net/dnscache: add better logging to bootstrap DNS path (#5640)

Change-Id: I4cde3a72e06dac18df856a0cfeac10ab7e3a9108
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/dnscache/dnscache.go

@@ -361,7 +361,7 @@ func (d *dialer) DialContext(ctx context.Context, network, address string) (retC
 	defer func() {
 		// On failure, consider that our DNS might be wrong and ask the DNS fallback mechanism for
 		// some other IPs to try.
-		if ret == nil || ctx.Err() != nil || d.dnsCache.LookupIPFallback == nil || dc.dnsWasTrustworthy() {
+		if !d.shouldTryBootstrap(ctx, ret, dc) {
 			return
 		}
 		ips, err := d.dnsCache.LookupIPFallback(ctx, host)
@@ -398,6 +398,40 @@ func (d *dialer) DialContext(ctx context.Context, network, address string) (retC
 	return dc.raceDial(ctx, ipsToTry)
 }
 
+func (d *dialer) shouldTryBootstrap(ctx context.Context, err error, dc *dialCall) bool {
+	// No need to do anything when we succeeded.
+	if err == nil {
+		return false
+	}
+
+	// Can't try bootstrap DNS if we don't have a fallback function
+	if d.dnsCache.LookupIPFallback == nil {
+		if debug {
+			log.Printf("dnscache: not using bootstrap DNS: no fallback")
+		}
+		return false
+	}
+
+	// We can't retry if the context is canceled, since any further
+	// operations with this context will fail.
+	if ctxErr := ctx.Err(); ctxErr != nil {
+		if debug {
+			log.Printf("dnscache: not using bootstrap DNS: context error: %v", ctxErr)
+		}
+		return false
+	}
+
+	wasTrustworthy := dc.dnsWasTrustworthy()
+	if wasTrustworthy {
+		if debug {
+			log.Printf("dnscache: not using bootstrap DNS: DNS was trustworthy")
+		}
+		return false
+	}
+
+	return true
+}
+
 // dialCall is the state around a single call to dial.
 type dialCall struct {
 	d                            *dialer