tiltfile: add fail function

docs/_build/html/api.html

@@ -222,6 +222,20 @@
 </table>
 </dd></dl>
 
+<dl class="function">
+<dt id="api.fail">
+<code class="descclassname">api.</code><code class="descname">fail</code><span class="sig-paren">(</span><em>msg</em><span class="sig-paren">)</span><a class="headerlink" href="#api.fail" title="Permalink to this definition">¶</a></dt>
+<dd><p>Raises an error that cannot be intercepted. Can be used anywhere in a Tiltfile.</p>
+<table class="docutils field-list" frame="void" rules="none">
+<col class="field-name" />
+<col class="field-body" />
+<tbody valign="top">
+<tr class="field-odd field"><th class="field-name">Return type:</th><td class="field-body"><code class="docutils literal notranslate"><span class="pre">None</span></code></td>
+</tr>
+</tbody>
+</table>
+</dd></dl>
+
 <dl class="function">
 <dt id="api.fast_build">
 <code class="descclassname">api.</code><code class="descname">fast_build</code><span class="sig-paren">(</span><em>img_name</em>, <em>dockerfile_path</em>, <em>entrypoint=''</em><span class="sig-paren">)</span><a class="headerlink" href="#api.fast_build" title="Permalink to this definition">¶</a></dt>

docs/_build/html/genindex.html

@@ -220,6 +220,10 @@ <h2 id="D">D</h2>
 
 <h2 id="F">F</h2>
 <table style="width: 100%" class="indextable genindextable"><tr>
+  <td style="width: 33%; vertical-align: top;"><ul>
+      <li><a href="api.html#api.fail">fail() (in module api)</a>
+</li>
+  </ul></td>
   <td style="width: 33%; vertical-align: top;"><ul>
       <li><a href="api.html#api.fast_build">fast_build() (in module api)</a>
 </li>

docs/api/api.py

@@ -110,3 +110,6 @@ def kustomize(pathToDir: str) -> Blob:
 
 def helm(pathToChartDir: Union[str, LocalPath]) -> Blob:
   """Run `helm template <https://docs.helm.sh/helm/#helm-template>`_ on a given directory that contains a chart and return the fully rendered YAML as a Blob"""
+
+def fail(msg: str) -> None:
+  """Raises an error that cannot be intercepted. Can be used anywhere in a Tiltfile."""

internal/tiltfile/fail.go

@@ -0,0 +1,17 @@
+package tiltfile
+
+import (
+	"fmt"
+
+	"go.starlark.net/starlark"
+)
+
+func (s *tiltfileState) fail(thread *starlark.Thread, fn *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+	var msg string
+	err := starlark.UnpackArgs(fn.Name(), args, kwargs, "msg", &msg)
+	if err != nil {
+		return nil, err
+	}
+
+	return nil, fmt.Errorf(msg)
+}

internal/tiltfile/tiltfile_state.go

@@ -91,6 +91,9 @@ const (
 	readFileN     = "read_file"
 	kustomizeN    = "kustomize"
 	helmN         = "helm"
+
+	// other functions
+	failN = "fail"
 )
 
 func (s *tiltfileState) builtins() starlark.StringDict {
@@ -117,6 +120,7 @@ func (s *tiltfileState) builtins() starlark.StringDict {
 	addBuiltin(r, localGitRepoN, s.localGitRepo)
 	addBuiltin(r, kustomizeN, s.kustomize)
 	addBuiltin(r, helmN, s.helm)
+	addBuiltin(r, failN, s.fail)
 
 	s.builtinsMap = r
 

internal/tiltfile/tiltfile_test.go

@@ -977,6 +977,19 @@ k8s_yaml('foo.yaml')
 	f.loadErrString("foo", "image gcr.io/foo:stable is not used in any resource")
 }
 
+func TestFail(t *testing.T) {
+	f := newFixture(t)
+	defer f.TearDown()
+
+	f.file("Tiltfile", `
+fail("this is an error")
+print("not this")
+fail("or this")
+`)
+
+	f.loadErrString("this is an error")
+}
+
 type fixture struct {
 	ctx context.Context
 	t   *testing.T