mirror of
https://github.com/ethereum/go-ethereum.git
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95 lines
2.9 KiB
Go
95 lines
2.9 KiB
Go
// Copyright 2015 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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package rlpx
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import (
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"errors"
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"fmt"
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"sync/atomic"
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"time"
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)
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var errAcquireTimeout = errors.New("acquisition timeout")
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// bufSema is a counting semaphore.
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type bufSema struct {
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val, cap, waiting uint32
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wakeup chan struct{}
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}
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func newBufSema(cap uint32) *bufSema {
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return &bufSema{cap: cap, val: cap, wakeup: make(chan struct{})}
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}
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func (sem *bufSema) get() uint32 {
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return atomic.LoadUint32(&sem.val)
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}
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// release increments sem, potentially unblocking a call to
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// waitAcquire if there is one. release never blocks.
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func (sem *bufSema) release(n uint32) {
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new := atomic.AddUint32(&sem.val, n)
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if new > sem.cap {
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panic(fmt.Sprintf("semaphore count %d exceeds cap after release(%d)", new, n))
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}
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// Wake up a pending waitAcquire call if there is one.
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if atomic.CompareAndSwapUint32(&sem.waiting, 1, 0) {
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sem.wakeup <- struct{}{}
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}
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}
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// waitAcquire decrements the semaphore by n. If less than
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// n units are available, waitAcquire blocks until release is called.
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// It may only be called from one goroutine at a time.
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func (sem *bufSema) waitAcquire(n uint32, timeout time.Duration) error {
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if n > sem.cap {
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return fmt.Errorf("requested amount %d exceeds semaphore cap of %d", n, sem.cap)
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}
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var timer *time.Timer
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for {
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// Set the waiting flag so release will try to wake us after
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// incrementing sem.val.
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if !atomic.CompareAndSwapUint32(&sem.waiting, 0, 1) {
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panic("concurrent call to waitAcquire")
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}
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// Decrement if sem.val if possible.
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if atomic.LoadUint32(&sem.val) >= n {
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atomic.AddUint32(&sem.val, ^(n - 1))
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// Gobble up wakeup signal in case release decremented sem.waiting.
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if !atomic.CompareAndSwapUint32(&sem.waiting, 1, 0) {
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<-sem.wakeup
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}
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return nil
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}
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// Start the timeout on the first iteration.
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if timer == nil {
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timer = time.NewTimer(timeout)
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defer timer.Stop()
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}
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select {
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case <-sem.wakeup:
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// Woken by release. It has decremented sem.waiting back to zero.
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case <-timer.C:
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// Gobble up wakeup signal in case release decremented sem.waiting.
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if !atomic.CompareAndSwapUint32(&sem.waiting, 1, 0) {
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<-sem.wakeup
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}
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return errAcquireTimeout
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}
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}
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return nil
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}
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