Subject: [PATCH v4 00/17] Refactor chunk-format into an API

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Thu, 18 Feb 2021 14:07:22 +0000

This is a restart on the topic previously submitted [1] but dropped because
ak/corrected-commit-date was still in progress. This version is based on
that branch.

[1]
https://lore.kernel.org/git/[email-masked]/

This version also changes the approach to use a more dynamic interaction
with a struct chunkfile pointer. This idea is credited to Taylor Blau [2],
but I started again from scratch. I also go further to make struct chunkfile
anonymous to API consumers. It is defined only in chunk-format.c, which
should hopefully deter future users from interacting with that data
directly.

[2] https://lore.kernel.org/git/[email-masked]/

This combined API is beneficial to reduce duplicated logic. Or rather, to
ensure that similar file formats have similar protections against bad data.
The multi-pack-index code did not have as many guards as the commit-graph
code did, but now they both share a common base that checks for things like
duplicate chunks or offsets outside the size of the file.

Here are some stats for the end-to-end change:

 * 570 insertions(+), 456 deletions(-).
 * commit-graph.c: 107 insertions(+), 192 deletions(-)
 * midx.c: 164 insertions(+), 260 deletions(-)

While there is an overall increase to the code size, the consumers do get
smaller. Boilerplate things like abstracting method to match chunk_write_fn
and chunk_read_fn make up a lot of these insertions. The "interesting" code
gets a lot smaller and cleaner.

Updates in V4
=============

 * Out-of-date macros in commit-graph.c and midx.c are removed in their
   appropriate patches.
 * Documentation around the read API is improved.

Updates in V3
=============

 * API methods use better types and changed their order to match internal
   data more closely.

 * Use hashfile_total() instead of internal data values.

 * The implementation of pair_chunk() uses read_chunk().

 * init_chunkfile() has an in-code doc comment warning against using the
   same struct chunkfile for reads and writes.

 * More multiplications are correctly cast in midx.c.

 * The chunk-format technical docs are expanded.

Updates in V2
=============

 * The method pair_chunk() now automatically sets a pointer while
   read_chunk() uses the callback. This greatly reduces the code size.

 * Pointer casts are now implicit instead of explicit.

 * Extra care is taken to not overflow when verifying chunk sizes on write.

Thanks, -Stolee

Derrick Stolee (17):
  commit-graph: anonymize data in chunk_write_fn
  chunk-format: create chunk format write API
  commit-graph: use chunk-format write API
  midx: rename pack_info to write_midx_context
  midx: use context in write_midx_pack_names()
  midx: add entries to write_midx_context
  midx: add pack_perm to write_midx_context
  midx: add num_large_offsets to write_midx_context
  midx: return success/failure in chunk write methods
  midx: drop chunk progress during write
  midx: use chunk-format API in write_midx_internal()
  chunk-format: create read chunk API
  commit-graph: use chunk-format read API
  midx: use chunk-format read API
  midx: use 64-bit multiplication for chunk sizes
  chunk-format: restore duplicate chunk checks
  chunk-format: add technical docs

 Documentation/technical/chunk-format.txt      | 116 +++++
 .../technical/commit-graph-format.txt         |   3 +
 Documentation/technical/pack-format.txt       |   3 +
 Makefile                                      |   1 +
 chunk-format.c                                | 179 ++++++++
 chunk-format.h                                |  68 +++
 commit-graph.c                                | 305 +++++-------
 midx.c                                        | 433 +++++++-----------
 t/t5318-commit-graph.sh                       |   2 +-
 t/t5319-multi-pack-index.sh                   |   6 +-
 10 files changed, 652 insertions(+), 464 deletions(-)
 create mode 100644 Documentation/technical/chunk-format.txt
 create mode 100644 chunk-format.c
 create mode 100644 chunk-format.h

base-commit: 5a3b130cad0d5c770f766e3af6d32b41766374c0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-848%2Fderrickstolee%2Fchunk-format%2Frefactor-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-848/derrickstolee/chunk-format/refactor-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/848

Range-diff vs v3:

