49 lines
2.4 KiB
Markdown
49 lines
2.4 KiB
Markdown
Started the day by getting the builds updated for yesterday's release. This
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included making it possible to build git-annex with Debian stable's version
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of cryptohash. Also updated the Debian stable backport to the previous
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release.
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----
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The [[design/roadmap]] has this month devoted to improving git-annex's
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support for recovering from disasters, broken repos, and so on. Today I've
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been working on the first thing on [[the_list|design/disaster_recovery]],
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stale git index lock files.
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It's unfortunate that git uses simple files for locking, and does not use
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fcntl or flock to prevent the stale lock file problem. Perhaps they want
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it to work on broken NFS systems? The problem with that line of thinking is
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is means all non-broken systems end up broken by stale lock files. Not a
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good tradeoff IMHO.
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There are actually two lock files that can end up stale when using
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git-annex; both `.git/index.lock` and `.git/annex/index.lock`. Today I
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concentrated on the latter, because I saw a way to prevent it from ever
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being a problem. All updates to that index file are done by git-annex when
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committing to the git-annex branch. git-annex already uses fcntl locking
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when manipulating its journal. So, that can be extended to also cover
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committing to the git-annex branch, and then the git `index.lock` file
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is irrelevant, and can just be removed if it exists when a commit is
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started.
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To prove this makes sense, I used the type system to prove that the journal
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locking was in effect everywhere I need it to be. Very happy I was able to
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do that, although I am very much a novice at using the type system for
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interesting proofs. But doing so made it very easily to build up to a point
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where I could unlink the `.git/annex/index.lock` and be sure it was safe to do
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that.
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----
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What about stale `.git/index.lock` files? I don't think it's appropriate
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for git-annex to generally recover from those, because it would change
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regular git command line behavior, and risks breaking something. However, I
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do want the assistant to be able to recover if such a file exists when it
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is starting up, since that would prevent it from running. Implemented that
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also today, although I am less happy with the way the assistant detects
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when this lock file is stale, which is somewhat heuristic (but should work
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even on networked filesystems with multiple writing machines).
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----
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Today's work was sponsored by Torbjørn Thorsen.
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