os: go1.24rc2 File.copyFile blocks forever in some cases

[r-hang]

Go version

go1.24rc2

Output of go env in your module/workspace:

go env

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE='on'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN='/home/user/gocode/bin'
GOCACHE='/home/user/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/user/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build276472280=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/user/gocode/src/code.uber.internal/go.mod'
GOMODCACHE='/home/user/gocode/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/user/gocode'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/user/gocode/src/github.com/golang/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/user/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/user/gocode/src/github.com/golang/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24rc2'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

go1.24rc2 seems to cause some terminal prompt testing programs to block forever. These programs previously ran fine on go1.23.5.
git bisect points to https://go-review.googlesource.com/c/go/+/603295. Specifically, the added syscall logic written, err, handled = poll.SendFile(&f.pfd, int(fd), remain) isn't returning.

I ran the go program provided below in go1.23.5 and it runs to completion with the expected result.

$ GO111MODULE=on /home/user/src/go/bin/go version
go version go1.23.5 linux/amd64

$ GO111MODULE=on /home/user/src/go/bin/go run .                                       
0xc000142780 0xc000142660 9223372036854775807
(0x0,0x0)
pass:  true , error:  <nil>

In go1.24rc2 this program blocks forever

$ GO111MODULE=on /home/user/gocode/src/github.com/golang/go/bin/go version
go version go1.24rc2 linux/amd64

$ GO111MODULE=on /home/user/gocode/src/github.com/golang/go/bin/go run .   
0xc0001b86c0 0xc0001b85a0 9223372036854775807
(0x0,0x0)

Standalone Reproducing Go Program

package main

import (
        "fmt"
        "os"

        expect "github.com/Netflix/go-expect"
        "github.com/creack/pty"
        "github.com/hinshun/vt10x"
        survey "github.com/AlecAivazis/survey/v2"
)

const (
        _prompt = "Name of the user"
        _response = "gopher"
)

// A program to test the interactive prompts of our cli tools.
func main() {
        tty := setupTTY()
        var res string
        err := survey.AskOne(&survey.Input{Message: _prompt}, &res, survey.WithStdio(tty, tty, tty))
        fmt.Println("pass: ", res == _response, ", error: ", err)
}

func setupTTY() *os.File {
        ptm, pts, err := pty.Open()
        if err != nil {
                panic(err)
        }

        term := vt10x.New(vt10x.WithWriter(pts))
        console, err := expect.NewConsole(
                        expect.WithStdin(ptm),
                        expect.WithStdout(term),
                        expect.WithCloser(pts, ptm),
        )
        if err != nil {
                panic(err)
        }

        go func() {
                // console.ExpectString blocks forever on go1.24rc2
                // git bisect points to https://go-review.googlesource.com/c/go/+/603295
                // Specifically, the syscall never returns.
                // ```written, err, handled = poll.SendFile(&f.pfd, int(fd), remain)```
                // never returns.
                _, err := console.ExpectString(_prompt)
                if err != nil {
                        panic(err)
                }
                _, err = console.SendLine(_response)
                if err != nil {
                        panic(err)
                }
                _, err = console.ExpectEOF()
                if err != nil {
                        panic(err)
                }
        }()

        return console.Tty()
}

PS: I tried creating a Go Playground but ran into timeouts.

What did you see happen?

On go1.24rc2 the program blocked.

What did you expect to see?

I expected the program to terminate with the expected result printed.

[seankhliao]

Reverting CL 603295 is now tracked in #71375

[gabyhelp]

Related Issues

• io: Copy to a pipe prevents process exit (Go 1.24rc2 on Linux regression) #71375
• os/exec: syscall.forkExec hang when spawning multiple processes concurrently on `darwin` #61080 (closed)
• os: Read() blocking on named pipe despite O_NONBLOCK #66239 (closed)
• cmd/go: tests timing out on linux-amd64 #21850 (closed)
• os/exec: cmd.Start got a deadlock when set cmd.Stdin #47061 (closed)
• cmd/go: get hangs indefinitely for go get -x -u ./... #60145 (closed)
• cmd/go: go get hangs #16104 (closed)
• runtime: all.bash intermittent hang running test for issue 19182 #23560 (closed)
• affected/package: os/exec, context not work. #53700 (closed)
• x/tools/gopls: unresponsive after CL 197117 #34519 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[kolyshkin]

It's the same under the hood as the reproducer in #71735, with io.Copy from a terminal to a pipe. Can be closed as duplicate I guess.