forked from TrueCloudLab/restic
910d917b71
... called backend/sema. I resisted the temptation to call the main type sema.Phore. Also, semaphores are now passed by value to skip a level of indirection when using them.
65 lines
1.4 KiB
Go
65 lines
1.4 KiB
Go
// Package sema implements semaphores.
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package sema
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import (
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"context"
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"io"
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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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// New returns a new semaphore with capacity n.
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func New(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() { s.ch <- struct{}{} }
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// ReleaseToken returns a token.
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func (s Semaphore) ReleaseToken() { <-s.ch }
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// ReleaseTokenOnClose wraps an io.ReadCloser to return a token on Close.
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// Before returning the token, cancel, if not nil, will be run
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// to free up context resources.
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func (s Semaphore) ReleaseTokenOnClose(rc io.ReadCloser, cancel context.CancelFunc) io.ReadCloser {
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return &wrapReader{ReadCloser: rc, sem: s, cancel: cancel}
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}
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type wrapReader struct {
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io.ReadCloser
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eofSeen bool
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sem Semaphore
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cancel context.CancelFunc
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}
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func (wr *wrapReader) Read(p []byte) (int, error) {
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if wr.eofSeen { // XXX Why do we do this?
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return 0, io.EOF
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}
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n, err := wr.ReadCloser.Read(p)
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if err == io.EOF {
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wr.eofSeen = true
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}
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return n, err
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}
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func (wr *wrapReader) Close() error {
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err := wr.ReadCloser.Close()
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if wr.cancel != nil {
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wr.cancel()
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}
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wr.sem.ReleaseToken()
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return err
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}
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