cmd/tailscale: don't duplicate log output

logtail.Log by default writes log output to stderr, but stderr is taken over by
filch's ReplaceStderr, resulting in duplicate logs sent to Tailscale.
ReplaceStderr is useful for capturing stack dumps from panics.

Configure logtail to route logs to the Android logger, which stops the
duplicate logging and replaces an existing MultiWriter setup for the same
purpose.

Reduce the scope of the logtail logger while here.

Fixes tailscale/tailscale#646

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/tailscale/backend.go

@@ -7,7 +7,6 @@ package main
 import (
 	"errors"
 	"fmt"
-	"io"
 	"log"
 	"path/filepath"
 	"reflect"
@@ -31,7 +30,6 @@ import (
 type backend struct {
 	engine   wgengine.Engine
 	backend  *ipn.LocalBackend
-	logger   logtail.Logger
 	devices  *multiTUN
 	settings func(*router.Config) error
 	lastCfg  *router.Config
@@ -283,6 +281,7 @@ func (b *backend) SetupLogs(logDir string, logID logtail.PrivateID) {
 	logcfg := logtail.Config{
 		Collection: "tailnode.log.tailscale.io",
 		PrivateID:  logID,
+		Stderr:     log.Writer(),
 	}
 	logcfg.LowMemory = true
 	drainCh := make(chan struct{})
@@ -310,13 +309,10 @@ func (b *backend) SetupLogs(logDir string, logID logtail.PrivateID) {
 	}
 
 	logf := wgengine.RusagePrefixLog(log.Printf)
-	b.logger = logtail.Log(logcfg, logf)
+	tlog := logtail.Log(logcfg, logf)
 
 	log.SetFlags(0)
-	log.SetOutput(io.MultiWriter(
+	log.SetOutput(tlog)
-		log.Writer(),
-		b.logger,
-	))
 
 	log.Printf("goSetupLogs: success")