Hi, I wonder why the two programs below have different behaviors? Both
start `yes` command, close write side of the pipe (because the main process
doesn't use it), and read from the read side of the pipe. The only
difference is one doesn't use goroutine and another uses it.
1) The first program doesn't use goroutine. It works as expected (you'll
need to press Ctl-C to temrinate the program).
```
package main
import (
"io"
"os"
"os/exec"
)
func main() {
rpipe, wpipe, err := os.Pipe()
if err != nil {
panic(err)
}
cmd := exec.Command("yes")
cmd.Stdout = wpipe
if err := cmd.Start(); err != nil {
panic(err)
}
wpipe.Close()
if _, err := io.Copy(os.Stdout, rpipe); err != nil {
panic(err)
}
}
```
2) The second program is almost same as the first one, except that it wraps
the `yes` command related code in a goroutine. It fails to read data from
pipe. The culprit is `wpipe.Close()` line.
```
package main
import (
"io"
"os"
"os/exec"
"fmt"
)
func main() {
rpipe, wpipe, err := os.Pipe()
if err != nil {
panic(err)
}
go func() {
cmd := exec.Command("yes")
cmd.Stdout = wpipe
if err := cmd.Start(); err != nil {
panic(err)
}
}()
// Commenting out this line would make the program works fine. Why?
wpipe.Close()
n, err := io.Copy(os.Stdout, rpipe)
if err != nil {
panic(err)
}
fmt.Printf("n: %d", n)
}
// Output:
// n: 0
```
Question 1: I wonder why the above code doesn't work?
I find two ways to make it work:
1) Remove `wpipe.Close()` line. I don't like this approach because I think
it's a convention in Unix programs to close unused file descriptors.
2) Move `wpipe.Close()` to the goroutine. Question 2: Why does this work?
Thanks for any explanation.
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