logtail: add Config.Disabled to suppress the startup banner

NewLogger unconditionally writes a "logtail started" banner before
it returns, which callers that later call Logger.SetEnabled(false)
have no way to suppress: the banner is already buffered for upload
by the time the caller gets the logger back.

Add Config.Disabled so callers that know up front they want the
logger to start disabled (e.g. Android's remote-logging opt-out)
can seed the state before NewLogger's internal Write. The process-
wide Disable kill switch still takes precedence; SetEnabled can
still flip the state at runtime.

Updates #13174
Updates tailscale/tailscale-android#695

Change-Id: Icc4fa88c198447cf0faa707264dac84e359fe52c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/config.go

@@ -64,4 +64,12 @@ type Config struct {
 	// being included in the logs. The sequence number is incremented for each
 	// log message sent, but is not persisted across process restarts.
 	IncludeProcSequence bool
+
+	// Disabled, if true, causes the returned [Logger] to start in the
+	// disabled state, dropping entries without buffering or uploading
+	// (equivalent to calling [Logger.SetEnabled] with false immediately).
+	// It applies before the internal startup banner is written, so no
+	// log entries are emitted until [Logger.SetEnabled] is called with
+	// true. The process-wide [Disable] kill switch still takes precedence.
+	Disabled bool
 }

logtail/logtail.go

@@ -132,6 +132,7 @@ func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
 	}
 	logger.SetSockstatsLabel(sockstats.LabelLogtailLogger)
 	logger.compressLogs = cfg.CompressLogs
+	logger.disabled.Store(cfg.Disabled)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	logger.uploadCancel = cancel

logtail/logtail_test.go

@@ -7,11 +7,16 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
+	"sync"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	"github.com/go-json-experiment/json/jsontext"
@@ -21,6 +26,23 @@ import (
 	"tailscale.com/util/must"
 )
 
+// TestMain installs a safety net that refuses non-localhost dials for any
+// test in this package. Config.BaseURL defaults to https://log.tailscale.com
+// and Config.HTTPC defaults to http.DefaultClient, so a test that forgets to
+// override either can otherwise silently hit the real logtail server.
+func TestMain(m *testing.M) {
+	tr := http.DefaultTransport.(*http.Transport)
+	orig := tr.DialContext
+	tr.DialContext = func(ctx context.Context, network, addr string) (net.Conn, error) {
+		host, _, err := net.SplitHostPort(addr)
+		if err == nil && (host == "127.0.0.1" || host == "::1" || host == "localhost") {
+			return orig(ctx, network, addr)
+		}
+		return nil, fmt.Errorf("logtail tests: refusing to dial non-localhost address %q; use httptest.Server or a custom Config.HTTPC", addr)
+	}
+	os.Exit(m.Run())
+}
+
 func TestFastShutdown(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
@@ -321,6 +343,59 @@ func TestLoggerWriteResult(t *testing.T) {
 	}
 }
 
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) { return f(r) }
+
+func TestNewLoggerDisabled(t *testing.T) { synctest.Test(t, synctestNewLoggerDisabled) }
+
+func synctestNewLoggerDisabled(t *testing.T) {
+	// When Config.Disabled is true, NewLogger must not emit the usual
+	// "logtail started" banner: the logger should start in the disabled
+	// state before the internal startup write, so nothing ever lands
+	// in the buffer for the upload goroutine to drain.
+	buf := NewMemoryBuffer(100)
+
+	// Any HTTP attempt indicates the banner leaked into the buffer and
+	// the upload goroutine tried to ship it. Report it once (so the
+	// retry spin doesn't drown the log), then block on the request
+	// context so synctest.Wait sees a durable block and Shutdown's
+	// uploadCancel can unblock us cleanly.
+	var once sync.Once
+	httpc := &http.Client{
+		Transport: roundTripperFunc(func(r *http.Request) (*http.Response, error) {
+			once.Do(func() {
+				t.Errorf("unexpected HTTP request while Disabled=true: %s", r.URL)
+			})
+			<-r.Context().Done()
+			return nil, r.Context().Err()
+		}),
+	}
+
+	logger := NewLogger(Config{
+		BaseURL:  "http://logtail.test.invalid",
+		HTTPC:    httpc,
+		Bus:      eventbustest.NewBus(t),
+		Buffer:   buf,
+		Disabled: true,
+	}, t.Logf)
+	defer func() {
+		// Pass an already-cancelled context so Shutdown invokes
+		// uploadCancel immediately; otherwise on the regression path
+		// (Disabled=false) the upload goroutine stays in its retry
+		// loop and synctest.Test never returns.
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+		logger.Shutdown(ctx)
+	}()
+
+	synctest.Wait()
+
+	if back, _ := buf.TryReadLine(); len(back) != 0 {
+		t.Errorf("Disabled logger buffered a startup entry: %q", back)
+	}
+}
+
 func TestLoggerSetEnabled(t *testing.T) {
 	buf := NewMemoryBuffer(100)
 	lg := &Logger{