net/http: HTTP/2 with MaxConnsPerHost hangs or crashes #34941

rhysh · 2019-10-16T21:33:01Z

What version of Go are you using (`go version`)?

$ go1.13 version
go version go1.13.1 darwin/amd64
$ go-tip version
go version devel +f6c624a22a Wed Oct 16 15:58:33 2019 +0000 darwin/amd64
$ go1.12 version
go version go1.12.10 darwin/amd64
$ go1.11 version
go version go1.11.13 darwin/amd64

Does this issue reproduce with the latest release?

Yes: The test hangs with Go 1.13. The test panics with tip.

What operating system and processor architecture are you using (`go env`)?

go env Output

$ go1.13 env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/rhys/Library/Caches/go-build"
GOENV="/Users/rhys/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY="*"
GONOSUMDB="*"
GOOS="darwin"
GOPATH="/Users/rhys/go"
GOPRIVATE="*"
GOPROXY="direct"
GOROOT="/Users/rhys/go/version/go1.13"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/Users/rhys/go/version/go1.13/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/49/zmds5zsn75z1283vtzxyfr5hj7yjq4/T/go-build216795429=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Using an http.Client with an http.Transport configured to use http/2 and with MaxConnsPerHost = 1, I started a large number of concurrent http/2 requests: more than the server would allow on a single TCP connection.

In the test environment, I did this by having the server delay its responses until I knew a large number of requests were either active on the connection or waiting for permission to use the connection, at which point I allowed the http.Handler to respond.

What did you expect to see?

I expected that every request I started would eventually be seen by the server, and that the client would receive the server's response.

What did you see instead?

In Go 1.11 where the default behavior is to use a single TCP connection for each http/2 authority, the requests queue up and once I allow the handler to return they are all processed. That's fine in this context.

In Go 1.12 where the http.Transport is correctly able to use more than one TCP connection for each http/2 authority (but doesn't yet know how to use MaxConnsPerHost), it passes all of the requests on to the server immediately. That's also fine in this context.

In Go 1.13, the server only sees the first 249 requests. The others appear stuck indefinitely. I see that as a bug.

In tip, the Transport panics. That's a second bug.

$ go1.13 test -v ./repro_test.go
=== RUN   TestMaxConns
2019/10/16 14:27:43 http: TLS handshake error from 127.0.0.1:50300: remote error: tls: bad certificate
--- FAIL: TestMaxConns (1.15s)
    repro_test.go:82: starts=249 finishes=0
    repro_test.go:82: starts=249 finishes=0
    repro_test.go:82: starts=249 finishes=0
    repro_test.go:82: starts=249 finishes=0
    repro_test.go:82: starts=249 finishes=0
    repro_test.go:82: starts=249 finishes=249
    repro_test.go:82: starts=249 finishes=249
    repro_test.go:82: starts=249 finishes=249
    repro_test.go:82: starts=249 finishes=249
    repro_test.go:82: starts=249 finishes=249
    repro_test.go:86: HTTP/2 requests started: 249 != 300
    repro_test.go:89: HTTP/2 requests completed: 249 != 300
FAIL
FAIL    command-line-arguments  2.287s
FAIL

$ go-tip test -v ./repro_test.go
=== RUN   TestMaxConns
2019/10/16 14:27:57 http: TLS handshake error from 127.0.0.1:50603: remote error: tls: bad certificate
panic: net/http: internal error: connCount underflow

goroutine 371 [running]:
net/http.(*Transport).decConnsPerHost(0xc0000d63c0, 0x0, 0x0, 0xc0000c6540, 0x5, 0xc0000160d0, 0xf, 0x0)
        /usr/local/go/src/net/http/transport.go:1318 +0x604
net/http.(*Transport).roundTrip(0xc0000d63c0, 0xc00024f200, 0x0, 0xc000626ac0, 0x100e518)
        /usr/local/go/src/net/http/transport.go:542 +0x762
net/http.(*Transport).RoundTrip(0xc0000d63c0, 0xc00024f200, 0xc0000d63c0, 0x0, 0x0)
        /usr/local/go/src/net/http/roundtrip.go:17 +0x35
net/http.send(0xc00024f200, 0x13bd4c0, 0xc0000d63c0, 0x0, 0x0, 0x0, 0xc000010338, 0x241ef00, 0x1, 0x0)
        /usr/local/go/src/net/http/client.go:252 +0x43e
net/http.(*Client).send(0xc000096e10, 0xc00024f200, 0x0, 0x0, 0x0, 0xc000010338, 0x0, 0x1, 0x20300000000000)
        /usr/local/go/src/net/http/client.go:176 +0xfa
net/http.(*Client).do(0xc000096e10, 0xc00024f200, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/http/client.go:692 +0x3c8
net/http.(*Client).Do(...)
        /usr/local/go/src/net/http/client.go:560
command-line-arguments.TestMaxConns.func2(0xc00040ca70, 0xc00009e2f8, 0xc00040e700, 0xc000590468, 0xc000086300)
        /tmp/repro_test.go:69 +0x1fb
created by command-line-arguments.TestMaxConns
        /tmp/repro_test.go:63 +0x451
FAIL    command-line-arguments  1.250s
FAIL

