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The SemaphoreBackend now uniformly enforces the limit of concurrent backend operations. In addition, it unifies the parameter validation. The List() methods no longer uses a semaphore. Restic already never runs multiple list operations in parallel. By managing the semaphore in a wrapper backend, the sections that hold a semaphore token grow slightly. However, the main bottleneck is IO, so this shouldn't make much of a difference. The key insight that enables the SemaphoreBackend is that all of the complex semaphore handling in `openReader()` still happens within the original call to `Load()`. Thus, getting and releasing the semaphore tokens can be refactored to happen directly in `Load()`. This eliminates the need for wrapping the reader in `openReader()` to release the token.
32 lines
712 B
Go
32 lines
712 B
Go
// Package sema implements semaphores.
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package sema
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import (
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"github.com/restic/restic/internal/debug"
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"github.com/restic/restic/internal/errors"
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)
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// A semaphore limits access to a restricted resource.
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type semaphore struct {
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ch chan struct{}
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}
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// newSemaphore returns a new semaphore with capacity n.
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func newSemaphore(n uint) (semaphore, error) {
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if n == 0 {
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return semaphore{}, errors.New("capacity must be a positive number")
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}
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return semaphore{
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ch: make(chan struct{}, n),
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}, nil
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}
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// GetToken blocks until a Token is available.
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func (s semaphore) GetToken() {
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s.ch <- struct{}{}
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debug.Log("acquired token")
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}
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// ReleaseToken returns a token.
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func (s semaphore) ReleaseToken() { <-s.ch }
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