tiltfile: add config.main_path, config.main_dir. Fixes #3507 (#3784)

internal/tiltfile/config/config.go

@@ -81,6 +81,19 @@ func (e *Extension) OnStart(env *starkit.Environment) error {
 	}
 
 	err := env.AddValue("config.tilt_subcommand", starlark.String(e.TiltSubcommand))
+	if err != nil {
+		return err
+	}
+
+	err = env.AddValue("config.main_path", starlark.String(env.StartPath()))
+	if err != nil {
+		return err
+	}
+
+	err = env.AddValue("config.main_dir", starlark.String(filepath.Dir(env.StartPath())))
+	if err != nil {
+		return err
+	}
 
 	return err
 }

internal/tiltfile/config/config_test.go

@@ -459,6 +459,44 @@ print(config.tilt_subcommand)
 	require.Equal(t, "foo\n", f.PrintOutput())
 }
 
+func TestTiltfilePath(t *testing.T) {
+	f := NewFixture(t, model.UserConfigState{}, "foo")
+	defer f.TearDown()
+
+	f.File("foo/Tiltfile", `
+print(config.main_path)
+`)
+	f.File("Tiltfile", `
+include('./foo/Tiltfile')
+print(config.main_path)
+`)
+
+	_, err := f.ExecFile("Tiltfile")
+	require.NoError(t, err)
+
+	val := f.JoinPath("Tiltfile")
+	require.Equal(t, fmt.Sprintf("%s\n%s\n", val, val), f.PrintOutput())
+}
+
+func TestTiltfileDir(t *testing.T) {
+	f := NewFixture(t, model.UserConfigState{}, "foo")
+	defer f.TearDown()
+
+	f.File("foo/Tiltfile", `
+print(config.main_dir)
+`)
+	f.File("Tiltfile", `
+include('./foo/Tiltfile')
+print(config.main_dir)
+`)
+
+	_, err := f.ExecFile("Tiltfile")
+	require.NoError(t, err)
+
+	val := f.Path()
+	require.Equal(t, fmt.Sprintf("%s\n%s\n", val, val), f.PrintOutput())
+}
+
 func NewFixture(tb testing.TB, userConfigState model.UserConfigState, tiltSubcommand model.TiltSubcommand) *starkit.Fixture {
 	ext := NewExtension(tiltSubcommand)
 	ext.UserConfigState = userConfigState

internal/tiltfile/starkit/environment.go

@@ -47,6 +47,7 @@ type Environment struct {
 	extensions       []Extension
 	fakeFileSystem   map[string]string
 	loadInterceptors []LoadInterceptor
+	startPath        string
 
 	builtinCalls []BuiltinCall
 }
@@ -70,6 +71,11 @@ func (e *Environment) SetArgUnpacker(unpackArgs ArgUnpacker) {
 	e.unpackArgs = unpackArgs
 }
 
+// The absolute path to the entrypoint of the environment.
+func (e *Environment) StartPath() string {
+	return e.startPath
+}
+
 // Add a builtin to the environment.
 //
 // All builtins will be wrapped to invoke OnBuiltinCall on every extension.
@@ -152,6 +158,8 @@ func (e *Environment) start(path string) (Model, error) {
 		return Model{}, errors.Wrap(err, "environment#start")
 	}
 
+	e.startPath = path
+
 	model := NewModel()
 	for _, ext := range e.extensions {
 		sExt, isStateful := ext.(StatefulExtension)