And for context, the passing results with Go 1.11 and Go 1.12:

$ go1.11 test -v ./repro_test.go
=== RUN   TestMaxConns
2019/10/16 14:27:50 http: TLS handshake error from 127.0.0.1:50307: remote error: tls: bad certificate
--- PASS: TestMaxConns (1.15s)
    repro_test.go:82: starts=155 finishes=0
    repro_test.go:82: starts=250 finishes=0
    repro_test.go:82: starts=250 finishes=0
    repro_test.go:82: starts=250 finishes=0
    repro_test.go:82: starts=250 finishes=0
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
PASS
ok      command-line-arguments  2.281s

$ go1.12 test -v ./repro_test.go
=== RUN   TestMaxConns
2019/10/16 14:27:35 http: TLS handshake error from 127.0.0.1:49994: remote error: tls: bad certificate
--- PASS: TestMaxConns (1.17s)
    repro_test.go:82: starts=84 finishes=0
    repro_test.go:82: starts=174 finishes=0
    repro_test.go:82: starts=263 finishes=0
    repro_test.go:82: starts=300 finishes=0
    repro_test.go:82: starts=300 finishes=0
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
    repro_test.go:82: starts=300 finishes=300
PASS
ok      command-line-arguments  3.505s

package repro

import (
	"context"
	"crypto/tls"
	"net/http"
	"net/http/httptest"
	"sync"
	"sync/atomic"
	"testing"
	"time"
)

func TestMaxConns(t *testing.T) {
	totalRequests := 300

	allow := make(chan struct{})
	var (
		starts   int64
		finishes int64
	)
	h := func(w http.ResponseWriter, r *http.Request) {
		if !r.ProtoAtLeast(2, 0) {
			t.Errorf("Request is not http/2: %q", r.Proto)
			return
		}
		atomic.AddInt64(&starts, 1)
		<-allow
	}

	s := httptest.NewUnstartedServer(http.HandlerFunc(h))
	s.TLS = &tls.Config{
		NextProtos: []string{"h2"},
	}
	s.StartTLS()
	defer s.Close()

	transport := s.Client().Transport.(*http.Transport)
	clientConfig := transport.TLSClientConfig
	transport.TLSClientConfig = nil

	// make a request to trigger HTTP/2 autoconfiguration
	resp, err := s.Client().Get(s.URL)
	if err == nil {
		resp.Body.Close()
	}
	// now allow the client to connect to the ad-hoc test server
	transport.TLSClientConfig.RootCAs = clientConfig.RootCAs

	transport.MaxConnsPerHost = 1

	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	var wg sync.WaitGroup
	for i := 0; i < totalRequests; i++ {
		req, err := http.NewRequest("GET", s.URL, nil)
		if err != nil {
			t.Fatalf("NewRequest: %s", err)
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			defer atomic.AddInt64(&finishes, 1)
			ctx, cancel := context.WithCancel(ctx)
			defer cancel()
			req = req.WithContext(ctx)
			resp, err := s.Client().Do(req)
			if err != nil {
				return
			}
			resp.Body.Close()
		}()
	}

	for i := 0; i < 10; i++ {
		if i == 5 {
			close(allow)
		}
		time.Sleep(100 * time.Millisecond)
		t.Logf("starts=%d finishes=%d", atomic.LoadInt64(&starts), atomic.LoadInt64(&finishes))
	}

	if have, want := atomic.LoadInt64(&starts), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests started: %d != %d", have, want)
	}
	if have, want := atomic.LoadInt64(&finishes), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests completed: %d != %d", have, want)
	}
}

