fix: resolve YAML anchors and aliases in Actions workflows (#38984) (#38996)

Backport https://github.com/go-gitea/gitea/pull/38984

Workflows using YAML anchors are rejected as invalid, because a workflow
is split into one document per job and an alias whose anchor lands in
another job's document no longer resolves. Node walkers such as `on:`
parsing have no alias case either.

Aliases are now expanded once, right after the workflow is parsed and
before anything reads or splits it, bounded like GitHub's parser so
nested aliases cannot expand without limit. Merge keys stay unsupported,
as they are upstream.

Fixes: https://github.com/go-gitea/gitea/issues/38983

modules/actions/jobparser/jobparser.go

@@ -4,7 +4,6 @@
 package jobparser
 
 import (
-	"bytes"
 	"fmt"
 	"slices"
 	"sort"
@@ -17,14 +16,21 @@ import (
 )
 
 func Parse(content []byte, options ...ParseOption) ([]*SingleWorkflow, error) {
-	origin, err := model.ReadWorkflow(bytes.NewReader(content))
+	// The workflow is split into one document per job below, which would strand an alias whose
+	// anchor lands in another one.
+	doc, err := resolveYamlAliases(content)
 	if err != nil {
-		return nil, fmt.Errorf("model.ReadWorkflow: %w", err)
+		return nil, fmt.Errorf("resolve aliases: %w", err)
+	}
+
+	origin, err := readWorkflowDoc(doc)
+	if err != nil {
+		return nil, fmt.Errorf("read workflow: %w", err)
 	}
 
 	workflow := &SingleWorkflow{}
-	if err := yaml.Unmarshal(content, workflow); err != nil {
-		return nil, fmt.Errorf("yaml.Unmarshal: %w", err)
+	if err := decodeYamlDoc(doc, workflow); err != nil {
+		return nil, fmt.Errorf("decode workflow: %w", err)
 	}
 
 	pc := &parseContext{}
@@ -156,9 +162,6 @@ func validateMatrixFilters(job *model.Job) error {
 			entries = value.Content
 		}
 		for _, entry := range entries {
-			if entry.Kind == yaml.AliasNode {
-				entry = entry.Alias
-			}
 			if entry.Kind != yaml.MappingNode {
 				return fmt.Errorf("matrix %s must be a list of mappings", name)
 			}