net: unify TCP keepalive behavior

CL 107196 introduced a default TCP keepalive interval for Dialer and
TCPListener (used by both ListenConfig and ListenTCP). Leaving DialTCP
out was likely an oversight.

DialTCP's documentation says it "acts like Dial". Therefore it's natural
to also expect DialTCP to enable TCP keepalive by default.

This commit addresses this disparity by moving the enablement logic down
to the newTCPConn function, which is used by both dialer and listener.

Fixes #49345

src/net/dial.go

@@ -437,21 +437,7 @@ func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn
 		primaries = addrs
 	}
 
-	c, err := sd.dialParallel(ctx, primaries, fallbacks)
-	if err != nil {
-		return nil, err
-	}
-
-	if tc, ok := c.(*TCPConn); ok && d.KeepAlive >= 0 {
-		setKeepAlive(tc.fd, true)
-		ka := d.KeepAlive
-		if d.KeepAlive == 0 {
-			ka = defaultTCPKeepAlive
-		}
-		setKeepAlivePeriod(tc.fd, ka)
-		testHookSetKeepAlive(ka)
-	}
-	return c, nil
+	return sd.dialParallel(ctx, primaries, fallbacks)
 }
 
 // dialParallel races two copies of dialSerial, giving the first a

src/net/file_plan9.go

@@ -100,7 +100,7 @@ func fileConn(f *os.File) (Conn, error) {
 
 	switch fd.laddr.(type) {
 	case *TCPAddr:
-		return newTCPConn(fd), nil
+		return newTCPConn(fd, defaultTCPKeepAlive, testHookSetKeepAlive), nil
 	case *UDPAddr:
 		return newUDPConn(fd), nil
 	}

src/net/file_unix.go

@@ -74,7 +74,7 @@ func fileConn(f *os.File) (Conn, error) {
 	}
 	switch fd.laddr.(type) {
 	case *TCPAddr:
-		return newTCPConn(fd), nil
+		return newTCPConn(fd, defaultTCPKeepAlive, testHookSetKeepAlive), nil
 	case *UDPAddr:
 		return newUDPConn(fd), nil
 	case *IPAddr:

src/net/tcpsock.go

@@ -217,10 +217,19 @@ func (c *TCPConn) SetNoDelay(noDelay bool) error {
 	return nil
 }
 
-func newTCPConn(fd *netFD) *TCPConn {
-	c := &TCPConn{conn{fd}}
-	setNoDelay(c.fd, true)
-	return c
+func newTCPConn(fd *netFD, keepAlive time.Duration, keepAliveHook func(time.Duration)) *TCPConn {
+	setNoDelay(fd, true)
+	if keepAlive == 0 {
+		keepAlive = defaultTCPKeepAlive
+	}
+	if keepAlive > 0 {
+		setKeepAlive(fd, true)
+		setKeepAlivePeriod(fd, keepAlive)
+		if keepAliveHook != nil {
+			keepAliveHook(keepAlive)
+		}
+	}
+	return &TCPConn{conn{fd}}
 }
 
 // DialTCP acts like Dial for TCP networks.

src/net/tcpsock_plan9.go

@@ -42,7 +42,7 @@ func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCP
 	if err != nil {
 		return nil, err
 	}
-	return newTCPConn(fd), nil
+	return newTCPConn(fd, sd.Dialer.KeepAlive, testHookSetKeepAlive), nil
 }
 
 func (ln *TCPListener) ok() bool { return ln != nil && ln.fd != nil && ln.fd.ctl != nil }
@@ -52,16 +52,7 @@ func (ln *TCPListener) accept() (*TCPConn, error) {
 	if err != nil {
 		return nil, err
 	}
-	tc := newTCPConn(fd)
-	if ln.lc.KeepAlive >= 0 {
-		setKeepAlive(fd, true)
-		ka := ln.lc.KeepAlive
-		if ln.lc.KeepAlive == 0 {
-			ka = defaultTCPKeepAlive
-		}
-		setKeepAlivePeriod(fd, ka)
-	}
-	return tc, nil
+	return newTCPConn(fd, ln.lc.KeepAlive, nil), nil
 }
 
 func (ln *TCPListener) close() error {

src/net/tcpsock_posix.go

@@ -107,7 +107,7 @@ func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCP
 	if err != nil {
 		return nil, err
 	}
-	return newTCPConn(fd), nil
+	return newTCPConn(fd, sd.Dialer.KeepAlive, testHookSetKeepAlive), nil
 }
 
 func selfConnect(fd *netFD, err error) bool {
@@ -149,16 +149,7 @@ func (ln *TCPListener) accept() (*TCPConn, error) {
 	if err != nil {
 		return nil, err
 	}
-	tc := newTCPConn(fd)
-	if ln.lc.KeepAlive >= 0 {
-		setKeepAlive(fd, true)
-		ka := ln.lc.KeepAlive
-		if ln.lc.KeepAlive == 0 {
-			ka = defaultTCPKeepAlive
-		}
-		setKeepAlivePeriod(fd, ka)
-	}
-	return tc, nil
+	return newTCPConn(fd, ln.lc.KeepAlive, nil), nil
 }
 
 func (ln *TCPListener) close() error {

src/net/tcpsock_test.go

@@ -808,3 +808,22 @@ func BenchmarkSetReadDeadline(b *testing.B) {
 		deadline = deadline.Add(1)
 	}
 }
+
+func TestDialTCPDefaultKeepAlive(t *testing.T) {
+	ln := newLocalListener(t, "tcp")
+	defer ln.Close()
+
+	got := time.Duration(-1)
+	testHookSetKeepAlive = func(d time.Duration) { got = d }
+	defer func() { testHookSetKeepAlive = func(time.Duration) {} }()
+
+	c, err := DialTCP("tcp", nil, ln.Addr().(*TCPAddr))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	if got != defaultTCPKeepAlive {
+		t.Errorf("got keepalive %v; want %v", got, defaultTCPKeepAlive)
+	}
+}