proxy: add LocalAddr() method

This method exposes the underlying listener's local address. It will be
useful to get the address if the listener binds to an address in the
form "127.0.0.1:0", which picks up a random port.

/xref planetscale/surfaces#111

proxy/client.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/tls"
 	"crypto/x509"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -52,6 +53,10 @@ type Client struct {
 	// configCache contains the TLS certificate chache for each indiviual
 	// database
 	configCache *tlsCache
+
+	listener net.Listener
+	// done is closed after a successfull net.Listen bind.
+	done chan struct{}
 }
 
 // Options are the options for creating a new Client.
@@ -89,6 +94,7 @@ func NewClient(opts Options) (*Client, error) {
 		remoteAddr:  opts.RemoteAddr,
 		instance:    opts.Instance,
 		configCache: newtlsCache(),
+		done:        make(chan struct{}, 0),
 	}
 
 	if opts.Logger != nil {
@@ -129,9 +135,24 @@ func (c *Client) Run(ctx context.Context) error {
 	}
 	defer c.log.Sync() // nolint: errcheck
 
+	c.listener = l
+	close(c.done)
+
 	return c.run(ctx, l)
 }
 
+// LocalAddr returns the address of the local listener. This is by default
+// blocking and will only return if the proxy is invoked with the Run() method.
+func (c *Client) LocalAddr() (net.Addr, error) {
+	<-c.done
+
+	if c.listener == nil {
+		return nil, errors.New("listener is not set")
+
+	}
+	return c.listener.Addr(), nil
+}
+
 func (c *Client) getListener() (net.Listener, error) {
 	if strings.HasPrefix(c.localAddr, "unix://") {
 		p := strings.TrimPrefix(c.localAddr, "unix://")