Subject: [PATCH 17/17] chunk-format: add technical docs

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Tue, 26 Jan 2021 16:01:26 +0000

From: Derrick Stolee <[email-masked]>

The chunk-based file format is now an API in the code, but we should
also take time to document it as a file format. Specifically, it matches
the CHUNK LOOKUP sections of the commit-graph and multi-pack-index
files, but there are some commonalities that should be grouped in this
document.

Signed-off-by: Derrick Stolee <[email-masked]>
---
 Documentation/technical/chunk-format.txt      | 54 +++++++++++++++++++
 .../technical/commit-graph-format.txt         |  3 ++
 Documentation/technical/pack-format.txt       |  3 ++
 3 files changed, 60 insertions(+)
 create mode 100644 Documentation/technical/chunk-format.txt

diff --git a/Documentation/technical/chunk-format.txt b/Documentation/technical/chunk-format.txt
new file mode 100644
index 00000000000..3db3792dea2
--- /dev/null
+++ b/Documentation/technical/chunk-format.txt
@@ -0,0 +1,54 @@
+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, including:
+
+ 1. Writing and reading the table of contents.
+
+ 2. Verifying that the data written in a chunk matches the expected size
+    that was recorded in the table of contents.
+
+ 3. Checking that a table of contents describes offsets properly within
+    the file boundaries.
diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt
index b6658eff188..87971c27dd7 100644
--- a/Documentation/technical/commit-graph-format.txt
+++ b/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.
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index f96b2e605f3..2fb1e60d29e 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -301,6 +301,9 @@ 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.
-- 
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