fix: schema validation must accept aliases and merges (#1012)

- aliases are an indirection to follow
  https://yaml.org/spec/1.2.1/#id2785586
- merges cannot be conveniently validated and are skipped
  https://yaml.org/type/merge.html

Resolves forgejo/runner#1011

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1012): <!--number 1012 --><!--line 0 --><!--description Zml4OiBzY2hlbWEgdmFsaWRhdGlvbiBtdXN0IGFjY2VwdCBhbGlhc2VzIGFuZCBtZXJnZXM=-->fix: schema validation must accept aliases and merges<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1012
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

act/schema/schema.go

@@ -237,6 +237,9 @@ func (s *Node) UnmarshalYAML(node *yaml.Node) error {
 	if node != nil && node.Kind == yaml.DocumentNode {
 		return s.UnmarshalYAML(node.Content[0])
 	}
+	if node != nil && node.Kind == yaml.AliasNode {
+		return s.UnmarshalYAML(node.Alias)
+	}
 	def := s.Schema.GetDefinition(s.Definition)
 	if s.Context == nil {
 		s.Context = def.Context
@@ -360,6 +363,15 @@ func (s *Node) checkMapping(node *yaml.Node, def Definition) error {
 	if node.Kind != yaml.MappingNode {
 		return fmt.Errorf("%sExpected a mapping got %v", formatLocation(node), getStringKind(node.Kind))
 	}
+	// merges cannot be conveniently validated and are skipped
+	// https://yaml.org/type/merge.html
+	for i, n := range node.Content {
+		if i%2 == 0 {
+			if n.Kind == yaml.ScalarNode && n.Value == "<<" && (n.Tag == "" || n.ShortTag() == "!!merge") {
+				return nil
+			}
+		}
+	}
 	insertDirective := regexp.MustCompile(`\${{\s*insert\s*}}`)
 	var allErrors error
 	for i, k := range node.Content {

act/schema/schema_test.go

@@ -243,3 +243,39 @@ runs:
 		})
 	}
 }
+
+// https://yaml.org/spec/1.2.1/#id2785586
+// An anchor is denoted by the “&” indicator. It marks a node for future reference.
+// https://yaml.org/type/merge.html
+// Specify one or more mappings to be merged with the current one.
+func TestSchema_AnchorAndReference(t *testing.T) {
+	var node yaml.Node
+	err := yaml.Unmarshal([]byte(`
+on: [push]
+jobs:
+  test1:
+    runs-on: docker
+    steps:
+      - &step
+        run: echo All good!
+      - *step
+  test2:
+    runs-on: docker
+    steps:
+      - << : *step
+        name: other name
+  test3:
+    runs-on: docker
+    steps:
+      - !!merge << : *step
+        name: other name
+`), &node)
+	if !assert.NoError(t, err) {
+		return
+	}
+	err = (&Node{
+		Definition: "workflow-root",
+		Schema:     GetWorkflowSchema(),
+	}).UnmarshalYAML(&node)
+	assert.NoError(t, err)
+}