net/dnscache: refactor from func-y closure-y state to types & methods

No behavior changes (intended, at least).

This is in prep for future changes to this package, which would get
too complicated in the current style.

Change-Id: Ic260f8e34ae2f64f34819d4a56e38bee8d8ac5ce
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dnscache/dnscache.go

@@ -278,53 +278,78 @@ type DialContextFunc func(ctx context.Context, network, address string) (net.Con
 
 // Dialer returns a wrapped DialContext func that uses the provided dnsCache.
 func Dialer(fwd DialContextFunc, dnsCache *Resolver) DialContextFunc {
-	return func(ctx context.Context, network, address string) (retConn net.Conn, ret error) {
-		host, port, err := net.SplitHostPort(address)
-		if err != nil {
-			// Bogus. But just let the real dialer return an error rather than
-			// inventing a similar one.
-			return fwd(ctx, network, address)
-		}
-		defer func() {
-			// On any failure, assume our DNS is wrong and try our fallback, if any.
-			if ret == nil || dnsCache.LookupIPFallback == nil {
-				return
-			}
-			ips, err := dnsCache.LookupIPFallback(ctx, host)
-			if err != nil {
-				// Return with original error
-				return
-			}
-			if c, err := raceDial(ctx, fwd, network, ips, port); err == nil {
-				retConn = c
-				ret = nil
-				return
-			}
-		}()
+	d := &dialer{
+		fwd:      fwd,
+		dnsCache: dnsCache,
+	}
+	return d.DialContext
+}
 
-		ip, ip6, allIPs, err := dnsCache.LookupIP(ctx, host)
+// dialer is the config and accumulated state for a dial func returned by Dialer.
+type dialer struct {
+	fwd      DialContextFunc
+	dnsCache *Resolver
+}
+
+func (d *dialer) DialContext(ctx context.Context, network, address string) (retConn net.Conn, ret error) {
+	host, port, err := net.SplitHostPort(address)
+	if err != nil {
+		// Bogus. But just let the real dialer return an error rather than
+		// inventing a similar one.
+		return d.fwd(ctx, network, address)
+	}
+	dc := &dialCall{
+		d:       d,
+		network: network,
+		address: address,
+		host:    host,
+		port:    port,
+	}
+	defer func() {
+		// On any failure, assume our DNS is wrong and try our fallback, if any.
+		if ret == nil || d.dnsCache.LookupIPFallback == nil {
+			return
+		}
+		ips, err := d.dnsCache.LookupIPFallback(ctx, host)
 		if err != nil {
-			return nil, fmt.Errorf("failed to resolve %q: %w", host, err)
+			// Return with original error
+			return
 		}
-		i4s := v4addrs(allIPs)
-		if len(i4s) < 2 {
-			dst := net.JoinHostPort(ip.String(), port)
-			if debug {
-				log.Printf("dnscache: dialing %s, %s for %s", network, dst, address)
-			}
-			c, err := fwd(ctx, network, dst)
-			if err == nil || ctx.Err() != nil || ip6 == nil {
-				return c, err
-			}
-			// Fall back to trying IPv6.
-			dst = net.JoinHostPort(ip6.String(), port)
-			return fwd(ctx, network, dst)
+		if c, err := dc.raceDial(ctx, ips); err == nil {
+			retConn = c
+			ret = nil
+			return
 		}
+	}()
 
-		// Multiple IPv4 candidates, and 0+ IPv6.
-		ipsToTry := append(i4s, v6addrs(allIPs)...)
-		return raceDial(ctx, fwd, network, ipsToTry, port)
+	ip, ip6, allIPs, err := d.dnsCache.LookupIP(ctx, host)
+	if err != nil {
+		return nil, fmt.Errorf("failed to resolve %q: %w", host, err)
+	}
+	i4s := v4addrs(allIPs)
+	if len(i4s) < 2 {
+		dst := net.JoinHostPort(ip.String(), port)
+		if debug {
+			log.Printf("dnscache: dialing %s, %s for %s", network, dst, address)
+		}
+		c, err := d.fwd(ctx, network, dst)
+		if err == nil || ctx.Err() != nil || ip6 == nil {
+			return c, err
+		}
+		// Fall back to trying IPv6.
+		dst = net.JoinHostPort(ip6.String(), port)
+		return d.fwd(ctx, network, dst)
 	}
+
+	// Multiple IPv4 candidates, and 0+ IPv6.
+	ipsToTry := append(i4s, v6addrs(allIPs)...)
+	return dc.raceDial(ctx, ipsToTry)
+}
+
+// dialCall is the state around a single call to dial.
+type dialCall struct {
+	d                            *dialer
+	network, address, host, port string
 }
 
 // fallbackDelay is how long to wait between trying subsequent
@@ -334,7 +359,12 @@ const fallbackDelay = 300 * time.Millisecond
 
 // raceDial tries to dial port on each ip in ips, starting a new race
 // dial every fallbackDelay apart, returning whichever completes first.
-func raceDial(ctx context.Context, fwd DialContextFunc, network string, ips []netaddr.IP, port string) (net.Conn, error) {
+func (dc *dialCall) raceDial(ctx context.Context, ips []netaddr.IP) (net.Conn, error) {
+	var (
+		fwd     = dc.d.fwd
+		network = dc.network
+		port    = dc.port
+	)
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()