net: use same TCP Keep Alive interval between dial and accept

Fixes #31510

Change-Id: I601d114b617a055380bf3c805e2d9a9b0795b656
Reviewed-on: https://go-review.googlesource.com/c/go/+/175259Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dial.go

@@ -12,6 +12,12 @@ import (
 	"time"
 )
 
+// defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
+// See golang.org/issue/31510
+const (
+	defaultTCPKeepAlive = 15 * time.Second
+)
+
 // A Dialer contains options for connecting to an address.
 //
 // The zero value for each field is equivalent to dialing
@@ -425,7 +431,7 @@ func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn
 		setKeepAlive(tc.fd, true)
 		ka := d.KeepAlive
 		if d.KeepAlive == 0 {
-			ka = 15 * time.Second
+			ka = defaultTCPKeepAlive
 		}
 		setKeepAlivePeriod(tc.fd, ka)
 		testHookSetKeepAlive(ka)

src/net/tcpsock_plan9.go

@@ -8,7 +8,6 @@ import (
 	"context"
 	"io"
 	"os"
-	"time"
 )
 
 func (c *TCPConn) readFrom(r io.Reader) (int64, error) {
@@ -50,7 +49,7 @@ func (ln *TCPListener) accept() (*TCPConn, error) {
 		setKeepAlive(fd, true)
 		ka := ln.lc.KeepAlive
 		if ln.lc.KeepAlive == 0 {
-			ka = 3 * time.Minute
+			ka = defaultTCPKeepAlive
 		}
 		setKeepAlivePeriod(fd, ka)
 	}

src/net/tcpsock_posix.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"os"
 	"syscall"
-	"time"
 )
 
 func sockaddrToTCP(sa syscall.Sockaddr) Addr {
@@ -146,7 +145,7 @@ func (ln *TCPListener) accept() (*TCPConn, error) {
 		setKeepAlive(fd, true)
 		ka := ln.lc.KeepAlive
 		if ln.lc.KeepAlive == 0 {
-			ka = 3 * time.Minute
+			ka = defaultTCPKeepAlive
 		}
 		setKeepAlivePeriod(fd, ka)
 	}