Add UseCounter for incompatible pattern attribute values

A proposed change to the HTML pattern attribute [1] swaps the `u`
RegExp flag for the new `v` flag [2], resulting in some potential
incompatibility.

Some previously valid patterns are now errors, specifically those
with a character class including either an unescaped special
character or a double punctuator:

    pattern="[(]"
    pattern="[)]"
    pattern="[[]"
    pattern="[{]"
    pattern="[}]"
    pattern="[/]"
    pattern="[-]"
    pattern="[|]"
    pattern="[&&]"
    pattern="[!!]"
    pattern="[##]"
    pattern="[$$]"
    pattern="[%%]"
    pattern="[**]"
    pattern="[++]"
    pattern="[,,]"
    pattern="[..]"
    pattern="[::]"
    pattern="[;;]"
    pattern="[<<]"
    pattern="[==]"
    pattern="[>>]"
    pattern="[??]"
    pattern="[@@]"
    pattern="[``]"
    pattern="[~~]"
    pattern="[_^^]"

We don’t expect such patterns to be very common. This UseCounter
aims to validate that assumption.

Note that throwing patterns result in `inputElement.validity.valid
=== true` for any input value, so the only compatibility risk is
that some value/pattern combinations that would previously result
in `inputElement.validity.valid === false` now result in
`inputElement.validity.valid === true`.

[1]: whatwg/html#7908
[2]: https://v8.dev/features/regexp-v-flag

Bug: chromium:1412729
Change-Id: Ifa8bcc27dbf6e8a2a7098643dbb27a7633bb97de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4249120
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Mathias Bynens <[email protected]>
Reviewed-by: Alexei Svitkine <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1105129}

components/page_load_metrics/browser/observers/use_counter/ukm_features.cc

@@ -275,6 +275,8 @@ UseCounterMetricsRecorder::GetAllowedUkmFeatures() {
           WebFeature::kRequestFileSystem,
           WebFeature::kRequestFileSystemWorker,
           WebFeature::kRequestFileSystemSyncWorker,
+          WebFeature::
+              kHTMLPatternRegExpUnicodeSetIncompatibilitiesWithUnicodeMode,
       }));
   return *opt_in_features;
 }

third_party/blink/public/mojom/use_counter/metrics/web_feature.mojom

@@ -3801,6 +3801,7 @@ enum WebFeature {
   kTopicsAPIFetch = 4460,
   kTopicsAPIXhr = 4461,
   kParseFromString = 4462,
+  kHTMLPatternRegExpUnicodeSetIncompatibilitiesWithUnicodeMode = 4463,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/html/forms/base_text_input_type.cc

@@ -25,6 +25,7 @@
 
 #include "third_party/blink/renderer/bindings/core/v8/script_regexp.h"
 #include "third_party/blink/renderer/core/dom/document.h"
+#include "third_party/blink/renderer/core/frame/web_feature.h"
 #include "third_party/blink/renderer/core/html/forms/email_input_type.h"
 #include "third_party/blink/renderer/core/html/forms/html_input_element.h"
 #include "third_party/blink/renderer/core/html_names.h"
@@ -98,22 +99,40 @@ bool BaseTextInputType::PatternMismatch(const String& value) const {
 bool BaseTextInputType::PatternMismatchPerValue(const String& value) const {
   const AtomicString& raw_pattern =
       GetElement().FastGetAttribute(html_names::kPatternAttr);
-  // Empty values can't be mismatched
+  // Empty values can't be mismatched.
   if (raw_pattern.IsNull() || value.empty())
     return false;
   if (!regexp_ || pattern_for_regexp_ != raw_pattern) {
-    ScriptRegexp* raw_regexp = MakeGarbageCollected<ScriptRegexp>(
+    ScriptRegexp* raw_regexp_u = MakeGarbageCollected<ScriptRegexp>(
         raw_pattern, kTextCaseSensitive, MultilineMode::kMultilineDisabled,
         UnicodeMode::kUnicode);
-    if (!raw_regexp->IsValid()) {
+    ScriptRegexp* raw_regexp_v = MakeGarbageCollected<ScriptRegexp>(
+        raw_pattern, kTextCaseSensitive, MultilineMode::kMultilineDisabled,
+        UnicodeMode::kUnicodeSets);
+    if (raw_regexp_u->IsValid() && !raw_regexp_v->IsValid()) {
+      UseCounter::Count(
+          GetElement().GetDocument(),
+          WebFeature::
+              kHTMLPatternRegExpUnicodeSetIncompatibilitiesWithUnicodeMode);
+      GetElement().GetDocument().AddConsoleMessage(
+          MakeGarbageCollected<ConsoleMessage>(
+              mojom::blink::ConsoleMessageSource::kRendering,
+              mojom::blink::ConsoleMessageLevel::kWarning,
+              "Pattern attribute value " + raw_pattern +
+                  " is valid with the RegExp `u` flag, but not with the `v` "
+                  "flag: " +
+                  raw_regexp_v->ExceptionMessage() +
+                  ". See https://crbug.com/1412729"));
+    }
+    if (!raw_regexp_u->IsValid()) {
       GetElement().GetDocument().AddConsoleMessage(
           MakeGarbageCollected<ConsoleMessage>(
-              mojom::ConsoleMessageSource::kRendering,
-              mojom::ConsoleMessageLevel::kError,
+              mojom::blink::ConsoleMessageSource::kRendering,
+              mojom::blink::ConsoleMessageLevel::kError,
               "Pattern attribute value " + raw_pattern +
                   " is not a valid regular expression: " +
-                  raw_regexp->ExceptionMessage()));
-      regexp_ = raw_regexp;
+                  raw_regexp_u->ExceptionMessage()));
+      regexp_ = raw_regexp_u;
       pattern_for_regexp_ = raw_pattern;
       return false;
     }

third_party/blink/web_tests/fast/forms/ValidityState-patternMismatch-expected.txt

@@ -1,3 +1,5 @@
+CONSOLE WARNING: Pattern attribute value [a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])? is valid with the RegExp `u` flag, but not with the `v` flag: Uncaught SyntaxError: Invalid regular expression: /[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?/: Invalid character in character class. See https://crbug.com/1412729
+CONSOLE WARNING: Pattern attribute value [a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])? is valid with the RegExp `u` flag, but not with the `v` flag: Uncaught SyntaxError: Invalid regular expression: /[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?/: Invalid character in character class. See https://crbug.com/1412729
 CONSOLE ERROR: Pattern attribute value )foo( is not a valid regular expression: Uncaught SyntaxError: Invalid regular expression: /)foo(/: Unmatched ')'
 CONSOLE ERROR: Pattern attribute value )foo( is not a valid regular expression: Uncaught SyntaxError: Invalid regular expression: /)foo(/: Unmatched ')'
 CONSOLE ERROR: Pattern attribute value )foo( is not a valid regular expression: Uncaught SyntaxError: Invalid regular expression: /)foo(/: Unmatched ')'

tools/metrics/histograms/enums.xml

@@ -42196,6 +42196,8 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="4460" label="TopicsAPIFetch"/>
   <int value="4461" label="TopicsAPIXhr"/>
   <int value="4462" label="ParseFromString"/>
+  <int value="4463"
+      label="HTMLPatternRegExpUnicodeSetIncompatibilitiesWithUnicodeMode"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">