[#11] dialer: Add event handler

Could be used for metrics and failover scenarios.

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

dialer.go

@@ -39,6 +39,8 @@ type dialer struct {
 	resolver net.Resolver
 	// See Config.FallbackDelay description.
 	fallbackDelay time.Duration
+	// Event handler.
+	eh EventHandler
 }
 
 // Subnet represents a single subnet, possibly routable from multiple source IPs.
@@ -47,6 +49,10 @@ type Subnet struct {
 	SourceIPs []netip.Addr
 }
 
+type EventHandler interface {
+	DialPerformed(sourceIP net.Addr, network, address string, err error)
+}
+
 // Config contains Multidialer configuration.
 type Config struct {
 	// Routable subnets.
@@ -71,6 +77,8 @@ type Config struct {
 	// DialContext is custom DialContext function.
 	// If not specified, default implemenattion is used (`d.DialContext(ctx, network, address)`).
 	DialContext func(d *net.Dialer, ctx context.Context, network, address string) (net.Conn, error)
+	// EventHandler defines event handler.
+	EventHandler EventHandler
 }
 
 // NewDialer ...
@@ -98,6 +106,12 @@ func NewDialer(c Config) (Dialer, error) {
 		d.customDialContext = c.DialContext
 	}
 
+	if c.EventHandler != nil {
+		d.eh = c.EventHandler
+	} else {
+		d.eh = noopEventHandler{}
+	}
+
 	return &d, nil
 }
 
@@ -279,10 +293,15 @@ func (d *dialer) dialAddr(ctx context.Context, network, address string, addr net
 }
 
 func (d *dialer) dialContext(ctx context.Context, nd *net.Dialer, network, address string) (net.Conn, error) {
+	var conn net.Conn
+	var err error
 	if h := d.customDialContext; h != nil {
-		return h(nd, ctx, network, address)
+		conn, err = h(nd, ctx, network, address)
+	} else {
+		conn, err = nd.DialContext(ctx, network, address)
 	}
-	return nd.DialContext(ctx, network, address)
+	d.eh.DialPerformed(nd.LocalAddr, network, address, err)
+	return conn, err
 }
 
 // splitByType divides an address list into two categories:
@@ -302,3 +321,7 @@ func splitByType(addrs []netip.AddrPort) (primaries []netip.AddrPort, fallbacks
 	}
 	return
 }
+
+type noopEventHandler struct{}
+
+func (s noopEventHandler) DialPerformed(net.Addr, string, string, error) {}