internal/debug: add --log.file option (ethereum#26149)

This adds an option to direct log output to a file. This feature has been
requested a lot. It's sometimes useful to have this available when running
geth in an environment that doesn't easily allow redirecting the output.

Notably, there is no support for log rotation with this change. The --log.file option
opens the file once on startup and then keeps writing to the file handle. 
This can become an issue when external log rotation tools are involved, so it's
best not to use them with this option for now.

internal/debug/flags.go

@@ -54,6 +54,11 @@ var (
 		Usage:    "Format logs with JSON",
 		Category: flags.LoggingCategory,
 	}
+	logFileFlag = &cli.StringFlag{
+		Name:     "log.file",
+		Usage:    "Write logs to a file",
+		Category: flags.LoggingCategory,
+	}
 	backtraceAtFlag = &cli.StringFlag{
 		Name:     "log.backtrace",
 		Usage:    "Request a stack trace at a specific logging statement (e.g. \"block.go:271\")",
@@ -110,6 +115,7 @@ var Flags = []cli.Flag{
 	verbosityFlag,
 	vmoduleFlag,
 	logjsonFlag,
+	logFileFlag,
 	backtraceAtFlag,
 	debugFlag,
 	pprofFlag,
@@ -121,7 +127,10 @@ var Flags = []cli.Flag{
 	traceFlag,
 }
 
-var glogger *log.GlogHandler
+var (
+	glogger         *log.GlogHandler
+	logOutputStream log.Handler
+)
 
 func init() {
 	glogger = log.NewGlogHandler(log.StreamHandler(os.Stderr, log.TerminalFormat(false)))
@@ -132,18 +141,30 @@ func init() {
 // Setup initializes profiling and logging based on the CLI flags.
 // It should be called as early as possible in the program.
 func Setup(ctx *cli.Context) error {
-	var ostream log.Handler
-	output := io.Writer(os.Stderr)
+	logFile := ctx.String(logFileFlag.Name)
+	useColor := logFile == "" && os.Getenv("TERM") != "dumb" && (isatty.IsTerminal(os.Stderr.Fd()) || isatty.IsCygwinTerminal(os.Stderr.Fd()))
+
+	var logfmt log.Format
 	if ctx.Bool(logjsonFlag.Name) {
-		ostream = log.StreamHandler(output, log.JSONFormat())
+		logfmt = log.JSONFormat()
 	} else {
-		usecolor := (isatty.IsTerminal(os.Stderr.Fd()) || isatty.IsCygwinTerminal(os.Stderr.Fd())) && os.Getenv("TERM") != "dumb"
-		if usecolor {
+		logfmt = log.TerminalFormat(useColor)
+	}
+
+	if logFile != "" {
+		var err error
+		logOutputStream, err = log.FileHandler(logFile, logfmt)
+		if err != nil {
+			return err
+		}
+	} else {
+		output := io.Writer(os.Stderr)
+		if useColor {
 			output = colorable.NewColorableStderr()
 		}
-		ostream = log.StreamHandler(output, log.TerminalFormat(usecolor))
+		logOutputStream = log.StreamHandler(output, logfmt)
 	}
-	glogger.SetHandler(ostream)
+	glogger.SetHandler(logOutputStream)
 
 	// logging
 	verbosity := ctx.Int(verbosityFlag.Name)
@@ -217,4 +238,7 @@ func StartPProf(address string, withMetrics bool) {
 func Exit() {
 	Handler.StopCPUProfile()
 	Handler.StopGoTrace()
+	if closer, ok := logOutputStream.(io.Closer); ok {
+		closer.Close()
+	}
 }