net: retrieve if unix network is available only once for AIX

The previous version was executing "oslevel -s" everytime testableNetwork
was called with unix/unixgram network. The current version retrieves if
the network is possible only once at the beginning of the tests.

This is clearly faster:
ok      net     74.045s
ok      net     5.098s

Change-Id: I12549da27721f85c007cf17cab5cfdbfeb839cf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/171717
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/platform_test.go

@@ -14,6 +14,23 @@ import (
 	"testing"
 )
 
+var unixEnabledOnAIX bool
+
+func init() {
+	if runtime.GOOS == "aix" {
+		// Unix network isn't properly working on AIX 7.2 with
+		// Technical Level < 2.
+		// The information is retrieved only once in this init()
+		// instead of everytime testableNetwork is called.
+		out, _ := exec.Command("oslevel", "-s").Output()
+		if len(out) >= len("7200-XX-ZZ-YYMM") { // AIX 7.2, Tech Level XX, Service Pack ZZ, date YYMM
+			aixVer := string(out[:4])
+			tl, _ := strconv.Atoi(string(out[5:7]))
+			unixEnabledOnAIX = aixVer > "7200" || (aixVer == "7200" && tl >= 2)
+		}
+	}
+}
+
 // testableNetwork reports whether network is testable on the current
 // platform configuration.
 func testableNetwork(network string) bool {
@@ -38,15 +55,7 @@ func testableNetwork(network string) bool {
 		case "android", "nacl", "plan9", "windows":
 			return false
 		case "aix":
-			// Unix network isn't properly working on AIX 7.2 with Technical Level < 2
-			out, err := exec.Command("oslevel", "-s").Output()
-			if err != nil {
-				return false
-			}
-			if tl, err := strconv.Atoi(string(out[5:7])); err != nil || tl < 2 {
-				return false
-			}
-			return true
+			return unixEnabledOnAIX
 		}
 		// iOS does not support unix, unixgram.
 		if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {