Reduce ParseWorkflow hot-path overhead in frontmatter parsing and import scanning (#27200)

Author: Copilot

pkg/parser/frontmatter_content.go

@@ -24,10 +24,15 @@ type FrontmatterResult struct {
 // ExtractFrontmatterFromContent parses YAML frontmatter from markdown content string
 func ExtractFrontmatterFromContent(content string) (*FrontmatterResult, error) {
 	log.Printf("Extracting frontmatter from content: size=%d bytes", len(content))
-	lines := strings.Split(content, "\n")
+	// Fast-path: inspect only the first line to determine whether frontmatter exists.
+	firstNewline := strings.IndexByte(content, '\n')
+	firstLine := content
+	if firstNewline >= 0 {
+		firstLine = content[:firstNewline]
+	}
 
-	// Check if file starts with frontmatter delimiter
-	if len(lines) == 0 || strings.TrimSpace(lines[0]) != "---" {
+	// Check if file starts with frontmatter delimiter.
+	if strings.TrimSpace(firstLine) != "---" {
 		log.Print("No frontmatter delimiter found, returning content as markdown")
 		// No frontmatter, return entire content as markdown
 		return &FrontmatterResult{
@@ -38,22 +43,52 @@ func ExtractFrontmatterFromContent(content string) (*FrontmatterResult, error) {
 		}, nil
 	}
 
-	// Find end of frontmatter
-	endIndex := -1
-	for i := 1; i < len(lines); i++ {
-		if strings.TrimSpace(lines[i]) == "---" {
-			endIndex = i
+	// Find end of frontmatter by scanning line-by-line without splitting the entire document.
+	searchStart := len(content)
+	if firstNewline >= 0 {
+		searchStart = firstNewline + 1
+	}
+	frontmatterEndStart := -1
+	markdownStart := len(content)
+	for cursor := searchStart; cursor <= len(content); {
+		lineStart := cursor
+		lineEnd := len(content)
+		nextCursor := len(content) + 1
+
+		if cursor < len(content) {
+			if relNewline := strings.IndexByte(content[cursor:], '\n'); relNewline >= 0 {
+				lineEnd = cursor + relNewline
+				nextCursor = lineEnd + 1
+			}
+		}
+
+		if strings.TrimSpace(content[lineStart:lineEnd]) == "---" {
+			frontmatterEndStart = lineStart
+			markdownStart = nextCursor
 			break
 		}
+
+		if nextCursor > len(content) {
+			break
+		}
+		cursor = nextCursor
 	}
 
-	if endIndex == -1 {
+	if frontmatterEndStart == -1 {
 		return nil, errors.New("frontmatter not properly closed")
 	}
 
 	// Extract frontmatter YAML
-	frontmatterLines := lines[1:endIndex]
-	frontmatterYAML := strings.Join(frontmatterLines, "\n")
+	frontmatterYAML := content[searchStart:frontmatterEndStart]
+	frontmatterLines := []string{}
+	if frontmatterYAML != "" {
+		frontmatterLines = strings.Split(frontmatterYAML, "\n")
+		// Preserve previous behavior from lines[1:endIndex]: a trailing newline before
+		// the closing delimiter does not create an additional empty frontmatter line.
+		if strings.HasSuffix(frontmatterYAML, "\n") {
+			frontmatterLines = frontmatterLines[:len(frontmatterLines)-1]
+		}
+	}
 
 	// Sanitize no-break whitespace characters (U+00A0) which break the YAML parser
 	frontmatterYAML = strings.ReplaceAll(frontmatterYAML, "\u00A0", " ")
@@ -73,11 +108,10 @@ func ExtractFrontmatterFromContent(content string) (*FrontmatterResult, error) {
 	}
 
 	// Extract markdown content (everything after the closing ---)
-	var markdownLines []string
-	if endIndex+1 < len(lines) {
-		markdownLines = lines[endIndex+1:]
+	markdown := ""
+	if markdownStart <= len(content) {
+		markdown = content[markdownStart:]
 	}
-	markdown := strings.Join(markdownLines, "\n")
 
 	log.Printf("Successfully extracted frontmatter: fields=%d, markdown_size=%d bytes", len(frontmatter), len(markdown))
 	return &FrontmatterResult{

pkg/parser/frontmatter_extraction_test.go

@@ -3,6 +3,7 @@
 package parser
 
 import (
+	"reflect"
 	"testing"
 )
 
