx.nr; i++) {
+		if (ctx.info[i].p) {
+			close_pack(ctx.info[i].p);
+			free(ctx.info[i].p);
 		}
-		free(packs.info[i].pack_name);
+		free(ctx.info[i].pack_name);
 	}
 
-	free(packs.info);
-	free(entries);
-	free(pack_perm);
+	free(ctx.info);
+	free(ctx.entries);
+	free(ctx.pack_perm);
 	free(midx_name);
 	return result;
 }

t/t5318-commit-graph.sh

@@ -585,7 +585,7 @@ test_expect_success 'detect bad hash version' '
 
 test_expect_success 'detect low chunk count' '
 	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\01" \
-		"missing the .* chunk"
+		"final chunk has non-zero id"
 '
 
 test_expect_success 'detect missing OID fanout chunk' '

t/t5319-multi-pack-index.sh

@@ -314,12 +314,12 @@ test_expect_success 'verify bad OID version' '
 
 test_expect_success 'verify truncated chunk count' '
 	corrupt_midx_and_verify $MIDX_BYTE_CHUNK_COUNT "\01" $objdir \
-		"missing required"
+		"final chunk has non-zero id"
 '
 
 test_expect_success 'verify extended chunk count' '
 	corrupt_midx_and_verify $MIDX_BYTE_CHUNK_COUNT "\07" $objdir \
-		"terminating multi-pack-index chunk id appears earlier than expected"
+		"terminating chunk id appears earlier than expected"
 '
 
 test_expect_success 'verify missing required chunk' '
@@ -329,7 +329,7 @@ test_expect_success 'verify missing required chunk' '
 
 test_expect_success 'verify invalid chunk offset' '
 	corrupt_midx_and_verify $MIDX_BYTE_CHUNK_OFFSET "\01" $objdir \
-		"invalid chunk offset (too large)"
+		"improper chunk offset(s)"
 '
 
 test_expect_success 'verify packnames out of order' '