Goroutine leaks when trying `ReadAll(response.Body)` where `response` is returned by http `client.Do(...)`

#php editor Baicao discovered a common goroutine leak when solving programming problems. When we try to use `ReadAll(response.Body)`, the returned `response` object is returned by the `client.Do(...)` method of the `http` package. However, this operation can lead to goroutine leakage problems. In order to avoid this happening, we need to take some measures to properly handle http responses.

Question content

The question comes from a very common scenario where all bytes are read from the response body after an http method call.

With the help of https://github.com/uber-go/goleak, I discovered an interesting goroutine leak.

To demonstrate the issue, run the following test code using go test, which results in found an unexpected goroutine:

package main

import (
    "io"
    "net/http"
    "testing"

    "go.uber.org/goleak"
    "gotest.tools/v3/assert"
)

func testleak(t *testing.t) {
    defer goleak.verifynone(t)

    request, err := http.newrequest(http.methodget, "https://google.com", nil)
    assert.nilerror(t, err)

    client := http.client{}

    response, err := client.do(request)
    assert.nilerror(t, err)

    defer response.body.close()

    _, err = io.readall(response.body)

    assert.nilerror(t, err)
}

Full test output:

$ go test
--- FAIL: TestLeak (1.26s)
    main_test.go:28: found unexpected goroutines:
        [Goroutine 14 in state IO wait, with internal/poll.runtime_pollWait on top of the stack:
        goroutine 14 [IO wait]:
        internal/poll.runtime_pollWait(0x7f945c1e0398, 0x72)
                /home/wxh/.local/go/src/runtime/netpoll.go:305 +0x89
        internal/poll.(*pollDesc).wait(0xc000154280?, 0xc0000ca000?, 0x0)
                /home/wxh/.local/go/src/internal/poll/fd_poll_runtime.go:84 +0x32
        internal/poll.(*pollDesc).waitRead(...)
                /home/wxh/.local/go/src/internal/poll/fd_poll_runtime.go:89
        internal/poll.(*FD).Read(0xc000154280, {0xc0000ca000, 0x2000, 0x2000})
                /home/wxh/.local/go/src/internal/poll/fd_unix.go:167 +0x25a
        net.(*netFD).Read(0xc000154280, {0xc0000ca000?, 0xc00017edd8?, 0xc0004d5808?})
                /home/wxh/.local/go/src/net/fd_posix.go:55 +0x29
        net.(*conn).Read(0xc00009c000, {0xc0000ca000?, 0xc0000c68f0?, 0x2c?})
                /home/wxh/.local/go/src/net/net.go:183 +0x45
        crypto/tls.(*atLeastReader).Read(0xc0002d4138, {0xc0000ca000?, 0x0?, 0x29f6540f238f2947?})
                /home/wxh/.local/go/src/crypto/tls/conn.go:787 +0x3d
        bytes.(*Buffer).ReadFrom(0xc0000a0278, {0x7c3860, 0xc0002d4138})
                /home/wxh/.local/go/src/bytes/buffer.go:202 +0x98
        crypto/tls.(*Conn).readFromUntil(0xc0000a0000, {0x7c3b80?, 0xc00009c000}, 0xc0000a41c0?)
                /home/wxh/.local/go/src/crypto/tls/conn.go:809 +0xe5
        crypto/tls.(*Conn).readRecordOrCCS(0xc0000a0000, 0x0)
                /home/wxh/.local/go/src/crypto/tls/conn.go:616 +0x116
        crypto/tls.(*Conn).readRecord(...)
                /home/wxh/.local/go/src/crypto/tls/conn.go:582
        crypto/tls.(*Conn).Read(0xc0000a0000, {0xc0001cf000, 0x1000, 0x11?})
                /home/wxh/.local/go/src/crypto/tls/conn.go:1287 +0x16f
        bufio.(*Reader).Read(0xc0001af1a0, {0xc0001aad60, 0x9, 0xc0004d5d18?})
                /home/wxh/.local/go/src/bufio/bufio.go:237 +0x1bb
        io.ReadAtLeast({0x7c3760, 0xc0001af1a0}, {0xc0001aad60, 0x9, 0x9}, 0x9)
                /home/wxh/.local/go/src/io/io.go:332 +0x9a
        io.ReadFull(...)
                /home/wxh/.local/go/src/io/io.go:351
        net/http.http2readFrameHeader({0xc0001aad60?, 0x9?, 0xc0004d5df0?}, {0x7c3760?, 0xc0001af1a0?})
                /home/wxh/.local/go/src/net/http/h2_bundle.go:1565 +0x6e
        net/http.(*http2Framer).ReadFrame(0xc0001aad20)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:1829 +0x95
        net/http.(*http2clientConnReadLoop).run(0xc0004d5f98)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:8875 +0x130
        net/http.(*http2ClientConn).readLoop(0xc00009e180)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:8771 +0x6f
        created by net/http.(*http2Transport).newClientConn
                /home/wxh/.local/go/src/net/http/h2_bundle.go:7478 +0xaaa
        