The text was updated successfully, but these errors were encountered:

julieqiu · 2019-10-18T19:19:25Z

/cc @bradfitz

bradfitz · 2019-10-18T19:25:43Z

@fraenkel, another MaxConnsPerHost issue. (sorry)

(The other is #34978).

fraenkel · 2019-10-19T01:12:58Z

As written, the test case as a data race.
I changed the test case slightly:

Fixing the data race, (transport.MaxConnsHost)
using ForceAttemptHTTP2 to simplifiy the setup
count successful finishes

The failure does still occur but I can get a few successful runs.
There is obviously some book keeping issue.

It is the same for 1.13.3 and tip (46aa835)

package issue34941

import (
	"context"
	"crypto/tls"
	"net/http"
	"net/http/httptest"
	"sync"
	"sync/atomic"
	"testing"
	"time"
)

func TestMaxConns(t *testing.T) {
	totalRequests := 300

	allow := make(chan struct{})
	var (
		starts   int64
		finishes int64
	)
	h := func(w http.ResponseWriter, r *http.Request) {
		if !r.ProtoAtLeast(2, 0) {
			t.Errorf("Request is not http/2: %q", r.Proto)
			return
		}
		atomic.AddInt64(&starts, 1)
		<-allow
	}

	s := httptest.NewUnstartedServer(http.HandlerFunc(h))
	s.TLS = &tls.Config{
		NextProtos: []string{"h2"},
	}
	s.StartTLS()
	defer s.Close()

	transport := s.Client().Transport.(*http.Transport)
	// clientConfig := transport.TLSClientConfig
	// transport.TLSClientConfig = nil
	transport.MaxConnsPerHost = 1
	transport.ForceAttemptHTTP2 = true

	// make a request to trigger HTTP/2 autoconfiguration
	// resp, err := s.Client().Get(s.URL)
	// if err == nil {
	// 	resp.Body.Close()
	// }
	// now allow the client to connect to the ad-hoc test server
	// transport.TLSClientConfig.RootCAs = clientConfig.RootCAs

	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	var wg sync.WaitGroup
	for i := 0; i < totalRequests; i++ {
		req, err := http.NewRequest("GET", s.URL, nil)
		if err != nil {
			t.Fatalf("NewRequest: %s", err)
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			ctx, cancel := context.WithCancel(ctx)
			defer cancel()
			req = req.WithContext(ctx)
			resp, err := s.Client().Do(req)
			if err != nil {
				return
			}
			resp.Body.Close()
			atomic.AddInt64(&finishes, 1)
		}()
	}

	for i := 0; i < 10; i++ {
		if i == 5 {
			close(allow)
		}
		time.Sleep(100 * time.Millisecond)
		t.Logf("starts=%d finishes=%d", atomic.LoadInt64(&starts), atomic.LoadInt64(&finishes))
	}

	if have, want := atomic.LoadInt64(&starts), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests started: %d != %d", have, want)
	}
	if have, want := atomic.LoadInt64(&finishes), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests completed: %d != %d", have, want)
	}
}

fraenkel · 2019-10-19T02:18:40Z

A fix is coming shortly. We cannot blindly decrement the conn count. We need to only decrement if we have removed the idle connection.

fraenkel · 2019-10-19T02:35:30Z

Bah. A simple tweak to our existing test for MaxConnsPerHost has uncovered yet another issue.
Just tweak the loop to 300 and tip will hit the same issue reported. My fix will fix it but now I have

--- FAIL: TestTransportMaxConnsPerHost (0.06s)
    transport_test.go:658: round 1: too many dials (http2): 2 != 1
    transport_test.go:661: round 1: too many get connections (http2): 2 != 1
    transport_test.go:664: round 1: too many tls handshakes (http2): 2 != 1

fraenkel · 2019-10-19T03:30:03Z

All I have determined at this point is that the http2 side starts sending back http.http2noCachedConnError after some time. The test passes when it doesn't occur which is not very often.

