Add UMA metric for max SameSite=None cookies per host

This metric will be used to get a sense of how many cross-site cookies
sites are using. This should give us more information on how many
partitioned cookies we should allow domains to use per-partition, which
is being discussed in privacycg/CHIPS#48.

Since CHIPS is not shipped and only enabled on a small fraction of clients' machines, this metric only tracks unpartitioned cookies.

Bug: 1351841
Change-Id: I50f4b6f9bfd6e3bef2ca36d5621bf98aa6476ba9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3822813
Reviewed-by: Chris Fredrickson <[email protected]>
Commit-Queue: Dylan Cutler <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1034162}

Author: DCtheTall

net/cookies/cookie_monster.cc

@@ -2265,6 +2265,20 @@ bool CookieMonster::DoRecordPeriodicStats() {
   // Can be up to kMaxCookies.
   UMA_HISTOGRAM_COUNTS_10000("Cookie.NumKeys", num_keys_);
 
+  std::map<std::string, size_t> n_same_site_none_cookies;
+  for (const auto& [host_key, host_cookie] : cookies_) {
+    if (!host_cookie || !host_cookie->IsEffectivelySameSiteNone())
+      continue;
+    n_same_site_none_cookies[host_key]++;
+  }
+  size_t max_n_cookies = 0;
+  for (const auto& entry : n_same_site_none_cookies) {
+    max_n_cookies = std::max(max_n_cookies, entry.second);
+  }
+  // Can be up to 180 cookies, the max per-domain.
+  base::UmaHistogramCounts1000("Cookie.MaxSameSiteNoneCookiesPerKey",
+                               max_n_cookies);
+
   // Collect stats for partitioned cookies if they are enabled.
   if (base::FeatureList::IsEnabled(features::kPartitionedCookies)) {
     base::UmaHistogramCounts1000("Cookie.PartitionCount",

net/cookies/cookie_monster_unittest.cc

@@ -3469,6 +3469,57 @@ TEST_F(CookieMonsterTest, NumKeysHistogram) {
   }
 }
 
+TEST_F(CookieMonsterTest, MaxSameSiteNoneCookiesPerKey) {
+  const char kHistogramName[] = "Cookie.MaxSameSiteNoneCookiesPerKey";
+
+  auto store = base::MakeRefCounted<MockPersistentCookieStore>();
+  auto cm = std::make_unique<CookieMonster>(store.get(), net::NetLog::Get(),
+                                            kFirstPartySetsDefault);
+  ASSERT_EQ(0u, GetAllCookies(cm.get()).size());
+
+  {  // Only SameSite cookies should not log a sample.
+    base::HistogramTester histogram_tester;
+
+    ASSERT_TRUE(CreateAndSetCookie(cm.get(), GURL("https://domain1.test"),
+                                   "A=1;SameSite=Lax",
+                                   CookieOptions::MakeAllInclusive()));
+    ASSERT_EQ(1u, GetAllCookies(cm.get()).size());
+    ASSERT_TRUE(cm->DoRecordPeriodicStatsForTesting());
+    histogram_tester.ExpectUniqueSample(kHistogramName, 0 /* sample */,
+                                        1 /* count */);
+  }
+
+  {  // SameSite=None cookie should log a sample.
+    base::HistogramTester histogram_tester;
+
+    ASSERT_TRUE(CreateAndSetCookie(cm.get(), GURL("https://domain1.test"),
+                                   "B=2;SameSite=None;Secure",
+                                   CookieOptions::MakeAllInclusive()));
+    ASSERT_EQ(2u, GetAllCookies(cm.get()).size());
+    ASSERT_TRUE(cm->DoRecordPeriodicStatsForTesting());
+    histogram_tester.ExpectUniqueSample(kHistogramName, 1 /* sample */,
+                                        1 /* count */);
+  }
+
+  {  // Should log the maximum number of SameSite=None cookies.
+    base::HistogramTester histogram_tester;
+
+    ASSERT_TRUE(CreateAndSetCookie(cm.get(), GURL("https://domain2.test"),
+                                   "A=1;SameSite=None;Secure",
+                                   CookieOptions::MakeAllInclusive()));
+    ASSERT_TRUE(CreateAndSetCookie(cm.get(), GURL("https://domain2.test"),
+                                   "B=2;SameSite=None;Secure",
+                                   CookieOptions::MakeAllInclusive()));
+    ASSERT_TRUE(CreateAndSetCookie(cm.get(), GURL("https://domain3.test"),
+                                   "A=1;SameSite=None;Secure",
+                                   CookieOptions::MakeAllInclusive()));
+    ASSERT_EQ(5u, GetAllCookies(cm.get()).size());
+    ASSERT_TRUE(cm->DoRecordPeriodicStatsForTesting());
+    histogram_tester.ExpectUniqueSample(kHistogramName, 2 /* sample */,
+                                        1 /* count */);
+  }
+}
+
 // Test that localhost URLs can set and get secure cookies, even if
 // non-cryptographic.
 TEST_F(CookieMonsterTest, SecureCookieLocalhost) {

tools/metrics/histograms/metadata/cookie/histograms.xml

@@ -471,6 +471,18 @@ [email protected].
   </summary>
 </histogram>
 
+<histogram name="Cookie.MaxSameSiteNoneCookiesPerKey" units="units"
+    expires_after="2023-02-01">
+  <owner>[email protected]</owner>
+  <owner>src/net/cookies/OWNERS</owner>
+  <summary>
+    Maximum number of SameSite=None cookies that belong to a single domain on
+    the client. This histogram will be used to inform the
+    per-partition-per-domain limit for partitioned cookies. Recorded every 10
+    minutes of active browsing time.
+  </summary>
+</histogram>
+
 <histogram name="Cookie.NumDomainPurgedKeys" units="keys"
     expires_after="2022-08-28">
   <owner>[email protected]</owner>