add __file__ variable to environment (#3705)

Co-authored-by: dgomez <dgomez@absolute.com>

internal/tiltfile/starkit/environment.go

@@ -241,7 +241,14 @@ func (e *Environment) doLoad(t *starlark.Thread, localPath string) (starlark.Str
 		contentBytes = []byte(contents)
 	}
 
-	return starlark.ExecFile(t, localPath, contentBytes, e.predeclared)
+	// Create a copy of predeclared variables so we can specify Tiltfile-specific values.
+	predeclared := starlark.StringDict{}
+	for k, v := range e.predeclared {
+		predeclared[k] = v
+	}
+	predeclared["__file__"] = starlark.String(localPath)
+
+	return starlark.ExecFile(t, localPath, contentBytes, predeclared)
 }
 
 type ArgUnpacker func(fnName string, args starlark.Tuple, kwargs []starlark.Tuple, pairs ...interface{}) error

internal/tiltfile/starkit/environment_test.go

@@ -198,3 +198,25 @@ load('../bar/Tiltfile', 'unused')
 	path := strings.TrimSpace(f.out.String())
 	require.Equal(t, "bar", filepath.Base(filepath.Dir(path)))
 }
+
+// Tiltfile loads Tiltfile2
+// 1 prints its __file__ and calls a method in 2 to do the same
+func TestUseMagicFileVar(t *testing.T) {
+	f := NewFixture(t, PwdExtension{})
+	f.File("Tiltfile2", `
+def print_mypath():
+  print(__file__)
+`)
+	f.File("Tiltfile", `
+load('Tiltfile2', 'print_mypath')
+print(__file__)
+print_mypath()
+`)
+
+	_, err := f.ExecFile("Tiltfile")
+	require.NoError(t, err)
+
+	paths := strings.Split(strings.TrimSpace(f.out.String()), "\n")
+	require.Equal(t, "Tiltfile", filepath.Base(paths[0]))
+	require.Equal(t, "Tiltfile2", filepath.Base(paths[1]))
+}