  1:  243dcec94368 =  1:  243dcec94368 commit-graph: anonymize data in chunk_write_fn
  2:  16c37d2370cf !  2:  da1fcc683576 chunk-format: create chunk format write API
     @@ chunk-format.c (new)
      +#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
     @@ chunk-format.c (new)
      +	uint64_t cur_offset = hashfile_total(cf->f);
      +
      +	/* Add the table of contents to the current offset */
     -+	cur_offset += (cf->chunks_nr + 1) * CHUNK_LOOKUP_WIDTH;
     ++	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);
     @@ chunk-format.h (new)
      +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);
  3:  e549e24d79af !  3:  ddebf2389880 commit-graph: use chunk-format write API
     @@ commit-graph.c
       
       void git_test_write_commit_graph_or_die(void)
       {
     +@@ commit-graph.c: 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)
     + 
      @@ commit-graph.c: static int write_graph_chunk_base(struct hashfile *f,
       	return 0;
       }
  4:  66ff49ed9309 =  4:  8fdaae017dab midx: rename pack_info to write_midx_context
  5:  1d7484c0cffa =  5:  c18f3738f5d4 midx: use context in write_midx_pack_names()
  6:  ea0e7d40e537 =  6:  2bb2847aad6e midx: add entries to write_midx_context
  7:  b283a38fb775 =  7:  f5d44c012ad5 midx: add pack_perm to write_midx_context
  8:  e7064512ab7f =  8:  f5baadf276ff midx: add num_large_offsets to write_midx_context
  9:  7aa3242e15b7 =  9:  fd008449fc89 midx: return success/failure in chunk write methods
 10:  70f68c95e479 = 10:  60e77e451800 midx: drop chunk progress during write
 11:  787cd7f18d2e ! 11:  0b95aa7e152f midx: use chunk-format API in write_midx_internal()
     @@ midx.c
       
       #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
       #define MIDX_VERSION 1
     +@@
     + #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" */
      @@ midx.c: static int write_midx_large_offsets(struct hashfile *f,
       static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
       			       struct string_list *packs_to_drop, unsigned flags)
 12:  366eb2afee83 ! 12:  3e0dbc45ce7f chunk-format: create read chunk API
     @@ chunk-format.c: int write_chunkfile(struct chunkfile *cf, void *data)
      +			return 1;
      +		}
      +
     -+		table_of_contents += CHUNK_LOOKUP_WIDTH;
     ++		table_of_contents += CHUNK_TOC_ENTRY_SIZE;
      +		next_chunk_offset = get_be64(table_of_contents + 4);
      +
      +		if (next_chunk_offset < chunk_offset ||
     @@ chunk-format.c: int write_chunkfile(struct chunkfile *cf, void *data)
      +}
      
       ## chunk-format.h ##
     -@@
     - struct hashfile;
     - struct chunkfile;
     +@@ chunk-format.h: struct chunkfile;
     + 
     + #define CHUNK_TOC_ENTRY_SIZE (sizeof(uint32_t) + sizeof(uint64_t))
       
      +/*
      + * Initialize a 'struct chunkfile' for writing _or_ reading a file
     @@ chunk-format.h
      + * If writing a file, supply a non-NULL 'struct hashfile *' that will
      + * be used to write.
      + *
     -+ * If reading a file, then supply the memory-mapped data to the
     ++ * 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
 13:  7838ad32e2e0 ! 13:  87f441fc5e15 commit-graph: use chunk-format read API
     @@ Commit message
          Signed-off-by: Derrick Stolee <[email-masked]>
      
       ## commit-graph.c ##
     +@@ commit-graph.c: 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)
      @@ commit-graph.c: static int verify_commit_graph_lite(struct commit_graph *g)
       	return 0;
       }
     @@ commit-graph.c: static int verify_commit_graph_lite(struct commit_graph *g)
       
       	if (!graph_map)
       		return NULL;
     +@@ commit-graph.c: 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);
      @@ commit-graph.c: struct commit_graph *parse_commit_graph(struct repository *r,
       		return NULL;
       	}
 14:  6bddd9e63b9b ! 14:  64010031db0e midx: use chunk-format read API
     @@ Commit message
          Signed-off-by: Derrick Stolee <[email-masked]>
      
       ## midx.c ##
     +@@
     + #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))
      @@ midx.c: static char *get_midx_filename(const char *object_dir)
       	return xstrfmt("%s/pack/multi-pack-index", object_dir);
       }
 15:  3cd97f389f1f = 15:  16205d8f8c40 midx: use 64-bit multiplication for chunk sizes
 16:  b9a1bddf615f = 16:  0e58218be418 chunk-format: restore duplicate chunk checks
 17:  4c7d751f1e39 = 17:  84bf6506dc12 chunk-format: add technical docs

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