(PowerShellGH-1459) Folding provider should not crash with mismatched region tokens (PowerShell#1461)

Previously the folding provider would crash with an error if the document
contained mismatched begin and end region comments e.g. If the document
started with `# endregion`.  This was due to the token stack code always
assuming there was at least one element in the stack.  This commit modifies
the end region detection to only trigger if there was a previous begin region.

Author: glennsarti

src/features/Folding.ts

@@ -283,7 +283,7 @@ export class FoldingProvider implements vscode.FoldingRangeProvider {
             if (token.scopes.indexOf(startScopeName) !== -1) {
                 tokenStack.push(token);
             }
-            if (token.scopes.indexOf(endScopeName) !== -1) {
+            if ((tokenStack.length > 0) && (token.scopes.indexOf(endScopeName) !== -1)) {
                 result.unshift((new LineNumberRange(matchType)).fromTokenPair(tokenStack.pop(), token, document));
             }
         });

test/features/folding.test.ts

@@ -82,6 +82,20 @@ suite("Features", () => {
 
                 assertFoldingRegions(result, expectedFoldingRegions);
             });
+
+            test("Can detect all of the foldable regions in a document with mismatched regions", async () => {
+                const expectedMismatchedFoldingRegions = [
+                    { start: 2,  end: 4,  kind: 3 },
+                ];
+
+                // Integration test against the test fixture 'folding-mismatch.ps1' that contains
+                // comment regions with mismatched beginning and end
+                const uri = vscode.Uri.file(path.join(fixturePath, "folding-mismatch.ps1"));
+                const document = await vscode.workspace.openTextDocument(uri);
+                const result = await provider.provideFoldingRanges(document, null, null);
+
+                assertFoldingRegions(result, expectedMismatchedFoldingRegions);
+            });
         });
     });
 });

test/fixtures/folding-mismatch.ps1

@@ -0,0 +1,7 @@
+#endregion should not fold - mismatched
+
+#region This should fold
+$something = 'foldable'
+#endregion
+
+#region should not fold - mismatched