Use a wrapper instead of UiWriter

command/agent/agent.go

@@ -162,10 +162,7 @@ func (c *cmd) run(args []string) int {
 
 	// FIXME: logs should always go to stderr, but previously they were sent to
 	// stdout, so continue to use Stdout for now, and fix this in a future release.
-	// UiWriter is being used here so that writes to Stdout do not error (e.g.
-	// Windows Services are non-interactive and do not have Stdout) and inadvertently
-	// stop writes to other logging outputs like syslogs or log files.
-	logGate := &logging.GatedWriter{Writer: &mcli.UiWriter{Ui: c.ui}}
+	logGate := &logging.GatedWriter{Writer: c.ui.Stdout()}
 	loader := func(source config.Source) (config.LoadResult, error) {
 		c.configLoadOpts.DefaultConfig = source
 		return config.Load(c.configLoadOpts)

logging/logger.go

@@ -45,6 +45,16 @@ const defaultRotateDuration = 24 * time.Hour
 
 type LogSetupErrorFn func(string)
 
+// noErrorWriter is a wrapper to suppress errors when writing to w.
+type noErrorWriter struct {
+	w io.Writer
+}
+
+func (w noErrorWriter) Write(p []byte) (n int, err error) {
+	n, _ = w.w.Write(p)
+	return n, nil
+}
+
 // Setup logging from Config, and return an hclog Logger.
 //
 // Logs may be written to out, and optionally to syslog, and a file.
@@ -55,7 +65,10 @@ func Setup(config Config, out io.Writer) (hclog.InterceptLogger, error) {
 			allowedLogLevels)
 	}
 
-	writers := []io.Writer{out}
+	// If out is os.Stdout and Consul is being run as a Windows Service, writes will
+	// fail silently, which may inadvertently prevent writes to other writers.
+	// noErrorWriter is used as a wrapper to suppress any errors when writing to out.
+	writers := []io.Writer{noErrorWriter{w: out}}
 
 	if config.EnableSyslog {
 		retries := 12