os: add LookupEnv, like Getenv but reports presence

Fixes #9676.

Change-Id: I32fe474cdfa09aff91daa4b10ac4df28ffdaa649
Reviewed-on: https://go-review.googlesource.com/9741Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env.go

@@ -81,6 +81,15 @@ func Getenv(key string) string {
 	return v
 }
 
+// LookupEnv retrieves the value of the environment variable named
+// by the key. If the variable is present in the environment the
+// value (which may be empty) is returned and the boolean is true.
+// Otherwise the returned value will be empty and the boolean will
+// be false.
+func LookupEnv(key string) (string, bool) {
+	return syscall.Getenv(key)
+}
+
 // Setenv sets the value of the environment variable named by the key.
 // It returns an error, if any.
 func Setenv(key, value string) error {

src/os/env_test.go

@@ -94,3 +94,15 @@ func TestUnsetenv(t *testing.T) {
 		t.Fatal("Unsetenv didn't clear TestUnsetenv")
 	}
 }
+
+func TestLookupEnv(t *testing.T) {
+	value, ok := LookupEnv("GOROOT") // Should be set.
+	if !ok {
+		t.Errorf("GOROOT is not set")
+	}
+	const v = "Variable That Does Not Exist"
+	value, ok = LookupEnv(v) // Should not be set.
+	if ok || value != "" {
+		t.Errorf("%s is set: %q", v, value)
+	}
+}