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cmd/go: add a GODEBUG to limit the number of concurrent network conne…
…ctions I implemented this in order to debug connection failures on a new-to-me VM development environment that uses Cloud NAT. It doesn't directly fix the bug, but perhaps folks will find it useful to diagnose port-exhaustion-related flakiness in other environments. For #52545. Change-Id: Icd3f13dcf62e718560c4f4a965a4df7c1bd785ce Reviewed-on: https://go-review.googlesource.com/c/go/+/473277 Run-TryBot: Bryan Mills <bcmills@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org> Auto-Submit: Bryan Mills <bcmills@google.com>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,84 @@ | ||
| // 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") | ||
|
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| // 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 | ||
| } | ||
|
|
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| 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) | ||
| }) | ||
|
|
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| return cap(netLimitSem), netLimitSem != nil | ||
| } | ||
|
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| // 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 | ||
| } | ||
|
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| checker := new(netTokenChecker) | ||
| runtime.SetFinalizer(checker, (*netTokenChecker).panicUnreleased) | ||
|
|
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| 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{} | ||
| ) | ||
|
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| 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 | ||
| } | ||
|
|
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| func (c *netTokenChecker) panicUnreleased() { | ||
| panic("internal error: net token acquired but not released") | ||
| } |
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