testscript: add env convenience functions

Manipulating Env.Vars is not completely trivial: you need to handle
last-entry-wins, invalid entries, and key normalization.

This commit adds Getenv and Setenv method to Env that function
identically to their counterparts in the standard os package.

testscript/testscript.go

@@ -69,6 +69,27 @@ func (e *Env) Defer(f func()) {
 	e.ts.Defer(f)
 }
 
+// Getenv retrieves the value of the environment variable named by the key. It
+// returns the value, which will be empty if the variable is not present.
+func (e *Env) Getenv(key string) string {
+	key = envvarname(key)
+	for i := len(e.Vars) - 1; i >= 0; i-- {
+		if pair := strings.SplitN(e.Vars[i], "=", 2); len(pair) == 2 && envvarname(pair[0]) == key {
+			return pair[1]
+		}
+	}
+	return ""
+}
+
+// Setenv sets the value of the environment variable named by the key. It
+// panics if key is invalid.
+func (e *Env) Setenv(key, value string) {
+	if key == "" || strings.IndexByte(key, '=') != -1 {
+		panic("Setenv: invalid argument")
+	}
+	e.Vars = append(e.Vars, key+"="+value)
+}
+
 // T returns the t argument passed to the current test by the T.Run method.
 // Note that if the tests were started by calling Run,
 // the returned value will implement testing.TB.

testscript/testscript_test.go

@@ -83,6 +83,52 @@ func TestCRLFInput(t *testing.T) {
 	})
 }
 
+func TestEnv(t *testing.T) {
+	e := &Env{
+		Vars: []string{
+			"HOME=/no-home",
+			"PATH=/usr/bin",
+			"PATH=/usr/bin:/usr/local/bin",
+			"INVALID",
+		},
+	}
+
+	if got, want := e.Getenv("HOME"), "/no-home"; got != want {
+		t.Errorf("e.Getenv(\"HOME\") == %q, want %q", got, want)
+	}
+
+	e.Setenv("HOME", "/home/user")
+	if got, want := e.Getenv("HOME"), "/home/user"; got != want {
+		t.Errorf("e.Getenv(\"HOME\") == %q, want %q", got, want)
+	}
+
+	if got, want := e.Getenv("PATH"), "/usr/bin:/usr/local/bin"; got != want {
+		t.Errorf("e.Getenv(\"PATH\") == %q, want %q", got, want)
+	}
+
+	if got, want := e.Getenv("INVALID"), ""; got != want {
+		t.Errorf("e.Getenv(\"INVALID\") == %q, want %q", got, want)
+	}
+
+	for _, key := range []string{
+		"",
+		"=",
+		"key=invalid",
+	} {
+		value := ""
+		var panicValue interface{}
+		func() {
+			defer func() {
+				panicValue = recover()
+			}()
+			e.Setenv(key, value)
+		}()
+		if panicValue == nil {
+			t.Errorf("e.Setenv(%q, %q) did not panic, want panic", key, value)
+		}
+	}
+}
+
 func TestScripts(t *testing.T) {
 	// TODO set temp directory.
 	testDeferCount := 0