From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-3.7 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_BLOCKED, SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by dcvr.yhbt.net (Postfix) with ESMTP id 4E7231F4B4 for ; Wed, 27 Jan 2021 15:36:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235687AbhA0Pez (ORCPT ); Wed, 27 Jan 2021 10:34:55 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52120 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235308AbhA0PFP (ORCPT ); Wed, 27 Jan 2021 10:05:15 -0500 Received: from mail-wr1-x42a.google.com (mail-wr1-x42a.google.com [IPv6:2a00:1450:4864:20::42a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DD599C061221 for ; Wed, 27 Jan 2021 07:02:17 -0800 (PST) Received: by mail-wr1-x42a.google.com with SMTP id h9so2227351wrr.9 for ; Wed, 27 Jan 2021 07:02:17 -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=RGKngy0xof48CDrWck0RiLoH8QLRTj9BkrR+HiH5rVb2jNbbsoQL2Y/nWKlpG2/uV3 aa03OoOfRql3W9pecV9qxd3z5uq4P1xb4/MLSbgmNNSSi4pvgnG327Feb1ruzR3WBa3b Kp+e8LEdvSy6wqyEEP3x4q8x/+lV5FbuXurGgLUr7KDmkaxot10KrB8kIaHkbZymboDP gXc8rwi9anrGFnTlPn9X0Mp2JrU6PeTN0jjN7N8mi+4I5qizXdg3EVBQrBSTG2XuO8RP VcUOo7wHX7kWImtYwhqJvLARwKkcLSPa2imaAWy7pObR5uyoEgttSx5q1SZUDaZDFF7E EsRg== 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=QajttKdzFceHo3h9Ggozh5bxqNn/ZT+V+e6aRvqwNbicvyVQfO1u+WyET6sLIBN9xM spIbTJzdirnoiqV56/y0qgUzUWxtgoYef4c/eQoevVHI4eOsHPDwuycOIqNCGzfp93z5 7AVGXo0sF1x45dL3U/iiQ8IbR+9X9iHNsvOFTGyljesfLJDCtFZwwxaKZv4NI9CjwjNE iflhEw51coEpFXekMbpJfeeN1s/8HtwQQnjqLKs/pqD3V6NHVC7/PqEk3BiNnZtzxDPY 5r69LmjbHGDUcjds5b68+WRwId+L5/sXKnhKQWFfcOieMCVBOUfQ6Afea1EhURksS2BB yEAA== X-Gm-Message-State: AOAM530I2GK0g9g7+aUWxvTE9tPzHMh4DutHDL7rEvP83+s8Sti220zo tK7u5C+nnOcpFeksHXpVfqcJuG1l7MU= X-Google-Smtp-Source: ABdhPJzPc7tCV6IT0sOPcvzSru6jbSJyf4D1j2miJSIxLSqD0hnUfgdap6h+kmoNBVLUdg/hUxPifw== X-Received: by 2002:a5d:6092:: with SMTP id w18mr11529833wrt.75.1611759736310; Wed, 27 Jan 2021 07:02:16 -0800 (PST) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id c20sm2646925wmb.38.2021.01.27.07.02.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Jan 2021 07:02:15 -0800 (PST) Message-Id: <8f3985ab5df3e4abc6de6db7f71f1adcbc16b4a8.1611759716.git.gitgitgadget@gmail.com> In-Reply-To: References: From: "Derrick Stolee via GitGitGadget" Date: Wed, 27 Jan 2021 15:01:56 +0000 Subject: [PATCH v2 17/17] chunk-format: add technical docs Fcc: Sent Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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, Chris Torek , Derrick Stolee , Derrick Stolee , Derrick Stolee Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org 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. -- gitgitgadget