gopherbot · 2019-10-19T03:33:02Z

Change https://golang.org/cl/202087 mentions this issue: net/http: only decrement connection count if we removed a connection

fraenkel · 2019-10-19T12:42:41Z

Part of the issue is the connection coordination between http and http2. When using 300 clients, the first 50+ compete to create the connection on the http side before the http2 side is aware of it. There is some glitch (still investigating) when the http2 side is aware, it actually causes a second connection.
By changing the test case to first do a single client request, the failure rate is greatly reduced but the problem still occurs. However, it shows that we will always reuse the first connection but may dial/tls handshake a second.
I don't think this will ever be perfect but I would at least like to better understand what is causing the second connection. We may need to relax the test case slightly.

fraenkel · 2019-10-19T20:09:11Z

The http2 server imposes a limit of 250 concurrent streams. Once we reach that number which does happen when we stampede with 300 requests, there is a point where a new connection is created.

mchtech · 2019-11-18T05:28:09Z

There is still a certain chance to crash with "internal error: connCount underflow" on golang 1.13.4 when go client enables HTTP2.
~~The "connCount underflow" error did not appeared when using golang 1.13.3~~

fraenkel · 2019-11-18T10:51:43Z

Can you also provide the stack trace? And the potential scenario if different than reported.
A new issue would be best.

mchtech · 2019-11-19T12:30:02Z

As written, the test case as a data race.
I changed the test case slightly:

Fixing the data race, (transport.MaxConnsHost)
using ForceAttemptHTTP2 to simplifiy the setup
count successful finishes

The failure does still occur but I can get a few successful runs.
There is obviously some book keeping issue.

It is the same for 1.13.3 and tip (46aa835)

package issue34941

import (
	"context"
	"crypto/tls"
	"net/http"
	"net/http/httptest"
	"sync"
	"sync/atomic"
	"testing"
	"time"
)

func TestMaxConns(t *testing.T) {
	totalRequests := 300

	allow := make(chan struct{})
	var (
		starts   int64
		finishes int64
	)
	h := func(w http.ResponseWriter, r *http.Request) {
		if !r.ProtoAtLeast(2, 0) {
			t.Errorf("Request is not http/2: %q", r.Proto)
			return
		}
		atomic.AddInt64(&starts, 1)
		<-allow
	}

	s := httptest.NewUnstartedServer(http.HandlerFunc(h))
	s.TLS = &tls.Config{
		NextProtos: []string{"h2"},
	}
	s.StartTLS()
	defer s.Close()

	transport := s.Client().Transport.(*http.Transport)
	// clientConfig := transport.TLSClientConfig
	// transport.TLSClientConfig = nil
	transport.MaxConnsPerHost = 1
	transport.ForceAttemptHTTP2 = true

	// make a request to trigger HTTP/2 autoconfiguration
	// resp, err := s.Client().Get(s.URL)
	// if err == nil {
	// 	resp.Body.Close()
	// }
	// now allow the client to connect to the ad-hoc test server
	// transport.TLSClientConfig.RootCAs = clientConfig.RootCAs

	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	var wg sync.WaitGroup
	for i := 0; i < totalRequests; i++ {
		req, err := http.NewRequest("GET", s.URL, nil)
		if err != nil {
			t.Fatalf("NewRequest: %s", err)
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			ctx, cancel := context.WithCancel(ctx)
			defer cancel()
			req = req.WithContext(ctx)
			resp, err := s.Client().Do(req)
			if err != nil {
				return
			}
			resp.Body.Close()
			atomic.AddInt64(&finishes, 1)
		}()
	}

	for i := 0; i < 10; i++ {
		if i == 5 {
			close(allow)
		}
		time.Sleep(100 * time.Millisecond)
		t.Logf("starts=%d finishes=%d", atomic.LoadInt64(&starts), atomic.LoadInt64(&finishes))
	}

	if have, want := atomic.LoadInt64(&starts), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests started: %d != %d", have, want)
	}
	if have, want := atomic.LoadInt64(&finishes), int64(totalRequests); have != want {
		t.Errorf("HTTP/2 requests completed: %d != %d", have, want)
	}
}

This test still fails on golang 1.13.4.
The smaller the value of transport.MaxConnsPerHost, the greater the probability that the test program will crash with panic: net/http: internal error: connCount underflow.

