fix: Handle the errors when the listener socket is closed gracefully.

internal/proxy/proxy.go

@@ -16,6 +16,7 @@ package proxy
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -637,7 +638,7 @@ func (c *Client) Serve(ctx context.Context, notify func()) error {
 	for _, m := range c.mnts {
 		go func(mnt *socketMount) {
 			err := c.serveSocketMount(ctx, mnt)
-			if err != nil {
+			if err != nil && errors.Is() {
 				select {
 				// Best effort attempt to send error.
 				// If this send fails, it means the reading goroutine has
@@ -731,50 +732,63 @@ func (c *Client) Close() error {
 
 // serveSocketMount persistently listens to the socketMounts listener and proxies connections to a
 // given Cloud SQL instance.
-func (c *Client) serveSocketMount(_ context.Context, s *socketMount) error {
+func (c *Client) serveSocketMount(ctx context.Context, s *socketMount) error {
 	for {
-		cConn, err := s.Accept()
-		if err != nil {
-			if nerr, ok := err.(net.Error); ok && nerr.Timeout() {
-				c.logger.Errorf("[%s] Error accepting connection: %v", s.inst, err)
-				// For transient errors, wait a small amount of time to see if it resolves itself
-				time.Sleep(10 * time.Millisecond)
-				continue
-			}
-			return err
-		}
-		// handle the connection in a separate goroutine
-		go func() {
-			c.logger.Infof("[%s] Accepted connection from %s", s.inst, cConn.RemoteAddr())
-
-			// A client has established a connection to the local socket. Before
-			// we initiate a connection to the Cloud SQL backend, increment the
-			// connection counter. If the total number of connections exceeds
-			// the maximum, refuse to connect and close the client connection.
-			count := atomic.AddUint64(&c.connCount, 1)
-			defer atomic.AddUint64(&c.connCount, ^uint64(0))
-
-			if c.conf.MaxConnections > 0 && count > c.conf.MaxConnections {
-				c.logger.Infof("max connections (%v) exceeded, refusing new connection", c.conf.MaxConnections)
-				if c.connRefuseNotify != nil {
-					go c.connRefuseNotify()
+		select {
+		case <-ctx.Done():
+			// If the context was canceled, do not accept any more connections,
+			// exit gracefully.
+			return nil
+		default:
+			// Wait to accept a connection. When s.Accept() returns io.EOF, exit
+			// gracefully.
+			cConn, err := s.Accept()
+			if err != nil {
+				if nerr, ok := err.(net.Error); ok && nerr.Timeout() {
+					c.logger.Errorf("[%s] Error accepting connection: %v", s.inst, err)
+					// For transient errors, wait a small amount of time to see if it resolves itself
+					time.Sleep(10 * time.Millisecond)
+					continue
+				} else if err == io.EOF {
+					// The socket was closed gracefully. Stop processing connections.
+					return nil
 				}
-				_ = cConn.Close()
-				return
+				return err
 			}
 
-			// give a max of 30 seconds to connect to the instance
-			ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
-			defer cancel()
+			// handle the connection in a separate goroutine
+			go func() {
+				c.logger.Infof("[%s] Accepted connection from %s", s.inst, cConn.RemoteAddr())
+
+				// A client has established a connection to the local socket. Before
+				// we initiate a connection to the Cloud SQL backend, increment the
+				// connection counter. If the total number of connections exceeds
+				// the maximum, refuse to connect and close the client connection.
+				count := atomic.AddUint64(&c.connCount, 1)
+				defer atomic.AddUint64(&c.connCount, ^uint64(0))
+
+				if c.conf.MaxConnections > 0 && count > c.conf.MaxConnections {
+					c.logger.Infof("max connections (%v) exceeded, refusing new connection", c.conf.MaxConnections)
+					if c.connRefuseNotify != nil {
+						go c.connRefuseNotify()
+					}
+					_ = cConn.Close()
+					return
+				}
 
-			sConn, err := c.dialer.Dial(ctx, s.inst, s.dialOpts...)
-			if err != nil {
-				c.logger.Errorf("[%s] failed to connect to instance: %v", s.inst, err)
-				_ = cConn.Close()
-				return
-			}
-			c.proxyConn(s.inst, cConn, sConn)
-		}()
+				// give a max of 30 seconds to connect to the instance
+				ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+				defer cancel()
+
+				sConn, err := c.dialer.Dial(ctx, s.inst, s.dialOpts...)
+				if err != nil {
+					c.logger.Errorf("[%s] failed to connect to instance: %v", s.inst, err)
+					_ = cConn.Close()
+					return
+				}
+				c.proxyConn(s.inst, cConn, sConn)
+			}()
+		}
 	}
 }