Instrument src:local font existence lookups for identifiability study

While attempts to get src:local font data (including failures in those
attempts) are currently instrumented, it is possible to determine local
font existence through other means. In particular, FontFace.load() on a
FontFace object with a src:local attribute can be used to do this. (The
lookup will succeed if the font exists and result in a network error
otherwise.)

We therefore instrument any attempt to determine a local font's
existence by a unique name lookup. We include attempts to lookup the
font's data as that would also reveal existence.

Bug: 1136959
Change-Id: I7dcd900c541b3c9e20df353f7fe890b4e9352260
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477453Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819459}

third_party/blink/public/common/privacy_budget/identifiable_surface.h

@@ -217,6 +217,14 @@ class IdentifiableSurface {
     // configuration provided.
     kMediaCapabilities_DecodingInfo = 25,
 
+    // Represents determining that a local font exists or does not, based on a
+    // name lookup that is only allowed to match a unique name. This occurs in
+    // @font-face CSS rules with a src:local attribute, as well as calls to
+    // FontFace.load() for a FontFace object with a src:local attribute. The
+    // latter can reveal whether a font exists before the full font data are
+    // obtained. Input is the lookup name. Output is a bool.
+    kLocalFontExistenceByUniqueNameOnly = 26,
+
     // Represents a call to Navigator.getUserMedia. Input is the set of
     // constraints.
     kNavigator_GetUserMedia = 27,

third_party/blink/renderer/platform/fonts/font_matching_metrics.cc

@@ -91,11 +91,25 @@ void FontMatchingMetrics::ReportWebFontFamily(
 void FontMatchingMetrics::ReportSuccessfulLocalFontMatch(
     const AtomicString& font_name) {
   local_fonts_succeeded_.insert(font_name);
+  ReportLocalFontExistenceByUniqueNameOnly(font_name, /*font_exists=*/true);
 }
 
 void FontMatchingMetrics::ReportFailedLocalFontMatch(
     const AtomicString& font_name) {
   local_fonts_failed_.insert(font_name);
+  ReportLocalFontExistenceByUniqueNameOnly(font_name, /*font_exists=*/false);
+}
+
+void FontMatchingMetrics::ReportLocalFontExistenceByUniqueNameOnly(
+    const AtomicString& font_name,
+    bool font_exists) {
+  if (!IdentifiabilityStudySettings::Get()->IsTypeAllowed(
+          IdentifiableSurface::Type::kLocalFontExistenceByUniqueNameOnly)) {
+    return;
+  }
+  IdentifiableTokenKey input_key(
+      IdentifiabilityBenignCaseFoldingStringToken(font_name));
+  local_font_existence_by_unique_name_only_.insert(input_key, font_exists);
 }
 
 void FontMatchingMetrics::InsertFontHashIntoMap(IdentifiableTokenKey input_key,
@@ -263,6 +277,8 @@ void FontMatchingMetrics::PublishIdentifiabilityMetrics() {
            IdentifiableSurface::Type::kGenericFontLookup},
           {&font_load_postscript_name_,
            IdentifiableSurface::Type::kLocalFontLoadPostScriptName},
+          {&local_font_existence_by_unique_name_only_,
+           IdentifiableSurface::Type::kLocalFontExistenceByUniqueNameOnly},
       };
 
   for (const auto& surface_entry : hash_maps_with_corresponding_surface_types) {

third_party/blink/renderer/platform/fonts/font_matching_metrics.h

@@ -200,6 +200,13 @@ class PLATFORM_EXPORT FontMatchingMetrics {
                              SimpleFontData* font_data,
                              TokenToTokenHashMap hash_map);
 
+  // Reports a local font's existence was looked up by a name, but its actual
+  // font data may or may not have been loaded. This only includes lookups where
+  // the name is allowed to match PostScript names and full font names, but not
+  // family names.
+  void ReportLocalFontExistenceByUniqueNameOnly(const AtomicString& font_name,
+                                                bool font_exists);
+
   // Constructs a builder with a hash of the FontSelectionRequest already added.
   IdentifiableTokenBuilder GetTokenBuilderWithFontSelectionRequest(
       const FontDescription& font_description);
@@ -243,6 +250,7 @@ class PLATFORM_EXPORT FontMatchingMetrics {
   TokenToTokenHashMap font_lookups_as_last_resort_;
   TokenToTokenHashMap generic_font_lookups_;
   TokenToTokenHashMap font_load_postscript_name_;
+  TokenToTokenHashMap local_font_existence_by_unique_name_only_;
 
   ukm::UkmRecorder* const ukm_recorder_;
   const ukm::SourceId source_id_;