You have at least three problems. The first is a race condition in the
second, goroutine, version of your program. Add a `time.Sleep(time.Second)`
just before the `wpipe.Close()` and you should see the same behavior as the
non-goroutine version. That provides a hint regarding the primary problem.
The second problem is you should not be closing the write side of the pipe
until the process writing to the pipe has terminated.
The third, and primary, problem is a misunderstanding of what it means to
assign a `os.File` object (such as from `os.Pipe`) to the `cmd.Stdout`
structure member and how that interacts with closing that file object.
Calling `cmd.Start` creates a second reference to the write side of the
pipe. If the write side of the pipe is open when `cmd.Start` is executed
calling `wpipe.Close` won't actually close the write side of the pipe. It
only removes the first reference to the write side of the pipe. The spawned
external process still holds a reference to the write side of the pipe. The
reason the second, goroutine, version of your program normally fails is
because your program will normally execute the `wpipe.Close` before the
goroutine spawns the external process and thus creates a second reference
to the write side of the pipe. Which is why adding a tiny sleep before the
close "fixes" the problem.
On Thu, Nov 7, 2024 at 9:36 PM huan xiong <huan.xi...@gmail.com> wrote:
> 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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> .
>
--
Kurtis Rader
Caretaker of the exceptional canines Junior and Hank
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