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Cheviiot
b366dfa9ac
combineDirTrees' merge logic assumes it's the only thing moving files into a given target at a time; two `vintner download`/`install` runs racing against the same --dest could otherwise interleave os.Rename calls and corrupt the tree instead of erroring cleanly. Take an exclusive, non-blocking flock(2) on the destination for the duration of each run, so a second invocation fails immediately with a clear message instead of silently colliding with the first.
50 lines
1.9 KiB
Go
50 lines
1.9 KiB
Go
// Package lock guards a destination directory against two `vintner
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// download`/`install` runs mutating it at the same time.
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package lock
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import (
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"fmt"
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"os"
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"path/filepath"
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"syscall"
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)
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// FileName is the advisory lock file `download` and `install` both take
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// out against their (usually shared) destination directory before
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// touching anything in it - concurrent download+install, or two
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// downloads, against the same dest could otherwise interleave badly:
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// combineDirTrees' merge logic assumes it's the only thing moving files
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// into a given target at a time, and two `os.Rename` calls racing for the
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// same destination path is exactly the kind of thing that corrupts a tree
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// instead of erroring cleanly.
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const FileName = ".vintner.lock"
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// Acquire takes an exclusive, non-blocking lock on dest (creating dest if
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// it doesn't exist yet) and returns a func to release it, which the caller
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// must defer. If another vintner process already holds the lock, returns
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// an error immediately instead of blocking - there's no reason a second
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// invocation should silently queue up and wait for the first to finish
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// touching the same directory; the caller should simply not have started
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// it yet. Uses flock(2), so a crashed holder's lock is released
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// automatically by the kernel when its file descriptor closes - never
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// needs manual cleanup, unlike a plain "does a file exist" lock
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// convention would.
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func Acquire(dest string) (unlock func(), err error) {
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if err := os.MkdirAll(dest, 0o755); err != nil {
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return nil, err
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}
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path := filepath.Join(dest, FileName)
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f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o644)
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if err != nil {
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return nil, err
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}
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if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
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f.Close()
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return nil, fmt.Errorf("another vintner download/install is already running against %s", dest)
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}
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return func() {
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syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
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f.Close()
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}, nil
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}
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