fix: return error instead of panic for missing frontmatter fence (#1137)

* fix: return error instead of panic for missing frontmatter fence

When a file has a closing frontmatter fence (---) without an opening fence,
the parser would panic with 'originalIM was set twice'. This change detects
that condition and returns a helpful diagnostic error instead.

Fixes #1133, #1038. May also help with #1094, #1107.

* Add test for location

Author: matthewp

.changeset/fix-missing-frontmatter-fence.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/compiler": patch
+---
+
+Fixes a panic when parsing files with a closing frontmatter fence (---) but no opening fence. The compiler now returns a helpful diagnostic error instead of crashing.

internal/loc/diagnostics.go

@@ -9,6 +9,7 @@ const (
 	ERROR_UNMATCHED_IMPORT            DiagnosticCode = 1003
 	ERROR_UNSUPPORTED_SLOT_ATTRIBUTE  DiagnosticCode = 1004
 	ERROR_UNTERMINATED_STRING         DiagnosticCode = 1005
+	ERROR_MISSING_FRONTMATTER_FENCE   DiagnosticCode = 1006
 	WARNING                           DiagnosticCode = 2000
 	WARNING_UNTERMINATED_HTML_COMMENT DiagnosticCode = 2001
 	WARNING_UNCLOSED_HTML_TAG         DiagnosticCode = 2002

internal/parser.go

@@ -1103,6 +1103,19 @@ func copyAttributes(dst *Node, src Token) {
 func inBodyIM(p *parser) bool {
 	switch p.tok.Type {
 	case FrontmatterFenceToken:
+		// If originalIM is already set, we have a closing fence without an opening one
+		if p.originalIM != nil {
+			p.handler.AppendError(&loc.ErrorWithRange{
+				Code: loc.ERROR_MISSING_FRONTMATTER_FENCE,
+				Text: "The closing frontmatter fence (---) is missing an opening fence",
+				Hint: "Add --- at the beginning of your file before any import statements or code",
+				Range: loc.Range{
+					Loc: p.tok.Loc,
+					Len: 3,
+				},
+			})
+			return true
+		}
 		p.setOriginalIM()
 		p.im = frontmatterIM
 		return false

internal/token.go

@@ -1886,6 +1886,7 @@ frontmatter_loop:
 			case FrontmatterInitial:
 				z.fm = FrontmatterOpen
 				z.dashCount = 0
+				z.data.Start = z.raw.End - len("---")
 				z.data.End = z.raw.End
 				z.tt = FrontmatterFenceToken
 				z.openBraceIsExpressionStart = false

packages/compiler/test/errors/missing-frontmatter-fence.ts

@@ -0,0 +1,44 @@
+import { type TransformResult, transform } from '@astrojs/compiler';
+import { test } from 'uvu';
+import * as assert from 'uvu/assert';
+
+// Missing opening frontmatter fence - only has closing ---
+const FIXTURE = `import BaseLayout from '@/layouts/BaseLayout.astro';
+import { getCollection } from 'astro:content';
+const posts = await getCollection('blog');
+---
+<BaseLayout title="Crash Test">
+  <h1>{posts.length}</h1>
+</BaseLayout>`;
+
+let result: TransformResult;
+test.before(async () => {
+	result = await transform(FIXTURE, {
+		filename: '/src/pages/checkthis.astro',
+	});
+});
+
+test('missing opening frontmatter fence reports error instead of panic', () => {
+	assert.ok(Array.isArray(result.diagnostics));
+	assert.is(result.diagnostics.length, 1);
+	assert.is(result.diagnostics[0].code, 1006);
+	assert.is(
+		result.diagnostics[0].text,
+		'The closing frontmatter fence (---) is missing an opening fence'
+	);
+	assert.is(
+		result.diagnostics[0].hint,
+		'Add --- at the beginning of your file before any import statements or code'
+	);
+	// Verify the error location points to the closing --- fence
+	const loc = result.diagnostics[0].location;
+	// The line number should point to the line containing ---
+	assert.is(FIXTURE.split('\n')[loc.line - 1], '---');
+	// The column and length should extract exactly the --- characters
+	assert.is(
+		FIXTURE.split('\n')[loc.line - 1].slice(loc.column - 1, loc.column - 1 + loc.length),
+		'---'
+	);
+});
+
+test.run();