         Goroutine 37 in state IO wait, with internal/poll.runtime_pollWait on top of the stack:
        goroutine 37 [IO wait]:
        internal/poll.runtime_pollWait(0x7f945c1e02a8, 0x72)
                /home/wxh/.local/go/src/runtime/netpoll.go:305 +0x89
        internal/poll.(*pollDesc).wait(0xc0003ca000?, 0xc000202000?, 0x0)
                /home/wxh/.local/go/src/internal/poll/fd_poll_runtime.go:84 +0x32
        internal/poll.(*pollDesc).waitRead(...)
                /home/wxh/.local/go/src/internal/poll/fd_poll_runtime.go:89
        internal/poll.(*FD).Read(0xc0003ca000, {0xc000202000, 0x1300, 0x1300})
                /home/wxh/.local/go/src/internal/poll/fd_unix.go:167 +0x25a
        net.(*netFD).Read(0xc0003ca000, {0xc000202000?, 0xc0004e6798?, 0xc0004c0808?})
                /home/wxh/.local/go/src/net/fd_posix.go:55 +0x29
        net.(*conn).Read(0xc000014028, {0xc000202000?, 0xc000136630?, 0x2c?})
                /home/wxh/.local/go/src/net/net.go:183 +0x45
        crypto/tls.(*atLeastReader).Read(0xc000276288, {0xc000202000?, 0x0?, 0x150fbb8ccb566149?})
                /home/wxh/.local/go/src/crypto/tls/conn.go:787 +0x3d
        bytes.(*Buffer).ReadFrom(0xc0000a0978, {0x7c3860, 0xc000276288})
                /home/wxh/.local/go/src/bytes/buffer.go:202 +0x98
        crypto/tls.(*Conn).readFromUntil(0xc0000a0700, {0x7c3b80?, 0xc000014028}, 0xc0004cc2c0?)
                /home/wxh/.local/go/src/crypto/tls/conn.go:809 +0xe5
        crypto/tls.(*Conn).readRecordOrCCS(0xc0000a0700, 0x0)
                /home/wxh/.local/go/src/crypto/tls/conn.go:616 +0x116
        crypto/tls.(*Conn).readRecord(...)
                /home/wxh/.local/go/src/crypto/tls/conn.go:582
        crypto/tls.(*Conn).Read(0xc0000a0700, {0xc00023c000, 0x1000, 0x11?})
                /home/wxh/.local/go/src/crypto/tls/conn.go:1287 +0x16f
        bufio.(*Reader).Read(0xc00022a2a0, {0xc00020e2e0, 0x9, 0xc0004c0d18?})
                /home/wxh/.local/go/src/bufio/bufio.go:237 +0x1bb
        io.ReadAtLeast({0x7c3760, 0xc00022a2a0}, {0xc00020e2e0, 0x9, 0x9}, 0x9)
                /home/wxh/.local/go/src/io/io.go:332 +0x9a
        io.ReadFull(...)
                /home/wxh/.local/go/src/io/io.go:351
        net/http.http2readFrameHeader({0xc00020e2e0?, 0x9?, 0xc0004c0df0?}, {0x7c3760?, 0xc00022a2a0?})
                /home/wxh/.local/go/src/net/http/h2_bundle.go:1565 +0x6e
        net/http.(*http2Framer).ReadFrame(0xc00020e2a0)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:1829 +0x95
        net/http.(*http2clientConnReadLoop).run(0xc0004c0f98)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:8875 +0x130
        net/http.(*http2ClientConn).readLoop(0xc00019e300)
                /home/wxh/.local/go/src/net/http/h2_bundle.go:8771 +0x6f
        created by net/http.(*http2Transport).newClientConn
                /home/wxh/.local/go/src/net/http/h2_bundle.go:7478 +0xaaa
        ]
FAIL
exit status 1

Does anyone know how to solve this problem?

Thanks!

Solution

In Go, http.Client uses Transport (http.RoundTripper interface). http.Transport is its default implementation. The default http.Transfer maintains a connection pool that holds connections that remain active or idle for some reason.

Basically all you need to do is make sure there are no connections in the pool* when you call goleak.Verify

This can be done by:

1. Disable keep-alive for all http clients before performing any requests:
   client := http.Client{Transport: &http.Transport{DisableKeepAlive: true}}
2. Close idle connections before calling goleak.Verify*:
   client.CloseIdleConnections() // Available since Go 1.12

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