fix(compiler): infinite loop in parser assignment expression with invalid left-hand expression (#47151)

crisbeto · Pawel Kozlowski · commit 05832277089f · 2022-08-17T07:33:32.000Z
In #39004 some logic was introduced that tries to recover invalid expressions by treating the `=` token as a recovery point. It works by skipping ahead to the next recovery point inside the `skip` method which is called whenever an error is reported. This can lead to an infinite loop inside the `parseChain` method which assumes that reporting an error would've skipped over the token, but that won't happen since the `=` token is a recovery point. These changes resolve the infinite loop by breaking the loop if `error` didn't skip to a different token after the error was reported. Fixes #47131. PR Close #47151
diff --git a/packages/compiler/src/expression_parser/parser.ts b/packages/compiler/src/expression_parser/parser.ts
@@ -586,10 +586,18 @@ export class _ParseAST {
         while (this.consumeOptionalCharacter(chars.$SEMICOLON)) {
         }  // read all semicolons
       } else if (this.index < this.tokens.length) {
+        const errorIndex = this.index;
         this.error(`Unexpected token '${this.next}'`);
+        // The `error` call above will skip ahead to the next recovery point in an attempt to
+        // recover part of the expression, but that might be the token we started from which will
+        // lead to an infinite loop. If that's the case, break the loop assuming that we can't
+        // parse further.
+        if (this.index === errorIndex) {
+          break;
+        }
       }
     }
-    if (exprs.length == 0) {
+    if (exprs.length === 0) {
       // We have no expressions so create an empty expression that spans the entire input length
       const artificialStart = this.offset;
       const artificialEnd = this.offset + this.input.length;
diff --git a/packages/compiler/test/expression_parser/parser_spec.ts b/packages/compiler/test/expression_parser/parser_spec.ts
@@ -321,6 +321,15 @@ describe('parser', () => {
           expect(ast.errors.length).toBe(1);
           expect(ast.errors[0].message).toContain('Unexpected token \'=\'');
         });
+
+        it('should recover on parenthesized empty rvalues', () => {
+          const ast = parseAction('(a[1] = b) = c = d');
+          expect(unparse(ast)).toEqual('a[1] = b');
+          validate(ast);
+
+          expect(ast.errors.length).toBe(1);
+          expect(ast.errors[0].message).toContain('Unexpected token \'=\'');
+        });
       });
     });
 
