don't call os.NewFile on unknown FDs

helper/wrappedstreams/streams.go

@@ -10,32 +10,29 @@ import (
 
 // Stdin returns the true stdin of the process.
 func Stdin() *os.File {
-	stdin := os.Stdin
-	if panicwrap.Wrapped(nil) {
-		stdin = wrappedStdin
-	}
-
+	stdin, _, _ := fds()
 	return stdin
 }
 
 // Stdout returns the true stdout of the process.
 func Stdout() *os.File {
-	stdout := os.Stdout
-	if panicwrap.Wrapped(nil) {
-		stdout = wrappedStdout
-	}
-
+	_, stdout, _ := fds()
 	return stdout
 }
 
 // Stderr returns the true stderr of the process.
 func Stderr() *os.File {
-	stderr := os.Stderr
+	_, _, stderr := fds()
+	return stderr
+}
+
+func fds() (stdin, stdout, stderr *os.File) {
+	stdin, stdout, stderr = os.Stdin, os.Stdout, os.Stderr
 	if panicwrap.Wrapped(nil) {
-		stderr = wrappedStderr
+		initPlatform()
+		stdin, stdout, stderr = wrappedStdin, wrappedStdout, wrappedStderr
 	}
-
-	return stderr
+	return
 }
 
 // These are the wrapped standard streams. These are setup by the
@@ -45,8 +42,3 @@ var (
 	wrappedStdout *os.File
 	wrappedStderr *os.File
 )
-
-func init() {
-	// Initialize the platform-specific code
-	initPlatform()
-}

helper/wrappedstreams/streams_other.go

@@ -4,11 +4,18 @@ package wrappedstreams
 
 import (
 	"os"
+	"sync"
 )
 
+var initOnce sync.Once
+
 func initPlatform() {
-	// The standard streams are passed in via extra file descriptors.
-	wrappedStdin = os.NewFile(uintptr(3), "stdin")
-	wrappedStdout = os.NewFile(uintptr(4), "stdout")
-	wrappedStderr = os.NewFile(uintptr(5), "stderr")
+	// These must be initialized lazily, once it's been determined that this is
+	// a wrapped process.
+	initOnce.Do(func() {
+		// The standard streams are passed in via extra file descriptors.
+		wrappedStdin = os.NewFile(uintptr(3), "stdin")
+		wrappedStdout = os.NewFile(uintptr(4), "stdout")
+		wrappedStderr = os.NewFile(uintptr(5), "stderr")
+	})
 }