all: cleanup unused code, part 2 (#10670)

And enable U1000 check in staticcheck.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

net/dns/direct_linux.go

@@ -4,9 +4,12 @@
 package dns
 
 import (
+	"bytes"
 	"context"
+	"errors"
 
 	"github.com/illarion/gonotify"
+	"tailscale.com/health"
 )
 
 func (m *directManager) runFileWatcher() {
@@ -55,6 +58,52 @@ func (m *directManager) runFileWatcher() {
 	}
 }
 
+var warnTrample = health.NewWarnable()
+
+// checkForFileTrample checks whether /etc/resolv.conf has been trampled
+// by another program on the system. (e.g. a DHCP client)
+func (m *directManager) checkForFileTrample() {
+	m.mu.Lock()
+	want := m.wantResolvConf
+	lastWarn := m.lastWarnContents
+	m.mu.Unlock()
+
+	if want == nil {
+		return
+	}
+
+	cur, err := m.fs.ReadFile(resolvConf)
+	if err != nil {
+		m.logf("trample: read error: %v", err)
+		return
+	}
+	if bytes.Equal(cur, want) {
+		warnTrample.Set(nil)
+		if lastWarn != nil {
+			m.mu.Lock()
+			m.lastWarnContents = nil
+			m.mu.Unlock()
+			m.logf("trample: resolv.conf again matches expected content")
+		}
+		return
+	}
+	if bytes.Equal(cur, lastWarn) {
+		// We already logged about this, so not worth doing it again.
+		return
+	}
+
+	m.mu.Lock()
+	m.lastWarnContents = cur
+	m.mu.Unlock()
+
+	show := cur
+	if len(show) > 1024 {
+		show = show[:1024]
+	}
+	m.logf("trample: resolv.conf changed from what we expected. did some other program interfere? current contents: %q", show)
+	warnTrample.Set(errors.New("Linux DNS config not ideal. /etc/resolv.conf overwritten. See https://tailscale.com/s/dns-fight"))
+}
+
 func (m *directManager) closeInotifyOnDone(ctx context.Context, in *gonotify.Inotify) {
 	<-ctx.Done()
 	in.Close()