don't call os.NewFile on unknown FDs

os.NewFile was called on file descriptors 3, 4, and 5 during every init,
in case this process happened to be running inside panicwrap. If the
runtime has already chosen one of these file descriptors to use
internally, starting polling on them can cause the runtime to crash.

Initialize the file descriptors lazily, only if we know that they belong
to us, after Wrapped is checked.

helper/wrappedstreams/streams.go

@@ -10,34 +10,31 @@ 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
-	if panicwrap.Wrapped(nil) {
-		stderr = wrappedStderr
-	}
-
+	_, _, stderr := fds()
 	return stderr
 }
 
+func fds() (stdin, stdout, stderr *os.File) {
+	stdin, stdout, stderr = os.Stdin, os.Stdout, os.Stderr
+	if panicwrap.Wrapped(nil) {
+		initPlatform()
+		stdin, stdout, stderr = wrappedStdin, wrappedStdout, wrappedStderr
+	}
+	return
+}
+
 // These are the wrapped standard streams. These are setup by the
 // platform specific code in initPlatform.
 var (
@@ -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")
+	})
 }