net: enable TCP keepalives by default

This is just the first step in attempting to make all network connection have
timeouts as a "safe default". TCP keepalives only protect against certain
classes of network and host issues (e.g. server/OS crash), but do nothing
against application-level issues (e.g. an application that accepts connections
but then fails to serve requests).

The actual keep-alive duration (15s) is chosen to cause broken connections
to be closed after 2~3 minutes (depending on the OS, see #23549 for details).
We don't make the actual default value part of the public API for a number of
reasons:
- because it's not very useful by itself: as discussed in #23549 the actual
  "timeout" after which the connection is torn down is duration*(KEEPCNT+1),
  and we use the OS-wide value for KEEPCNT because there's currently no way
  to set it from Go.
- because it may change in the future: if users need to rely on a specific
  value they should explicitly set this value instead of relying on the default.

Fixes #23459

Change-Id: I348c03be97588d5001e6de0f377e7a93b51957fd
Reviewed-on: https://go-review.googlesource.com/c/107196
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 17 insertions, 10 deletions

diff --git a/src/net/dial_test.go b/src/net/dial_test.go
index 983338885d..3a2c59a2d1 100644
--- a/src/net/dial_test.go
+++ b/src/net/dial_test.go
@@ -729,22 +729,29 @@ func TestDialerKeepAlive(t *testing.T) {
 	if err := ls.buildup(handler); err != nil {
 		t.Fatal(err)
 	}
-	defer func() { testHookSetKeepAlive = func() {} }()
+	defer func() { testHookSetKeepAlive = func(time.Duration) {} }()
 
-	for _, keepAlive := range []bool{false, true} {
-		got := false
-		testHookSetKeepAlive = func() { got = true }
-		var d Dialer
-		if keepAlive {
-			d.KeepAlive = 30 * time.Second
-		}
+	tests := []struct {
+		ka       time.Duration
+		expected time.Duration
+	}{
+		{-1, -1},
+		{0, 15 * time.Second},
+		{5 * time.Second, 5 * time.Second},
+		{30 * time.Second, 30 * time.Second},
+	}
+
+	for _, test := range tests {
+		var got time.Duration = -1
+		testHookSetKeepAlive = func(d time.Duration) { got = d }
+		d := Dialer{KeepAlive: test.ka}
 		c, err := d.Dial("tcp", ls.Listener.Addr().String())
 		if err != nil {
 			t.Fatal(err)
 		}
 		c.Close()
-		if got != keepAlive {
-			t.Errorf("Dialer.KeepAlive = %v: SetKeepAlive called = %v, want %v", d.KeepAlive, got, !got)
+		if got != test.expected {
+			t.Errorf("Dialer.KeepAlive = %v: SetKeepAlive set to %v, want %v", d.KeepAlive, got, test.expected)
 		}
 	}
 }