@@ -287,3 +288,71 @@ This is markdown.`,
 		})
 	}
 }
+
+func TestExtractFrontmatterFromContent_FrontmatterLinesAndStart(t *testing.T) {
+	tests := []struct {
+		name                 string
+		content              string
+		wantFrontmatterLines []string
+		wantFrontmatterStart int
+	}{
+		{
+			name: "no trailing blank frontmatter line without blank before closing delimiter",
+			content: `---
+on: workflow_dispatch
+permissions:
+  contents: read
+---
+# Body
+`,
+			wantFrontmatterLines: []string{
+				"on: workflow_dispatch",
+				"permissions:",
+				"  contents: read",
+			},
+			wantFrontmatterStart: 2,
+		},
+		{
+			name: "preserve intentional blank line before closing delimiter",
+			content: `---
+on: workflow_dispatch
+permissions:
+  contents: read
+
+---
+# Body
+`,
+			wantFrontmatterLines: []string{
+				"on: workflow_dispatch",
+				"permissions:",
+				"  contents: read",
+				"",
+			},
+			wantFrontmatterStart: 2,
+		},
+		{
+			name: "no frontmatter keeps empty frontmatter metadata",
+			content: `# Body without frontmatter
+`,
+			wantFrontmatterLines: []string{},
+			wantFrontmatterStart: 0,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := ExtractFrontmatterFromContent(tt.content)
+			if err != nil {
+				t.Fatalf("ExtractFrontmatterFromContent() error = %v", err)
+			}
+
+			if !reflect.DeepEqual(result.FrontmatterLines, tt.wantFrontmatterLines) {
+				t.Errorf("ExtractFrontmatterFromContent() FrontmatterLines = %#v, want %#v", result.FrontmatterLines, tt.wantFrontmatterLines)
+			}
+
+			if result.FrontmatterStart != tt.wantFrontmatterStart {
+				t.Errorf("ExtractFrontmatterFromContent() FrontmatterStart = %d, want %d", result.FrontmatterStart, tt.wantFrontmatterStart)
+			}
+		})
+	}
+}

pkg/workflow/compiler_orchestrator_engine.go

@@ -159,8 +159,9 @@ func (c *Compiler) setupEngineAndImports(result *parser.FrontmatterResult, clean
 		if idx := strings.Index(importFilePath, "#"); idx >= 0 {
 			importFilePath = importFilePath[:idx]
 		}
-		// Only scan markdown files — .yml imports are YAML config, not markdown content
-		if !strings.HasSuffix(importFilePath, ".md") {
+		// Only scan non-builtin markdown imports.
+		// Builtin imports are trusted project assets and are validated in-source.
+		if !shouldScanImportedMarkdown(importFilePath) {
 			continue
 		}
 		// Resolve the import path to a full filesystem path
@@ -358,6 +359,15 @@ func (c *Compiler) setupEngineAndImports(result *parser.FrontmatterResult, clean
 	}, nil
 }
 
+// shouldScanImportedMarkdown reports whether an import path should be processed by
+// markdown security scanning.
+func shouldScanImportedMarkdown(importFilePath string) bool {
+	if !strings.HasSuffix(importFilePath, ".md") {
+		return false
+	}
+	return !strings.HasPrefix(importFilePath, parser.BuiltinPathPrefix)
+}
+
 // isStringFormEngine reports whether the "engine" field in the given frontmatter is a
 // plain string (e.g. "engine: copilot"), as opposed to an object with an "id" or
 // "runtime" sub-key.

pkg/workflow/compiler_orchestrator_engine_test.go

@@ -218,6 +218,47 @@ strict: false
 	}
 }
 
+func TestShouldScanImportedMarkdown(t *testing.T) {
+	tests := []struct {
+		name           string
+		importFilePath string
+		want           bool
+	}{
+		{
+			name:           "scans regular markdown imports",
+			importFilePath: "shared/workflow.md",
+			want:           true,
+		},
+		{
+			name:           "skips builtin markdown imports",
+			importFilePath: parser.BuiltinPathPrefix + "engines/claude.md",
+			want:           false,
+		},
+		{
+			name:           "skips non-markdown imports",
+			importFilePath: "shared/workflow.lock.yml",
+			want:           false,
+		},
+		{
+			name:           "skips uppercase markdown extension",
+			importFilePath: "shared/workflow.MD",
+			want:           false,
+		},
+		{
+			name:           "skips builtin non-markdown imports",
+			importFilePath: parser.BuiltinPathPrefix + "engines/claude.yml",
+			want:           false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := shouldScanImportedMarkdown(tt.importFilePath)
+			assert.Equal(t, tt.want, got, "shouldScanImportedMarkdown(%q)", tt.importFilePath)
+		})
+	}
+}
+
 // TestSetupEngineAndImports_NetworkMerging tests network permissions merging from imports
 func TestSetupEngineAndImports_NetworkMerging(t *testing.T) {
 	tmpDir := testutil.TempDir(t, "engine-network")