panic: net/http: internal error: connCount underflow

goroutine 119 [running]:
net/http.(*Transport).decConnsPerHost(0xc0000e2780, 0x0, 0x0, 0xc0000b4780, 0x5, 0xc00020e010, 0xf, 0x0)
	/usr/local/opt/go/libexec/src/net/http/transport.go:1334 +0x604
net/http.(*Transport).roundTrip(0xc0000e2780, 0xc000346a00, 0x0, 0xc000185b08, 0x100e7b8)
	/usr/local/opt/go/libexec/src/net/http/transport.go:546 +0x77f
net/http.(*Transport).RoundTrip(0xc0000e2780, 0xc000346a00, 0xc0000e2780, 0x0, 0x0)
	/usr/local/opt/go/libexec/src/net/http/roundtrip.go:17 +0x35
net/http.send(0xc000346a00, 0x1593fe0, 0xc0000e2780, 0x0, 0x0, 0x0, 0xc0001be2b0, 0xc0001dbcd0, 0x1, 0x0)
	/usr/local/opt/go/libexec/src/net/http/client.go:250 +0x443
net/http.(*Client).send(0xc0000a18f0, 0xc000346a00, 0x0, 0x0, 0x0, 0xc0001be2b0, 0x0, 0x1, 0x10)
	/usr/local/opt/go/libexec/src/net/http/client.go:174 +0xfa
net/http.(*Client).do(0xc0000a18f0, 0xc000346a00, 0x0, 0x0, 0x0)
	/usr/local/opt/go/libexec/src/net/http/client.go:641 +0x3ce
net/http.(*Client).Do(...)
	/usr/local/opt/go/libexec/src/net/http/client.go:509
test.TestMaxConns.func2(0xc0000ab210, 0xc00009db00, 0xc0000a6358, 0xc0000a8480, 0xc0000aa848)
	/Volumes/data/code/go/src/test/main_test.go:68 +0x1ad
created by test.TestMaxConns
	/Volumes/data/code/go/src/test/main_test.go:63 +0x3e4

fraenkel · 2019-11-20T11:50:01Z

The fix only went into 1.14.
The powers that be would have to decide if it gets backported.

creker · 2020-01-15T09:31:58Z

Why isn't this backported? Maybe we need to ping someone? It's clearly a bug and regression in 1.13. 1.14 should be released fairly soon but that's a major release with tons of changes (the runtime in particular) that may come with its own bag of problems.

networkimprov · 2020-01-15T09:50:45Z

cc @toothrot @dmitshur re backport

networkimprov · 2020-01-15T20:05:37Z

@gopherbot please backport to 1.13

This is a regression, as documented in the issue comments.

gopherbot · 2020-01-15T20:05:47Z

Backport issue(s) opened: #36583 (for 1.13).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

michaeldorner · 2020-01-19T17:10:10Z

I fear that this issue is not resolved completely:

In Go version go1.13.6 darwin/amd64 I still get

panic: net/http: internal error: connCount underflow

goroutine 247 [running]:
net/http.(*Transport).decConnsPerHost(0xc0000963c0, 0x0, 0x0, 0xc00038caf0, 0x5, 0xc0000f9660, 0x11, 0x0)
	/usr/local/go/src/net/http/transport.go:1334 +0x604
net/http.(*Transport).roundTrip(0xc0000963c0, 0xc000312e00, 0xc000374600, 0xc0001e040c, 0xc0001e0450)
	/usr/local/go/src/net/http/transport.go:546 +0x77f
net/http.(*Transport).RoundTrip(0xc0000963c0, 0xc000312e00, 0xc0000963c0, 0xbf8140581cdfd498, 0xe2c22493c)
	/usr/local/go/src/net/http/roundtrip.go:17 +0x35
net/http.send(0xc000312c00, 0x1356060, 0xc0000963c0, 0xbf8140581cdfd498, 0xe2c22493c, 0x15254e0, 0xc0000fc1f8, 0xbf8140581cdfd498, 0x1, 0x0)
	/usr/local/go/src/net/http/client.go:250 +0x443
net/http.(*Client).send(0xc000082c00, 0xc000312c00, 0xbf8140581cdfd498, 0xe2c22493c, 0x15254e0, 0xc0000fc1f8, 0x0, 0x1, 0x0)
	/usr/local/go/src/net/http/client.go:174 +0xfa
net/http.(*Client).do(0xc000082c00, 0xc000312c00, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/http/client.go:641 +0x3ce
net/http.(*Client).Do(...)
	/usr/local/go/src/net/http/client.go:509
main.main.func2.1(0xc000082c00, 0xc0001c94c0, 0xc000312c00)
	/Users/michaeldorner/Code/Go/src/main.go:80 +0x4d
created by main.main.func2
	/Users/michaeldorner/Code/Go/src/main.go:79 +0x8c
exit status 2

