From patchwork Tue Jan 26 16:01:26 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Derrick Stolee X-Patchwork-Id: 12047291 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3E777C433DB for ; Tue, 26 Jan 2021 16:04:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0AE882245C for ; Tue, 26 Jan 2021 16:04:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404310AbhAZQDv (ORCPT ); Tue, 26 Jan 2021 11:03:51 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37606 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2404570AbhAZQCu (ORCPT ); Tue, 26 Jan 2021 11:02:50 -0500 Received: from mail-wr1-x435.google.com (mail-wr1-x435.google.com [IPv6:2a00:1450:4864:20::435]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 91573C0698CB for ; Tue, 26 Jan 2021 08:01:45 -0800 (PST) Received: by mail-wr1-x435.google.com with SMTP id 6so17019896wri.3 for ; Tue, 26 Jan 2021 08:01:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=message-id:in-reply-to:references:from:date:subject:fcc :content-transfer-encoding:mime-version:to:cc; bh=/31wU3H+Rpd5f02JkLU2apACd46gCGYTclTqM21pbrU=; b=WH1gy189bG8syuAuMMA8X5of+0Qp3VhR7nIfFuXmQHgMJXW+Xidwca+Obgzhx2C+f+ 5nB76y3MODoeRmZtKTIvJzlAebEzB8BP5PhyPOcpY527MHmiEqtuKn4WH7qmL1X2bLpN RsoH2WAF0OQIPCdip1GS49MXJ3AVDPzBP+Mu4zd0Uqo2Itn6FGLwtpARubn4weIRXMeN 99D1jWX7lm+iLbsZBjvAOJEMRCYIF6bCLl1nAce5yX1rU18b13ZFEBFms9bHgWatFjkz iOW96gMNqfIhZPWGUe1AT7YbO//cW7r6ZzGRv0V3g0IJdnC6zganhTMuaS1TsQMDWb64 4vHg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:message-id:in-reply-to:references:from:date :subject:fcc:content-transfer-encoding:mime-version:to:cc; bh=/31wU3H+Rpd5f02JkLU2apACd46gCGYTclTqM21pbrU=; b=qAMMLyQCu65WlR0uiXQyiswtHNe9x3hzEAfHSLhvrVbYLFSt6tkVaWvu0MZvXkEasg JNJHHwHnuEymgOVUtV4n9RD9l7DHB7CB1Fq0kOM8F9e6XCAPIsIy+R5qj08r6DMIZgNP k/sl+OBmt0EwWxkDcE4Yp50ydHBXPne9UeixwROq9bE6k0H+YuuYYzFNpQEv48zHsBJm PpzxXDFRKX3q6tTOcjimNicyI7J/7UPAiFkaKVi0R1YdepA4hPxfE3r8ARAEXGzC/HD7 OgUnbIpC4sEXvCt8kauUtUFd5KcaHmo4HehcINPa6qCKSI6eVRvtEzC/EDhl6PozIkMm jSTQ== X-Gm-Message-State: AOAM53352ZPvpgq9shwEdSeKH4VjV8jjkc1nR8PifzzQ+dlvJhtq3Ve9 ExXD98TkDOtyJMM+yV9QO3aUpa+n2QU= X-Google-Smtp-Source: ABdhPJwAZHOMvDFOmAqpszmk6mDC4AyHbP7tRBFDrBw7r+k+wYUaKSlQ2ZRHsbuhrJ2j/jfFWzyoCA== X-Received: by 2002:adf:f909:: with SMTP id b9mr7028152wrr.111.1611676904149; Tue, 26 Jan 2021 08:01:44 -0800 (PST) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id w4sm3670237wmc.13.2021.01.26.08.01.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 26 Jan 2021 08:01:43 -0800 (PST) Message-Id: <05cbd0a8d93a3e54d868a549fe76e16cb75ba6d6.1611676886.git.gitgitgadget@gmail.com> In-Reply-To: References: Date: Tue, 26 Jan 2021 16:01:26 +0000 Subject: [PATCH 17/17] chunk-format: add technical docs Fcc: Sent MIME-Version: 1.0 To: git@vger.kernel.org Cc: me@ttaylorr.com, gitster@pobox.com, l.s.r@web.de, szeder.dev@gmail.com, Derrick Stolee , Derrick Stolee Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org From: Derrick Stolee From: Derrick Stolee 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 --- 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.