[#4] dialer: Support hostnames

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

dialer.go

@@ -8,6 +8,11 @@ import (
 	"net/netip"
 	"sort"
 	"sync"
+	"time"
+)
+
+const (
+	defaultFallbackDelay = 300 * time.Millisecond
 )
 
 // Dialer contains the single most important method from the net.Dialer.
@@ -38,6 +43,10 @@ type dialer struct {
 	balancer balancer
 	// Hook used in tests, overrides `net.Dialer.DialContext()`
 	testHookDialContext func(d *net.Dialer, ctx context.Context, network, address string) (net.Conn, error)
+	// Hostname resolver
+	resolver net.Resolver
+	// See Config.FallbackDelay description.
+	fallbackDelay time.Duration
 }
 
 // Subnet represents a single subnet, possibly routable from multiple interfaces.
@@ -65,6 +74,15 @@ type Config struct {
 	Dialer net.Dialer
 	// Balancer specifies algorithm used to pick source address.
 	Balancer BalancerType
+	// FallbackDelay specifies the length of time to wait before
+	// spawning a RFC 6555 Fast Fallback connection. That is, this
+	// is the amount of time to wait for IPv6 to succeed before
+	// assuming that IPv6 is misconfigured and falling back to
+	// IPv4.
+	//
+	// If zero, a default delay of 300ms is used.
+	// A negative value disables Fast Fallback support.
+	FallbackDelay time.Duration
 }
 
 // NewDialer ...
@@ -105,6 +123,11 @@ func NewDialer(c Config) (Multidialer, error) {
 	}
 
 	d.restrict = c.Restrict
+	d.resolver.Dial = d.dialContextIP
+	d.fallbackDelay = c.FallbackDelay
+	if d.fallbackDelay == 0 {
+		d.fallbackDelay = defaultFallbackDelay
+	}
 
 	return &d, nil
 }
@@ -168,10 +191,164 @@ func processSubnet(subnet string, sources []iface) (Subnet, error) {
 // Hostnames for address are currently not supported.
 func (d *dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	addr, err := netip.ParseAddrPort(address)
+	if err != nil { //try resolve as hostname
+		return d.dialContextHostname(ctx, network, address)
+	}
+	return d.dialAddr(ctx, network, address, addr)
+}
+
+func (d *dialer) dialContextIP(ctx context.Context, network, address string) (net.Conn, error) {
+	addr, err := netip.ParseAddrPort(address)
+	if err != nil {
+		return nil, err
+	}
+	return d.dialAddr(ctx, network, address, addr)
+}
+
+func (d *dialer) dialContextHostname(ctx context.Context, network, address string) (net.Conn, error) {
+	// https://github.com/golang/go/blob/release-branch.go1.21/src/net/dial.go#L488
+	addrPorts, err := d.resolveHostIPs(ctx, address, network)
 	if err != nil {
 		return nil, err
 	}
 
