gpu/internal/convertshaders: don't wait for winepath to exit

Apparently, exec.Command.Output waits for winepath's grandchildren to
exit. However, that may take several seconds if wineserver was started
by winepath.

exec.Command.StdoutPipe works better, in that it is closed when the
winepath process exits.

A similar change may help run the fxc.exe tool under Wine, if that ever
turns out to have the same problem.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/convertshaders/hlsl.go

@@ -3,7 +3,9 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os/exec"
 	"path/filepath"
@@ -121,11 +123,20 @@ func winepath(paths ...*string) error {
 	for _, path := range paths {
 		winepath.Args = append(winepath.Args, *path)
 	}
-	out, err := winepath.Output()
+	// Use a pipe instead of Output, because winepath may have left wineserver
+	// running for several seconds as a grandchild.
+	out, err := winepath.StdoutPipe()
 	if err != nil {
-		return fmt.Errorf("unable to run `winepath --windows`: %w", err)
+		return fmt.Errorf("unable to start winepath: %w", err)
 	}
-	winPaths := strings.Split(strings.TrimSpace(string(out)), "\n")
+	if err := winepath.Start(); err != nil {
+		return fmt.Errorf("unable to start winepath: %w", err)
+	}
+	var buf bytes.Buffer
+	if _, err := io.Copy(&buf, out); err != nil {
+		return fmt.Errorf("unable to run winepath: %w", err)
+	}
+	winPaths := strings.Split(strings.TrimSpace(buf.String()), "\n")
 	for i, path := range paths {
 		*path = winPaths[i]
 	}