os/exec: Stdout/Stderr not closed after data copied #22610
Description
Problem
os/exec: when command.Stdout or command.Stderr are provided an io.Writer which is not an *os.File, the goroutine that is created to copy output to the Writer never Closes that Writer.
This manifests unexpectedly in situations like the example below, where the bufio.Scanner blocks forever on a Read from an io.Pipe.
Passing a compress/gzip.Writer to command.Stdout/Stderr behaves similarly because its Close method needs to be called before the data and footer are written to its output.
The gzip.Writer case can be trivially worked around by explicitly calling its Close method after cmd.Wait, but is unintuitive. The io.Pipe example below is not easy to work around.
What version of Go are you using (go version)?
1.9
master
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
darwin/amd64
linux/amd64
What did you do?
package main
import (
"bufio"
"fmt"
"io"
"os/exec"
)
func main() {
cmd := exec.Command("echo", "hello, world")
pr, pw := io.Pipe()
cmd.Stdout = pw
cmd.Stderr = pw
done := make(chan struct{})
go func() {
s := bufio.NewScanner(pr)
for s.Scan() {
fmt.Printf("output: %v\n", s.Text())
}
done <- struct{}{}
}()
if err := cmd.Start(); err != nil {
fmt.Println(err)
return
}
// Wait for all output to be read before calling Wait.
<-done
fmt.Println("finished reading output")
if err := cmd.Wait(); err != nil {
fmt.Println(err)
}
}
What did you expect to see?
% go run -race example.go
output: hello, world
finished reading output
<program terminates>
What did you see instead?
% go run -race example.go
output: hello, world
<program hangs indefinitely>