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authorLibravatar Jeff King <peff@peff.net>2016-05-18 18:45:37 -0400
committerLibravatar Junio C Hamano <gitster@pobox.com>2016-06-02 15:22:24 -0700
commit20b20a22f8f7c1420e259c97ef790cb93091f475 (patch)
tree9b4262a0b8bf6de105f5f24773406c511712d686 /Documentation/git-replace.txt
parentt1308: do not get fooled by symbolic links to the source tree (diff)
downloadtgif-20b20a22f8f7c1420e259c97ef790cb93091f475.tar.xz
upload-pack: provide a hook for running pack-objects
When upload-pack serves a client request, it turns to pack-objects to do the heavy lifting of creating a packfile. There's no easy way to intercept the call to pack-objects, but there are a few good reasons to want to do so: 1. If you're debugging a client or server issue with fetching, you may want to store a copy of the generated packfile. 2. If you're gathering data from real-world fetches for performance analysis or debugging, storing a copy of the arguments and stdin lets you replay the pack generation at your leisure. 3. You may want to insert a caching layer around pack-objects; it is the most CPU- and memory-intensive part of serving a fetch, and its output is a pure function[1] of its input, making it an ideal place to consolidate identical requests. This patch adds a simple "hook" interface to intercept calls to pack-objects. The new test demonstrates how it can be used for debugging (using it for caching is a straightforward extension; the tricky part is writing the actual caching layer). This hook is unlike the normal hook scripts found in the "hooks/" directory of a repository. Because we promise that upload-pack is safe to run in an untrusted repository, we cannot execute arbitrary code or commands found in the repository (neither in hooks/, nor in the config). So instead, this hook is triggered from a config variable that is explicitly ignored in the per-repo config. The config variable holds the actual shell command to run as the hook. Another approach would be to simply treat it as a boolean: "should I respect the upload-pack hooks in this repo?", and then run the script from "hooks/" as we usually do. However, that isn't as flexible; there's no way to run a hook approved by the site administrator (e.g., in "/etc/gitconfig") on a repository whose contents are not trusted. The approach taken by this patch is more fine-grained, if a little less conventional for git hooks (it does behave similar to other configured commands like diff.external, etc). [1] Pack-objects isn't _actually_ a pure function. Its output depends on the exact packing of the object database, and if multi-threading is used for delta compression, can even differ racily. But for the purposes of caching, that's OK; of the many possible outputs for a given input, it is sufficient only that we output one of them. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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