go: net/http: connCount underflow panic with MaxConnsPerHost

I’m splitting this off from https://github.com/golang/go/issues/34941 because it seems to be a separate issue, although with the same symptom.

$ go version
go version go1.14 darwin/amd64

I compiled this binary on macOS, but I’m running it on Linux amd64.

Here is the stack trace:

panic: net/http: internal error: connCount underflow

goroutine 2114 [running]:
net/http.(*Transport).decConnsPerHost(0x1094ca0, 0x0, 0x0, 0xc001f110b5, 0x4, 0xc001a27320, 0x14, 0x0)
		/usr/local/go/src/net/http/transport.go:1391 +0x590
net/http.(*persistConn).closeLocked(0xc0000cd320, 0xb8a620, 0xc000096b60)
		/usr/local/go/src/net/http/transport.go:2584 +0xf5
net/http.(*persistConn).close(0xc0000cd320, 0xb8a620, 0xc000096b60)
		/usr/local/go/src/net/http/transport.go:2574 +0x83
net/http.(*persistConn).readLoop.func1(0xc0000cd320, 0xc0073b4d60)
		/usr/local/go/src/net/http/transport.go:1946 +0x41
net/http.(*persistConn).readLoop(0xc0000cd320)
		/usr/local/go/src/net/http/transport.go:2122 +0x12ae
created by net/http.(*Transport).dialConn
		/usr/local/go/src/net/http/transport.go:1647 +0xc56

The usage is in a forward proxy server (github.com/andybalholm/redwood). We have about 2000 users running through the server I have been watching these errors on the most closely. So the usage is heavy, and probably about as close to random as you are likely to find.

I thought that switching to two Transports (one for HTTP/1.1 and one for HTTP/2) instead of one http.Transport that was configured to automatically switch protocols would get rid of the panics. But it didn’t seem to help. (So it seems that the panic doesn’t depend on having HTTP/2 enabled on the Transport.)

I had Transport.MaxConnsPerHost set to 8. Setting it to 0 (unlimited) made the panics go away.

About this issue

Original URL
State: closed
Created 4 years ago
Comments: 19 (17 by maintainers)

Most upvoted comments

I can’t run it with -race on the server where I was getting the panics; it’s too heavily loaded. But I tried it on a local server with just me proxied through it, and that was enough:

WARNING: DATA RACE
Read at 0x000001587540 by goroutine 87:
  net/http.(*Transport).onceSetNextProtoDefaults()
      /usr/local/go/src/net/http/transport.go:344 +0x61
  net/http.(*Transport).onceSetNextProtoDefaults-fm()
      /usr/local/go/src/net/http/transport.go:343 +0x41
  sync.(*Once).doSlow()
      /usr/local/go/src/sync/once.go:66 +0x103
  sync.(*Once).Do()
      /usr/local/go/src/sync/once.go:57 +0x68
  net/http.(*Transport).roundTrip()
      /usr/local/go/src/net/http/transport.go:485 +0x8b
  net/http.(*Transport).RoundTrip()
      /usr/local/go/src/net/http/roundtrip.go:17 +0x42
  main.proxyHandler.ServeHTTP()
      /root/src/redwood/proxy.go:328 +0x17fb
  main.(*proxyHandler).ServeHTTP()
      <autogenerated>:1 +0x114
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/src/net/http/server.go:2807 +0xce
  net/http.initALPNRequest.ServeHTTP()
      /usr/local/go/src/net/http/server.go:3381 +0xfc
  net/http.(*initALPNRequest).ServeHTTP()
      <autogenerated>:1 +0xa6
  net/http.Handler.ServeHTTP-fm()
      /usr/local/go/src/net/http/server.go:87 +0x64
  golang.org/x/net/http2.(*serverConn).runHandler()
      /root/go/pkg/mod/golang.org/x/net@v0.0.0-20190213061140-3a22650c66bd/http2/server.go:2095 +0xc0

Previous write at 0x000001587540 by main goroutine:
  golang.org/x/net/http2.configureTransport()
      /root/go/pkg/mod/golang.org/x/net@v0.0.0-20190213061140-3a22650c66bd/http2/transport.go:174 +0x420
  golang.org/x/net/http2.ConfigureTransport()
      /root/go/pkg/mod/golang.org/x/net@v0.0.0-20190213061140-3a22650c66bd/http2/transport.go:136 +0x7ca
  main.main()
      /root/src/redwood/redwood.go:88 +0x7ab

It’s been several years since I tried running with -race. I didn’t find anything then, and I figured only a heavily-loaded production server would have enough concurrency to expose races if there were any. But now I know that running with -race locally is worth doing.

Reordering the startup steps gets rid of the race.

andybalholm on Apr 10, 2020

@andybalholm I am confused. You mention that you set MaxConnsPerHost = 8, but the above does not show you setting it. Where/When did you do that.

fraenkel on Apr 9, 2020

@bcmills Here is my transport:

var dialer = &net.Dialer{
	Timeout:   30 * time.Second,
	KeepAlive: 30 * time.Second,
	DualStack: true,
}

var httpTransport = &http.Transport{
	Proxy:                 http.ProxyFromEnvironment,
	DialContext:           dialer.DialContext,
	TLSHandshakeTimeout:   10 * time.Second,
	ExpectContinueTimeout: 1 * time.Second,
}

The environment doesn’t specify a proxy. I call http2.ConfigureTransport(httpTransport) elsewhere in the code. (But not making that call doesn’t fix the problem.)

andybalholm on Apr 7, 2020

Building with

go version devel +620208790e Wed Apr 1 19:32:57 2020 +0000 darwin/amd64

I still get the panic:

panic: net/http: internal error: connCount underflow

goroutine 67124 [running]:
net/http.(*Transport).decConnsPerHost(0x108df80, 0x0, 0x0, 0xc00b2800e4, 0x4, 0xc002061900, 0x17, 0x0)
	/usr/local/go/src/net/http/transport.go:1388 +0x590
net/http.(*persistConn).closeLocked(0xc00af806c0, 0xb847e0, 0xc00006cb60)
	/usr/local/go/src/net/http/transport.go:2586 +0xf5
net/http.(*persistConn).close(0xc00af806c0, 0xb847e0, 0xc00006cb60)
	/usr/local/go/src/net/http/transport.go:2576 +0x83
net/http.(*persistConn).closeConnIfStillIdle(0xc00af806c0)
	/usr/local/go/src/net/http/transport.go:1890 +0xcb
created by time.goFunc
	/usr/local/go/src/time/sleep.go:169 +0x44

andybalholm on Apr 1, 2020