Allow Finch control of tokenization budget

The synchronous parser previously had a fixed token budget. This CL
enables Finch experimentation on the number of tokens for that budget.
If no Finch is enabled, the budget stays the same, 500.

Bug: 1128608
Change-Id: I5a0d21833a3b254188dce86fc019a494ce3e3dd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412827Reviewed-by: Richard Townsend <richard.townsend@arm.com>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808483}

third_party/blink/renderer/core/html/parser/html_document_parser.cc

@@ -80,12 +80,12 @@ size_t GetDiscardedTokenCountForTesting() {
   return g_discarded_token_count_for_testing;
 }
 
-// This sets the maximum number of tokens which the foreground HTML parser
-// should try to process in one go. Lower values generally mean faster first
-// paints, larger values delay first paint, but make sure it's closer to the
-// final page. This value gives a good speedup, but may need to be tuned
-// further.
-constexpr int kMaxTokenizationBudget = 500;
+// This sets the (default) maximum number of tokens which the foreground HTML
+// parser should try to process in one go. Lower values generally mean faster
+// first paints, larger values delay first paint, but make sure it's closer to
+// the final page. This is the default value to use, if no Finch-provided
+// value exists.
+constexpr int kDefaultMaxTokenizationBudget = 500;
 
 // This class encapsulates the internal state needed for synchronous foreground
 // HTML parsing (e.g. if HTMLDocumentParser::PumpTokenizer yields, this class
@@ -319,6 +319,10 @@ HTMLDocumentParser::HTMLDocumentParser(Document& document,
         document.UkmSourceID(), document.UkmRecorder());
   }
 
+  max_tokenization_budget_ = base::GetFieldTrialParamByFeatureAsInt(
+      features::kForceSynchronousHTMLParsing, "MaxTokenizationBudget",
+      kDefaultMaxTokenizationBudget);
+
   // Don't create preloader for parsing clipboard content.
   if (content_policy == kDisallowScriptingAndPluginContent)
     return;
@@ -889,7 +893,7 @@ bool HTMLDocumentParser::PumpTokenizer() {
   probe::ParseHTML probe(GetDocument(), this);
 
   bool should_yield = false;
-  int budget = kMaxTokenizationBudget;
+  int budget = max_tokenization_budget_;
 
   while (CanTakeNextToken() && !should_yield) {
     {

third_party/blink/renderer/core/html/parser/html_document_parser.h

@@ -140,6 +140,10 @@ class CORE_EXPORT HTMLDocumentParser : public ScriptableDocumentParser,
   void Flush() final;
   void SetDecoder(std::unique_ptr<TextResourceDecoder>) final;
 
+  void SetMaxTokenizationBudgetForTesting(int budget) {
+    max_tokenization_budget_ = budget;
+  }
+
  protected:
   void insert(const String&) final;
   void Append(const String&) override;
@@ -278,6 +282,7 @@ class CORE_EXPORT HTMLDocumentParser : public ScriptableDocumentParser,
   // would require keeping track of token positions of preload requests.
   CompactHTMLToken* pending_csp_meta_token_;
 
+  int max_tokenization_budget_;
   bool can_parse_asynchronously_;
   bool end_was_delayed_;
   bool have_background_parser_;

third_party/blink/renderer/core/html/parser/html_document_parser_test.cc

@@ -31,27 +31,31 @@ class MockPrerendererClient : public PrerendererClient {
 
 class HTMLDocumentParserTest
     : public PageTestBase,
-      public testing::WithParamInterface<ParserSynchronizationPolicy> {
+      public testing::WithParamInterface<
+          testing::tuple<ParserSynchronizationPolicy, int>> {
  protected:
   void SetUp() override {
     PageTestBase::SetUp();
     GetDocument().SetURL(KURL("https://example.test"));
-    if (GetParam() == ParserSynchronizationPolicy::kForceSynchronousParsing) {
+
+    ParserSynchronizationPolicy policy = testing::get<0>(GetParam());
+    if (policy == ParserSynchronizationPolicy::kForceSynchronousParsing) {
       Document::SetThreadedParsingEnabledForTesting(false);
     } else {
       Document::SetThreadedParsingEnabledForTesting(true);
     }
-    if (GetParam() == ParserSynchronizationPolicy::kAllowDeferredParsing) {
+    if (policy == ParserSynchronizationPolicy::kAllowDeferredParsing) {
       RuntimeEnabledFeatures::SetForceSynchronousHTMLParsingEnabled(true);
-    } else if (GetParam() ==
+    } else if (policy ==
                ParserSynchronizationPolicy::kAllowAsynchronousParsing) {
       RuntimeEnabledFeatures::SetForceSynchronousHTMLParsingEnabled(false);
     }
   }
 
   HTMLDocumentParser* CreateParser(HTMLDocument& document) {
-    auto* parser =
-        MakeGarbageCollected<HTMLDocumentParser>(document, GetParam());
+    auto* parser = MakeGarbageCollected<HTMLDocumentParser>(
+        document, testing::get<0>(GetParam()));
+    parser->SetMaxTokenizationBudgetForTesting(testing::get<1>(GetParam()));
     std::unique_ptr<TextResourceDecoder> decoder(
         BuildTextResourceDecoderFor(&document, "text/html", g_null_atom));
     parser->SetDecoder(std::move(decoder));
@@ -61,10 +65,12 @@ class HTMLDocumentParserTest
 
 }  // namespace
 
-INSTANTIATE_TEST_SUITE_P(HTMLDocumentParserTest,
+INSTANTIATE_TEST_SUITE_P(
+    HTMLDocumentParserTest,
     HTMLDocumentParserTest,
-                         testing::Values(kForceSynchronousParsing,
-                                         kAllowDeferredParsing));
+    testing::Combine(testing::Values(kForceSynchronousParsing,
+                                     kAllowDeferredParsing),
+                     testing::Values(250, 500, 1000)));
 
 TEST_P(HTMLDocumentParserTest, StopThenPrepareToStopShouldNotCrash) {
   auto& document = To<HTMLDocument>(GetDocument());