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2020-06-09reftable: define version 2 of the spec to accomodate SHA256Libravatar Han-Wen Nienhuys1-37/+45
Version appends a hash ID to the file header, making it slightly larger. This commit also changes "SHA-1" into "object ID" in many places. Signed-off-by: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-06-09reftable: clarify how empty tables should be writtenLibravatar Han-Wen Nienhuys1-0/+6
The format allows for some ambiguity, as a lone footer also starts with a valid file header. However, the current JGit code will barf on this. This commit codifies this behavior into the standard. Signed-off-by: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-06-09reftable: file format documentationLibravatar Jonathan Nieder1-0/+1069
Shawn Pearce explains: Some repositories contain a lot of references (e.g. android at 866k, rails at 31k). The reftable format provides: - Near constant time lookup for any single reference, even when the repository is cold and not in process or kernel cache. - Near constant time verification if a SHA-1 is referred to by at least one reference (for allow-tip-sha1-in-want). - Efficient lookup of an entire namespace, such as `refs/tags/`. - Support atomic push `O(size_of_update)` operations. - Combine reflog storage with ref storage. This file format spec was originally written in July, 2017 by Shawn Pearce. Some refinements since then were made by Shawn and by Han-Wen Nienhuys based on experiences implementing and experimenting with the format. (All of this was in the context of our work at Google and Google is happy to contribute the result to the Git project.) Imported from JGit[1]'s current version (c217d33ff, "Documentation/technical/reftable: improve repo layout", 2020-02-04) of Documentation/technical/reftable.md and converted to asciidoc by running pandoc -t asciidoc -f markdown reftable.md >reftable.txt using pandoc 2.2.1. The result required the following additional minor changes: - removed the [TOC] directive to add a table of contents, since asciidoc does not support it - replaced git-scm.com/docs links with linkgit: directives that link to other pages within Git's documentation [1] https://eclipse.googlesource.com/jgit/jgit Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>