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The existing chunk formats use the table of contents at the beginning of the file. This is intended as a way to speed up the initial loading of the file, but comes at a cost during writes. Each example needs to fully compute how big each chunk will be in advance, which usually requires storing the full file contents in memory. Future file formats may want to use the chunk format API in cases where the writing stage is critical to performance, so we may want to stream updates from an existing file and then only write the table of contents at the end. Add a new 'flags' parameter to write_chunkfile() that allows this behavior. When this is specified, the defensive programming that checks that the chunks are written with the precomputed sizes is disabled. Then, the table of contents is written in reverse order at the end of the hashfile, so a parser can read the chunk list starting from the end of the file (minus the hash). The parsing of these table of contents will come in a later change. Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Taylor Blau <me@ttaylorr.com>
79 lines
2.1 KiB
C
79 lines
2.1 KiB
C
#ifndef CHUNK_FORMAT_H
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#define CHUNK_FORMAT_H
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#include "git-compat-util.h"
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#include "hash.h"
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struct hashfile;
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struct chunkfile;
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#define CHUNK_TOC_ENTRY_SIZE (sizeof(uint32_t) + sizeof(uint64_t))
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/*
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* Initialize a 'struct chunkfile' for writing _or_ reading a file
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* with the chunk format.
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*
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* If writing a file, supply a non-NULL 'struct hashfile *' that will
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* be used to write.
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*
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* If reading a file, use a NULL 'struct hashfile *' and then call
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* read_table_of_contents(). Supply the memory-mapped data to the
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* pair_chunk() or read_chunk() methods, as appropriate.
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*
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* DO NOT MIX THESE MODES. Use different 'struct chunkfile' instances
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* for reading and writing.
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*/
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struct chunkfile *init_chunkfile(struct hashfile *f);
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void free_chunkfile(struct chunkfile *cf);
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int get_num_chunks(struct chunkfile *cf);
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typedef int (*chunk_write_fn)(struct hashfile *f, void *data);
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void add_chunk(struct chunkfile *cf,
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uint32_t id,
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size_t size,
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chunk_write_fn fn);
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enum chunkfile_flags {
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CHUNKFILE_TRAILING_TOC = (1 << 0),
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};
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int write_chunkfile(struct chunkfile *cf,
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enum chunkfile_flags flags,
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void *data);
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int read_table_of_contents(struct chunkfile *cf,
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const unsigned char *mfile,
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size_t mfile_size,
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uint64_t toc_offset,
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int toc_length);
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#define CHUNK_NOT_FOUND (-2)
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/*
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* Find 'chunk_id' in the given chunkfile and assign the
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* given pointer to the position in the mmap'd file where
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* that chunk begins.
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*
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* Returns CHUNK_NOT_FOUND if the chunk does not exist.
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*/
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int pair_chunk(struct chunkfile *cf,
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uint32_t chunk_id,
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const unsigned char **p);
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typedef int (*chunk_read_fn)(const unsigned char *chunk_start,
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size_t chunk_size, void *data);
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/*
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* Find 'chunk_id' in the given chunkfile and call the
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* given chunk_read_fn method with the information for
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* that chunk.
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*
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* Returns CHUNK_NOT_FOUND if the chunk does not exist.
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*/
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int read_chunk(struct chunkfile *cf,
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uint32_t chunk_id,
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chunk_read_fn fn,
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void *data);
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uint8_t oid_version(const struct git_hash_algo *algop);
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#endif
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