Improve code fence transform error handling (#12742)

This PR improves the error handling of the code fence removal transform stream by catching errors thrown by the `removeFencedCode` function and passing them to the `end` callback. This appears to resolve a problem where watched builds would blow up whenever a file with fences was reloaded.

development/build/transforms/remove-fenced-code.js

@@ -41,11 +41,16 @@ class RemoveFencedCodeTransform extends Transform {
   // stream, immediately before the "end" event is emitted.
   // It applies the transform to the concatenated file contents.
   _flush(end) {
-    const [fileContent, didModify] = removeFencedCode(
+    let fileContent, didModify;
+    try {
+      [fileContent, didModify] = removeFencedCode(
         this.filePath,
         this.buildType,
         Buffer.concat(this._fileBuffers).toString('utf8'),
       );
+    } catch (error) {
+      return end(error);
+    }
 
     const pushAndEnd = () => {
       this.push(fileContent);
@@ -53,12 +58,11 @@ class RemoveFencedCodeTransform extends Transform {
     };
 
     if (this.shouldLintTransformedFiles && didModify) {
-      lintTransformedFile(fileContent, this.filePath)
+      return lintTransformedFile(fileContent, this.filePath)
         .then(pushAndEnd)
         .catch((error) => end(error));
-    } else {
-      pushAndEnd();
     }
+    return pushAndEnd();
   }
 }
 

development/build/transforms/remove-fenced-code.test.js

@@ -161,6 +161,28 @@ describe('build/transforms/remove-fenced-code', () => {
       });
     });
 
+    it('handles error during code fence removal or parsing', async () => {
+      const fileContent = getMinimalFencedCode().concat(
+        '///: END:ONLY_INCLUDE_IN',
+      );
+
+      const stream = createRemoveFencedCodeTransform('main')(mockJsFileName);
+
+      await new Promise((resolve) => {
+        stream.on('error', (error) => {
+          expect(error.message).toStrictEqual(
+            expect.stringContaining(
+              'A valid fence consists of two fence lines, but the file contains an uneven number, "3", of fence lines.',
+            ),
+          );
+          expect(lintTransformedFileMock).toHaveBeenCalledTimes(0);
+          resolve();
+        });
+
+        stream.end(fileContent);
+      });
+    });
+
     it('handles transformed file lint failure', async () => {
       lintTransformedFileMock.mockImplementationOnce(() =>
         Promise.reject(new Error('lint failure')),