os/exec:

[chrisFang5]

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

$ go version
go version go1.15.2 linux/amd64

Does this issue reproduce with the latest release?

1.15.2 is latest release now(2020/11/24)

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fanghongwei/.cache/go-build"
GOENV="/home/fanghongwei/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/fanghongwei/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/fanghongwei/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn/"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/fanghongwei/go/src/mycode/fuzz/gdb/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build641017757=/tmp/go-build -gno-record-gcc-switches"

What did you do?

my code as below:

package main

import (
    "context"
    "io"
    "log"
    "os/exec"
    "sync"
    "time"
)

func main() {
    test()
}

func test() {
    var err error

    ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*100000)
    // cmd := exec.CommandContext(ctx, "/usr/bin/gdb")
    cmd := exec.CommandContext(ctx, "/usr/bin/gdb", "-ex", "run", "./must_panic")
    log.Printf("command:%s", cmd.String())

    defer cancel()

    out, err := cmd.StdoutPipe()
    must(err)
    // defer out.Close()

    fderr, err := cmd.StderrPipe()
    must(err)
    // defer fderr.Close()

    err = cmd.Start()
    must(err)

    wg := sync.WaitGroup{}
    wg.Add(1)
    go func() {
        defer wg.Done()
        log.Printf("wait result:%v", cmd.Wait())
    }()

    wg.Add(1)
    go func() {
        defer wg.Done()
        handle("stdout", out)
    }()

    wg.Add(1)
    go func() {
        defer wg.Done()
        handle("stderr", fderr)
    }()

    wg.Wait()
}

func handle(tag string, reader io.Reader) {
    buffer := make([]byte, 0, 4096)
    for {
        n, err := reader.Read(buffer)
        log.Printf("%s, n:%d, err:%v", tag, n, err)
        if n > 0 {
            log.Printf("***************%s:%s***************", tag, buffer[:n])
        }
        if io.EOF == err {
            log.Printf("fderr touch the end")
            break
        }
        if nil != err {
            break
        }
    }
}

func must(err error) {
    if nil != err {
        panic(err)
    }
}

output in cli:

....
....
2020/11/24 18:40:48 stdout, n:0, err:<nil>
2020/11/24 18:40:48 stdout, n:0, err:<nil>
2020/11/24 18:40:48 stdout, n:0, err:<nil>
2020/11/24 18:40:48 stdout, n:0, err:<nil>
2020/11/24 18:40:48 stdout, n:0, err:read |0: file already closed
2020/11/24 18:40:48 wait result:<nil>
2020/11/24 18:40:48 stderr, n:0, err:<nil>
2020/11/24 18:40:48 stderr, n:0, err:read |0: file already closed

must_panic is a golang process which must panic when executed

[fanghongwei@5d527337538b gdb]$ ./must_panic 
2020/11/24 18:52:28 user:uesr, passwd:passwd
panic: my friends and I get up early every day, so it is honored for us!

goroutine 1 [running]:
main.main()
        /home/fanghongwei/go/src/mycode/fuzz/gdb/panic/main.go:17 +0x14c

while I use another way to get stdout/err, this could get what I expected.

func main() {
    test()
}

func test() {
    var err error

    ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*100000)
    // cmd := exec.CommandContext(ctx, "/usr/bin/gdb")
    cmd := exec.CommandContext(ctx, "/usr/bin/gdb", "-ex", "run", "./must_panic")
    log.Printf("command:%s", cmd.String())

    defer cancel()

    var sout bytes.Buffer
    var serr bytes.Buffer

    cmd.Stdout = &sout
    cmd.Stderr = &serr

    err = cmd.Start()
    must(err)

    log.Printf("wait result:%v", cmd.Wait())
    log.Printf("out:%s***********************", sout.Bytes())
    log.Printf("err:%s***********************", serr.Bytes())

}

What did you expect to see?

2020/11/24 18:46:46 command:/usr/bin/gdb -ex run ./must_panic
2020/11/24 18:46:46 wait result:<nil>
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./must_panic...done.
To enable execution of this file add
        add-auto-load-safe-path /usr/local/go/src/runtime/runtime-gdb.py
line to your configuration file "/home/fanghongwei/.gdbinit".
To completely disable this security protection add
        set auto-load safe-path /
line to your configuration file "/home/fanghongwei/.gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
        info "(gdb)Auto-loading safe path"
Starting program: /home/fanghongwei/go/src/mycode/fuzz/gdb/must_panic 
[New LWP 11420]
[New LWP 11421]
[New LWP 11422]
[LWP 11420 exited]
[LWP 11416 exited]
[LWP 11421 exited]
[Inferior 1 (process 11416) exited with code 02]
(gdb) quit
***********************
2020/11/24 18:46:46 err:warning: File "/usr/local/go/src/runtime/runtime-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
2020/11/24 18:46:46 user:uesr, passwd:passwd
panic: my friends and I get up early every day, so it is honored for us!

goroutine 1 [running]:
main.main()
        /home/fanghongwei/go/src/mycode/fuzz/gdb/panic/main.go:17 +0x14c
***********************

[mvdan]

The Go project doesn't use the issue tracker for questions. See https://golang.org/wiki/Questions. You can come back to the issue tracker if or when you're pretty certain that you have found a specific bug in Go.