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// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package base
import (
"fmt"
"internal/godebug"
"runtime"
"strconv"
"sync"
)
var NetLimitGodebug = godebug.New("#cmdgonetlimit")
// NetLimit returns the limit on concurrent network operations
// configured by GODEBUG=cmdgonetlimit, if any.
//
// A limit of 0 (indicated by 0, true) means that network operations should not
// be allowed.
func NetLimit() (int, bool) {
netLimitOnce.Do(func() {
s := NetLimitGodebug.Value()
if s == "" {
return
}
n, err := strconv.Atoi(s)
if err != nil {
Fatalf("invalid %s: %v", NetLimitGodebug.Name(), err)
}
if n < 0 {
// Treat negative values as unlimited.
return
}
netLimitSem = make(chan struct{}, n)
})
return cap(netLimitSem), netLimitSem != nil
}
// AcquireNet acquires a semaphore token for a network operation.
func AcquireNet() (release func(), err error) {
hasToken := false
if n, ok := NetLimit(); ok {
if n == 0 {
return nil, fmt.Errorf("network disabled by %v=%v", NetLimitGodebug.Name(), NetLimitGodebug.Value())
}
netLimitSem <- struct{}{}
hasToken = true
}
checker := new(netTokenChecker)
runtime.SetFinalizer(checker, (*netTokenChecker).panicUnreleased)
return func() {
if checker.released {
panic("internal error: net token released twice")
}
checker.released = true
if hasToken {
<-netLimitSem
}
runtime.SetFinalizer(checker, nil)
}, nil
}
var (
netLimitOnce sync.Once
netLimitSem chan struct{}
)
type netTokenChecker struct {
released bool
// We want to use a finalizer to check that all acquired tokens are returned,
// so we arbitrarily pad the tokens with a string to defeat the runtime's
// “tiny allocator”.
unusedAvoidTinyAllocator string
}
func (c *netTokenChecker) panicUnreleased() {
panic("internal error: net token acquired but not released")
}