crypto/tls: enable TLS 1.3 by default

Updates #30055

Change-Id: I3e79dd7592673c5d76568b0bcded6c391c3be6b3
Reviewed-on: https://go-review.googlesource.com/c/163081
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/tls/common.go

@@ -776,7 +776,7 @@ func (c *Config) supportedVersions(isClient bool) []uint16 {
 		if isClient && v < VersionTLS10 {
 			continue
 		}
-		// TLS 1.3 is opt-in in Go 1.12.
+		// TLS 1.3 is opt-out in Go 1.13.
 		if v == VersionTLS13 && !isTLS13Supported() {
 			continue
 		}
@@ -791,11 +791,11 @@ var tls13Support struct {
 	cached bool
 }
 
-// isTLS13Supported returns whether the program opted into TLS 1.3 via
-// GODEBUG=tls13=1. It's cached after the first execution.
+// isTLS13Supported returns whether the program enabled TLS 1.3 by not opting
+// out with GODEBUG=tls13=0. It's cached after the first execution.
 func isTLS13Supported() bool {
 	tls13Support.Do(func() {
-		tls13Support.cached = goDebugString("tls13") == "1"
+		tls13Support.cached = goDebugString("tls13") != "0"
 	})
 	return tls13Support.cached
 }

src/crypto/tls/tls.go

@@ -5,14 +5,9 @@
 // Package tls partially implements TLS 1.2, as specified in RFC 5246,
 // and TLS 1.3, as specified in RFC 8446.
 //
-// TLS 1.3 is available only on an opt-in basis in Go 1.12. To enable
+// TLS 1.3 is available on an opt-out basis in Go 1.13. To disable
 // it, set the GODEBUG environment variable (comma-separated key=value
-// options) such that it includes "tls13=1". To enable it from within
-// the process, set the environment variable before any use of TLS:
-//
-//     func init() {
-//         os.Setenv("GODEBUG", os.Getenv("GODEBUG")+",tls13=1")
-//     }
+// options) such that it includes "tls13=0".
 package tls
 
 // BUG(agl): The crypto/tls package only implements some countermeasures

src/crypto/tls/tls_test.go

@@ -23,13 +23,6 @@ import (
 	"time"
 )
 
-func init() {
-	// TLS 1.3 is opt-in for Go 1.12, but we want to run most tests with it enabled.
-	// TestTLS13Switch below tests the disabled behavior. See Issue 30055.
-	tls13Support.Do(func() {}) // defuse the sync.Once
-	tls13Support.cached = true
-}
-
 var rsaCertPEM = `-----BEGIN CERTIFICATE-----
 MIIB0zCCAX2gAwIBAgIJAI/M7BYjwB+uMA0GCSqGSIb3DQEBBQUAMEUxCzAJBgNV
 BAYTAkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEwHwYDVQQKDBhJbnRlcm5ldCBX