+	var primaries, fallbacks []netip.AddrPort
+	if d.fallbackDelay >= 0 && network == "tcp" {
+		primaries, fallbacks = splitByType(addrPorts)
+	} else {
+		primaries = addrPorts
+	}
+
+	return d.dialParallel(ctx, network, primaries, fallbacks)
+}
+
+func (d *dialer) dialSerial(ctx context.Context, network string, addrs []netip.AddrPort) (net.Conn, error) {
+	var firstErr error
+	for _, addr := range addrs {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		conn, err := d.dialAddr(ctx, network, addr.String(), addr)
+		if err == nil {
+			return conn, nil
+		}
+		if firstErr == nil {
+			firstErr = err
+		}
+	}
+	return nil, fmt.Errorf("failed to connect any resolved address: %w", firstErr)
+}
+
+func (d *dialer) dialParallel(ctx context.Context, network string, primaries, fallbacks []netip.AddrPort) (net.Conn, error) {
+	if len(fallbacks) == 0 {
+		return d.dialSerial(ctx, network, primaries)
+	}
+
+	returned := make(chan struct{})
+	defer close(returned)
+
+	type dialResult struct {
+		net.Conn
+		error
+		primary bool
+		done    bool
+	}
+	results := make(chan dialResult)
+
+	dialerFunc := func(ctx context.Context, primary bool) {
+		addrs := primaries
+		if !primary {
+			addrs = fallbacks
+		}
+		c, err := d.dialSerial(ctx, network, addrs)
+		select {
+		case results <- dialResult{Conn: c, error: err, primary: primary, done: true}:
+		case <-returned:
+			if c != nil {
+				c.Close()
+			}
+		}
+	}
+
+	var primary, fallback dialResult
+
+	primaryCtx, primaryCancel := context.WithCancel(ctx)
+	defer primaryCancel()
+	go dialerFunc(primaryCtx, true)
+
+	fallbackTimer := time.NewTimer(d.fallbackDelay)
+	defer fallbackTimer.Stop()
+
+	for {
+		select {
+		case <-fallbackTimer.C:
+			fallbackCtx, fallbackCancel := context.WithCancel(ctx)
+			defer fallbackCancel()
+			go dialerFunc(fallbackCtx, false)
+
+		case res := <-results:
+			if res.error == nil {
+				return res.Conn, nil
+			}
+			if res.primary {
+				primary = res
+			} else {
+				fallback = res
+			}
+			if primary.done && fallback.done {
+				return nil, primary.error
+			}
+			if res.primary && fallbackTimer.Stop() {
+				// If we were able to stop the timer, that means it
+				// was running (hadn't yet started the fallback), but
+				// we just got an error on the primary path, so start
+				// the fallback immediately (in 0 nanoseconds).
+				fallbackTimer.Reset(0)
+			}
+		}
+	}
+}
+
+func (d *dialer) resolveHostIPs(ctx context.Context, address string, network string) ([]netip.AddrPort, error) {
+	host, port, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, fmt.Errorf("invalid address format: %w", err)
+	}
+
+	portnum, err := d.resolver.LookupPort(ctx, network, port)
+	if err != nil {
+		return nil, fmt.Errorf("invalid port format: %w", err)
+	}
+
+	ips, err := d.resolver.LookupHost(ctx, host)
+	if err != nil {
+		return nil, fmt.Errorf("failed to resolve host address: %w", err)
+	}
+
+	if len(ips) == 0 {
+		return nil, fmt.Errorf("failed to resolve address for [%s]%s", network, address)
+	}
+
+	var ipAddrs []netip.Addr
+	for _, ip := range ips {
+		ipAddr, err := netip.ParseAddr(ip)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse ip address '%s': %w", ip, err)
+		}
+		ipAddrs = append(ipAddrs, ipAddr)
+	}
+
+	var addrPorts []netip.AddrPort
+	for _, ipAddr := range ipAddrs {
+		addrPorts = append(addrPorts, netip.AddrPortFrom(ipAddr, uint16(portnum)))
+	}
+	return addrPorts, nil
+}
+
+func (d *dialer) dialAddr(ctx context.Context, network, address string, addr netip.AddrPort) (net.Conn, error) {
 	d.mtx.RLock()
 	defer d.mtx.RUnlock()
 
@@ -217,3 +394,21 @@ func (d *dialer) UpdateInterface(iface string, addr netip.Addr, up bool) error {
 	}
 	return nil
 }
+
+// splitByType divides an address list into two categories:
+// the first address, and any with same type, are returned as
+// primaries, while addresses with the opposite type are returned
+// as fallbacks.
+func splitByType(addrs []netip.AddrPort) (primaries []netip.AddrPort, fallbacks []netip.AddrPort) {
+	var primaryLabel bool
+	for i, addr := range addrs {
+		label := addr.Addr().Is4()
+		if i == 0 || label == primaryLabel {
+			primaryLabel = label
+			primaries = append(primaries, addr)
+		} else {
+			fallbacks = append(fallbacks, addr)
+		}
+	}
+	return
+}