refactor: move stderr filter to unix build-tagged file

cmd/olcrtc/main.go

@@ -6,7 +6,6 @@
 package main
 
 import (
-	"bufio"
 	"bytes"
 	"context"
 	"errors"
@@ -16,7 +15,6 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
-	"sync"
 	"syscall"
 	"time"
 
@@ -47,8 +45,6 @@ var runSession = session.Run
 //nolint:gochecknoglobals // Tests replace gen runner with a stub.
 var runGen = execGen
 
-var stderrFilterOnce sync.Once //nolint:gochecknoglobals // process-wide stderr fd filter
-
 // loadedConfig bundles the parsed YAML file and the derived session config.
 type loadedConfig struct {
 	scfg       session.Config
@@ -339,46 +335,6 @@ func (f filteredWriter) Write(p []byte) (int, error) {
 	return n, nil
 }
 
-func installStderrFilter() {
-	stderrFilterOnce.Do(func() {
-		origFD, err := syscall.Dup(int(os.Stderr.Fd()))
-		if err != nil {
-			return
-		}
-		reader, writer, err := os.Pipe()
-		if err != nil {
-			_ = syscall.Close(origFD)
-			return
-		}
-		if err := syscall.Dup2(int(writer.Fd()), int(os.Stderr.Fd())); err != nil {
-			_ = reader.Close()
-			_ = writer.Close()
-			_ = syscall.Close(origFD)
-			return
-		}
-		_ = writer.Close()
-		os.Stderr = os.NewFile(uintptr(syscall.Stderr), "/dev/stderr")
-		orig := os.NewFile(uintptr(origFD), "/dev/stderr-original")
-		go copyFilteredStderr(reader, orig)
-	})
-}
-
-func copyFilteredStderr(reader *os.File, out io.Writer) {
-	defer func() { _ = reader.Close() }()
-	br := bufio.NewReader(reader)
-	for {
-		line, err := br.ReadBytes('\n')
-		if len(line) > 0 && !isNoisyLogLine(line) {
-			if _, writeErr := out.Write(line); writeErr != nil {
-				return
-			}
-		}
-		if err != nil {
-			return
-		}
-	}
-}
-
 func isNoisyLogLine(line []byte) bool {
 	for _, prefix := range noisyPrefixes {
 		if bytes.Contains(line, prefix) {

cmd/olcrtc/stderr_filter_unix.go

@@ -0,0 +1,54 @@
+//go:build !windows
+
+package main
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"sync"
+
+	"golang.org/x/sys/unix"
+)
+
+var stderrFilterOnce sync.Once //nolint:gochecknoglobals // process-wide stderr fd filter
+
+func installStderrFilter() {
+	stderrFilterOnce.Do(func() {
+		origFD, err := unix.Dup(int(os.Stderr.Fd()))
+		if err != nil {
+			return
+		}
+		reader, writer, err := os.Pipe()
+		if err != nil {
+			_ = unix.Close(origFD)
+			return
+		}
+		if err := unix.Dup2(int(writer.Fd()), int(os.Stderr.Fd())); err != nil {
+			_ = reader.Close()
+			_ = writer.Close()
+			_ = unix.Close(origFD)
+			return
+		}
+		_ = writer.Close()
+		os.Stderr = os.NewFile(uintptr(unix.Stderr), "/dev/stderr")
+		orig := os.NewFile(uintptr(origFD), "/dev/stderr-original")
+		go copyFilteredStderr(reader, orig)
+	})
+}
+
+func copyFilteredStderr(reader *os.File, out io.Writer) {
+	defer func() { _ = reader.Close() }()
+	br := bufio.NewReader(reader)
+	for {
+		line, err := br.ReadBytes('\n')
+		if len(line) > 0 && !isNoisyLogLine(line) {
+			if _, writeErr := out.Write(line); writeErr != nil {
+				return
+			}
+		}
+		if err != nil {
+			return
+		}
+	}
+}

cmd/olcrtc/stderr_filter_windows.go

@@ -0,0 +1,5 @@
+//go:build windows
+
+package main
+
+func installStderrFilter() {}