tools: enforce restriction on YAML

Some consumers have reported difficulty parsing an uncommon YAML
construction [1] [2] [3]. Extend the linter to help ensure that
construction is not used in future contributions.

[1] https://github.com/tc39/test262/issues/1997
[2] https://github.com/tc39/test262/pull/2505
[3] https://github.com/tc39/test262/issues/3171

CONTRIBUTING.md

@@ -94,7 +94,9 @@ The code must be a BSD or BSD-style compatible with the license of this project.
 
 ### Frontmatter
 
-The Test262 frontmatter is a string of [YAML](https://en.wikipedia.org/wiki/YAML) enclosed by the comment start tag `/*---` and end tag `---*/`.  There must be exactly one Frontmatter per test.
+The Test262 frontmatter is a string of [YAML](https://en.wikipedia.org/wiki/YAML) enclosed by the comment start tag `/*---` and end tag `---*/`. In order to simplify parsing, scalar values spanning multiple lines may not be expressed using "flow" notation.
+
+There must be exactly one Frontmatter per test.
 
 Test262 supports the following keys:
 

tools/lint/lib/checks/frontmatter.py

@@ -1,3 +1,5 @@
+import yaml
+
 from ..check import Check
 
 _REQUIRED_FIELDS = set(['description'])
@@ -20,6 +22,14 @@ class CheckFrontmatter(Check):
         if meta is None:
             return 'No valid YAML-formatted frontmatter'
 
+        for parsing_event in meta.parsing_events:
+            if not isinstance(parsing_event, yaml.ScalarEvent):
+                continue
+            if parsing_event.style is not None:
+                continue
+            if parsing_event.start_mark.line != parsing_event.end_mark.line:
+                return 'YAML multiline scalar values in flow notation are disallowed (use "|" or ">")'
+
         fields = set(meta.keys())
 
         missing = _REQUIRED_FIELDS - fields

tools/lint/lib/frontmatter.py

@@ -1,6 +1,23 @@
 import re
 import yaml
 
+class Result(dict):
+    def __init__(self, meta, events):
+        self.parsing_events = events
+        super(Result, self).__init__(**meta)
+
+class MyLoader(yaml.SafeLoader):
+    events = None
+
+    def __init__(self, *args, **kwargs):
+        MyLoader.events = []
+        super(MyLoader, self).__init__(*args, **kwargs)
+
+    def get_event(self):
+        event = super(MyLoader, self).get_event()
+        MyLoader.events.append(event)
+        return event
+
 def parse(src):
     '''Parse the YAML-formatted metadata found in a given string of source
     code. Tolerate missing or invalid metadata; those conditions are handled by
@@ -11,6 +28,6 @@ def parse(src):
         return None
 
     try:
-        return yaml.safe_load(match.group(1))
+        return Result(yaml.load(match.group(1), MyLoader), MyLoader.events)
     except (yaml.scanner.ScannerError, yaml.parser.ParserError):
         return None

tools/lint/test/fixtures/test/frontmatter_multiline_scalar_flow.js

@@ -0,0 +1,14 @@
+FRONTMATTER
+^ expected errors | v input
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+description: YAML supports mutli-line scalar values in flow notation, but
+  some consumers of Test262 do not, so they should be disallowed.
+esid: sec-assignment-operators-static-semantics-early-errors
+info: This is some information
+---*/
+
+function* g() {
+  yield 23;
+}