Adding upstream version 1:2.39.2.upstream/1%2.39.2upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 133 insertions, 0 deletions

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+gitformat-chunk(5)
+==================
+
+NAME
+----
+gitformat-chunk - Chunk-based file formats
+
+SYNOPSIS
+--------
+
+Used by linkgit:gitformat-commit-graph[5] and the "MIDX" format (see
+the pack format documentation in linkgit:gitformat-pack[5]).
+
+DESCRIPTION
+-----------
+
+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
+the `multi-pack-index` format in linkgit:gitformat-pack[5] and
+the `commit-graph` format in linkgit:gitformat-commit-graph[5] 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
+  the commit-graph file format in linkgit:gitformat-commit-graph[5].
+
+* *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
+  the multi-pack-index file format section of linkgit:gitformat-pack[5].
+
+GIT
+---
+Part of the linkgit:git[1] suite