Lines 1332 and 1333 in https://golang.org/src/net/http/transport.go explain that this
Shouldn't happen, but if it does, the counting is buggy and could easily lead to a silent deadlock, so report the problem loudly. which I do hereby. :)

Please let me know if I shall create a new issue.

fraenkel · 2020-01-19T17:23:44Z

@michaeldorner This has not be backported to 1.13 yet.

michaeldorner · 2020-01-19T17:38:01Z

@fraenkel Oh sorry, my bad. Thanks for the fast reply.

andybalholm · 2020-03-30T15:21:37Z

I'm still getting internal error: connCount underflow sometimes on 1.14.1.

fraenkel · 2020-03-31T00:03:33Z

@andybalholm can you attach the stack trace?
Is there anything special in your usage?

andybalholm · 2020-03-31T00:50:36Z

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.

fraenkel · 2020-03-31T00:55:42Z

@andybalholm Thanks for the stacktrace. Please open a separate issue to track this. It is a different path than the one fixed.

rhysh mentioned this issue Oct 17, 2019

net/http: performance collapse when http/2 requests wait for connection #34944

Open

julieqiu added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Oct 18, 2019

bradfitz added this to the Go1.14 milestone Oct 18, 2019

gopherbot closed this as completed in 1a058cd Oct 29, 2019

ameshkov mentioned this issue Nov 25, 2019

panic: net/http: internal error: connCount underflow AdguardTeam/AdguardForAndroid#3224

Closed

gopherbot mentioned this issue Jan 15, 2020

net/http: HTTP/2 with MaxConnsPerHost hangs or crashes [1.13 backport] #36583

Closed

andybalholm mentioned this issue Mar 31, 2020

net/http: connCount underflow panic with MaxConnsPerHost #38172

Closed

golang locked and limited conversation to collaborators Mar 31, 2021

gopherbot added the FrozenDueToAge label Mar 31, 2021

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net/http: HTTP/2 with MaxConnsPerHost hangs or crashes #34941

net/http: HTTP/2 with MaxConnsPerHost hangs or crashes #34941

rhysh commented Oct 16, 2019

julieqiu commented Oct 18, 2019

bradfitz commented Oct 18, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

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fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

mchtech commented Nov 18, 2019 •

edited

fraenkel commented Nov 18, 2019

mchtech commented Nov 19, 2019

fraenkel commented Nov 20, 2019

creker commented Jan 15, 2020 •

edited

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michaeldorner commented Jan 19, 2020 •

edited

fraenkel commented Jan 19, 2020

michaeldorner commented Jan 19, 2020

andybalholm commented Mar 30, 2020

fraenkel commented Mar 31, 2020

andybalholm commented Mar 31, 2020 •

edited

fraenkel commented Mar 31, 2020

net/http: HTTP/2 with MaxConnsPerHost hangs or crashes #34941

net/http: HTTP/2 with MaxConnsPerHost hangs or crashes #34941

Comments

rhysh commented Oct 16, 2019

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

julieqiu commented Oct 18, 2019

bradfitz commented Oct 18, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

gopherbot commented Oct 19, 2019

fraenkel commented Oct 19, 2019

fraenkel commented Oct 19, 2019

mchtech commented Nov 18, 2019 • edited

fraenkel commented Nov 18, 2019

mchtech commented Nov 19, 2019

fraenkel commented Nov 20, 2019

creker commented Jan 15, 2020 • edited

networkimprov commented Jan 15, 2020

networkimprov commented Jan 15, 2020

gopherbot commented Jan 15, 2020

michaeldorner commented Jan 19, 2020 • edited

fraenkel commented Jan 19, 2020

michaeldorner commented Jan 19, 2020

andybalholm commented Mar 30, 2020

fraenkel commented Mar 31, 2020

andybalholm commented Mar 31, 2020 • edited

fraenkel commented Mar 31, 2020

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

mchtech commented Nov 18, 2019 •

edited

creker commented Jan 15, 2020 •

edited

michaeldorner commented Jan 19, 2020 •

edited

andybalholm commented Mar 31, 2020 •

edited