net/dnscache: don't do bootstrap DNS lookup after most failed dials

If we've already connected to a certain name's IP in the past, don't
assume the problem was DNS related. That just puts unnecessarily load
on our bootstrap DNS servers during regular restarts of Tailscale
infrastructure components.

Also, if we do do a bootstrap DNS lookup and it gives the same IP(s)
that we already tried, don't try them again.

Change-Id: I743e8991a7f957381b8e4c1508b8e9d0df1782fe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dnscache/dnscache_test.go

@@ -6,10 +6,14 @@ package dnscache
 
 import (
 	"context"
+	"errors"
 	"flag"
 	"net"
+	"reflect"
 	"testing"
 	"time"
+
+	"inet.af/netaddr"
 )
 
 var dialTest = flag.String("dial-test", "", "if non-empty, addr:port to test dial")
@@ -31,3 +35,78 @@ func TestDialer(t *testing.T) {
 	t.Logf("dialed in %v", time.Since(t0))
 	c.Close()
 }
+
+func TestDialCall_DNSWasTrustworthy(t *testing.T) {
+	type step struct {
+		ip  netaddr.IP // IP we pretended to dial
+		err error      // the dial error or nil for success
+	}
+	mustIP := netaddr.MustParseIP
+	errFail := errors.New("some connect failure")
+	tests := []struct {
+		name  string
+		steps []step
+		want  bool
+	}{
+		{
+			name: "no-info",
+			want: false,
+		},
+		{
+			name: "previous-dial",
+			steps: []step{
+				{mustIP("2003::1"), nil},
+				{mustIP("2003::1"), errFail},
+			},
+			want: true,
+		},
+		{
+			name: "no-previous-dial",
+			steps: []step{
+				{mustIP("2003::1"), errFail},
+			},
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			d := &dialer{
+				pastConnect: map[netaddr.IP]time.Time{},
+			}
+			dc := &dialCall{
+				d: d,
+			}
+			for _, st := range tt.steps {
+				dc.noteDialResult(st.ip, st.err)
+			}
+			got := dc.dnsWasTrustworthy()
+			if got != tt.want {
+				t.Errorf("got %v; want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestDialCall_uniqueIPs(t *testing.T) {
+	dc := &dialCall{}
+	mustIP := netaddr.MustParseIP
+	errFail := errors.New("some connect failure")
+	dc.noteDialResult(mustIP("2003::1"), errFail)
+	dc.noteDialResult(mustIP("2003::2"), errFail)
+	got := dc.uniqueIPs([]netaddr.IP{
+		mustIP("2003::1"),
+		mustIP("2003::2"),
+		mustIP("2003::2"),
+		mustIP("2003::3"),
+		mustIP("2003::3"),
+		mustIP("2003::4"),
+		mustIP("2003::4"),
+	})
+	want := []netaddr.IP{
+		mustIP("2003::3"),
+		mustIP("2003::4"),
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+}