feat: filter noisy log lines from stderr at the fd level

cmd/olcrtc/main.go

@@ -6,6 +6,7 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"errors"
@@ -15,6 +16,7 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"sync"
 	"syscall"
 	"time"
 
@@ -45,6 +47,8 @@ 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
@@ -73,6 +77,7 @@ func run() error {
 
 func runWithArgs(args []string) error {
 	logger.DisableNoisyPionLogs()
+	installStderrFilter()
 	session.RegisterDefaults()
 
 	if len(args) != 1 || args[0] == "-h" || args[0] == "--help" || args[0] == "-help" {
@@ -334,7 +339,57 @@ 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) {
+			return true
+		}
+	}
+	return false
+}
+
 func configureLogging(debug bool) {
+	installStderrFilter()
 	log.SetOutput(filteredWriter{w: os.Stderr})
 	logger.DisableNoisyPionLogs()
 	if debug {