fix: avoid panic when a schema document decodes to null (#356)

pkg/validator/validator.go

@@ -372,6 +372,13 @@ func downloadSchema(registries []registry.Registry, l jsonschema.SchemeURLLoader
 	for _, reg := range registries {
 		path, s, err = reg.DownloadSchema(kind, version, k8sVersion)
 		if err == nil {
+			// A schema document that decodes to nil (for example a file whose
+			// entire content is the literal "null") is accepted by AddResource
+			// but makes Compile dereference a nil root and panic. Treat it as a
+			// non-parseable response and try the next registry instead.
+			if s == nil {
+				continue
+			}
 			c := jsonschema.NewCompiler()
 			c.RegisterFormat(&jsonschema.Format{"duration", validateDuration})
 			c.UseLoader(l)

pkg/validator/validator_test.go

@@ -460,6 +460,40 @@ interval: test
 			Invalid,
 			[]ValidationError{{Path: "/interval", Msg: "'test' is not valid duration: must start with P"}},
 		},
+		{
+			// A schema file whose entire content is the literal "null" decodes to
+			// a nil document. It used to reach jsonschema's Compile and panic with
+			// a nil pointer dereference (issue #337); it must now be treated as a
+			// missing schema instead.
+			"schema document is null",
+			[]byte(`
+kind: name
+apiVersion: v1
+firstName: foo
+`),
+			[]byte(`null`),
+			nil,
+			true,
+			false,
+			Skipped,
+			[]ValidationError{},
+		},
+		{
+			// Same as above but with IgnoreMissingSchemas disabled: a null schema
+			// document must surface as a graceful Error, not a panic (issue #337).
+			"schema document is null, do not ignore missing",
+			[]byte(`
+kind: name
+apiVersion: v1
+firstName: foo
+`),
+			[]byte(`null`),
+			nil,
+			false,
+			false,
+			Error,
+			[]ValidationError{},
+		},
 	} {
 		val := v{
 			opts: Opts{