html/template: prevent panic while escaping pipelines

AFAIK, the documentation does not explicitly state whether
variables can store a callable entity or not. I believe the
current implementation in text/template assumes they cannot
though. The call builtin function is supposed to be used for
this purpose.

Template "{{0|$}}" should generate an error at runtime,
instead of a panic.

Similarly, template "{{0|(nil)}}" should not generate
a panic.

This CL aborts the sanitization process for a given pipeline
when no identifier can be derived from the selected node.
It happens with malformed pipelines.

We now have the following errors:

{{ 0 | $ }}
template: foo:1:10: executing "foo" at <$>: can't give argument to non-function $

{{ 0 | (nil) }}
template: foo:1:11: executing "foo" at <nil>: nil is not a command

Fixes #11118
Fixes #11356

Change-Id: Idae52f806849f4c9ab7aca1b4bb4b59a74723d0e
Reviewed-on: https://go-review.googlesource.com/10823Reviewed-by: Rob Pike <r@golang.org>

src/html/template/escape.go

@@ -215,7 +215,7 @@ func allIdents(node parse.Node) []string {
 	case *parse.ChainNode:
 		return node.Field
 	}
-	panic("unidentified node type in allIdents")
+	return nil
 }
 
 // ensurePipelineContains ensures that the pipeline has commands with

src/html/template/escape_test.go

@@ -1586,6 +1586,28 @@ func TestEnsurePipelineContains(t *testing.T) {
 	}
 }
 
+func TestEscapeMalformedPipelines(t *testing.T) {
+	tests := []string{
+		"{{ 0 | $ }}",
+		"{{ 0 | $ | urlquery }}",
+		"{{ 0 | $ | urlquery | html }}",
+		"{{ 0 | (nil) }}",
+		"{{ 0 | (nil) | html }}",
+		"{{ 0 | (nil) | html | urlquery }}",
+	}
+	for _, test := range tests {
+		var b bytes.Buffer
+		tmpl, err := New("test").Parse(test)
+		if err != nil {
+			t.Errorf("failed to parse set: %q", err)
+		}
+		err = tmpl.Execute(&b, nil)
+		if err == nil {
+			t.Errorf("Expected error for %q", test)
+		}
+	}
+}
+
 func TestEscapeErrorsNotIgnorable(t *testing.T) {
 	var b bytes.Buffer
 	tmpl, _ := New("dangerous").Parse("<a")