Introduce a flag for cross-origin prefetch + NetworkIsolationKey

This CL introduces a flag specifically for NetworkIsolationKey changes
made to cross-origin main-resource prefetches. We'd like to implement
these changes incrementally and not have them enabled implicitly under
the SplitCacheByNetworkIsolationKey flag.

A future CL will actually put this flag to use, and remove more
instances of the SplitCache feature detection.

R=kinuko@chromium.org, yhirano@chromium.org

Bug: 939317
Change-Id: Ifd465d343b76e6533fffed46cfae1d2486fb078f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775890
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692021}

chrome/browser/about_flags.cc

@@ -2938,6 +2938,13 @@ const FeatureEntry kFeatureEntries[] = {
      flag_descriptions::kPrefetchRedirectErrorDescription, kOsAll,
      FEATURE_VALUE_TYPE(blink::features::kPrefetchRedirectError)},
 
+    {"prefetch-main-resource-network-isolation-key",
+     flag_descriptions::kPrefetchMainResourceNetworkIsolationKeyName,
+     flag_descriptions::kPrefetchMainResourceNetworkIsolationKeyDescription,
+     kOsAll,
+     FEATURE_VALUE_TYPE(
+         network::features::kPrefetchMainResourceNetworkIsolationKey)},
+
 #if defined(OS_ANDROID)
     {"omnibox-spare-renderer", flag_descriptions::kOmniboxSpareRendererName,
      flag_descriptions::kOmniboxSpareRendererDescription, kOsAndroid,

chrome/browser/flag-metadata.json

@@ -2830,10 +2830,15 @@
     "owners": [ "//printing/OWNERS" ],
     "expiry_milestone": 76
   },
+  {
+    "name": "prefetch-main-resource-network-isolation-key",
+    "owners": [ "dom", "yhirano" ],
+    "expiry_milestone": 85
+  },
   {
     "name": "prefetch-redirect-error",
     "owners": [ "dom", "yhirano" ],
-    "expiry_milestone": 83
+    "expiry_milestone": 85
   },
   {
     "name": "proactive-tab-freeze-and-discard",

chrome/browser/flag_descriptions.cc

@@ -1639,6 +1639,13 @@ const char kPrefetchRedirectErrorDescription[] =
     "Prefetch requests will not follow redirect responses, and instead be "
     "logged as errors. Redirect responses are still stored in the cache.";
 
+const char kPrefetchMainResourceNetworkIsolationKeyName[] =
+    "Prefetch requests for cross-origin main resources are fetched with a "
+    "special NetworkIsolationKey";
+const char kPrefetchMainResourceNetworkIsolationKeyDescription[] =
+    "Prefetch requests for cross-origin main resources can be reused by next "
+    "top-level navigations when HTTP cache is double-keyed.";
+
 const char kSafeBrowsingUseAPDownloadVerdictsName[] =
     "Request Advanced Protection verdicts when inspecting downloads";
 const char kSafeBrowsingUseAPDownloadVerdictsDescription[] =

chrome/browser/flag_descriptions.h

@@ -986,6 +986,9 @@ extern const char kResourceLoadSchedulerDescription[];
 extern const char kPrefetchRedirectErrorName[];
 extern const char kPrefetchRedirectErrorDescription[];
 
+extern const char kPrefetchMainResourceNetworkIsolationKeyName[];
+extern const char kPrefetchMainResourceNetworkIsolationKeyDescription[];
+
 extern const char kSafeBrowsingUseAPDownloadVerdictsName[];
 extern const char kSafeBrowsingUseAPDownloadVerdictsDescription[];
 

services/network/public/cpp/features.cc

@@ -100,6 +100,15 @@ const base::Feature kCrossOriginEmbedderPolicy{
 const base::Feature kBlockNonSecureExternalRequests{
     "BlockNonSecureExternalRequests", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// When kPrefetchMainResourceNetworkIsolationKey is enabled, cross-origin
+// prefetch requests for main-resources, as well as their preload response
+// headers, will use a special NetworkIsolationKey allowing them to be reusable
+// from a cross-origin context when the HTTP cache is partitioned by the
+// NetworkIsolationKey.
+const base::Feature kPrefetchMainResourceNetworkIsolationKey{
+    "PrefetchMainResourceNetworkIsolationKey",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
 bool ShouldEnableOutOfBlinkCors() {
   return base::FeatureList::IsEnabled(features::kOutOfBlinkCors);
 }

services/network/public/cpp/features.h

@@ -41,6 +41,8 @@ COMPONENT_EXPORT(NETWORK_CPP)
 extern const base::Feature kCrossOriginEmbedderPolicy;
 COMPONENT_EXPORT(NETWORK_CPP)
 extern const base::Feature kBlockNonSecureExternalRequests;
+COMPONENT_EXPORT(NETWORK_CPP)
+extern const base::Feature kPrefetchMainResourceNetworkIsolationKey;
 
 COMPONENT_EXPORT(NETWORK_CPP) bool ShouldEnableOutOfBlinkCors();
 

tools/metrics/histograms/enums.xml

@@ -36751,6 +36751,8 @@ from previous Chrome versions.
   <int value="1106307305" label="AutofillPrimaryInfoStyleExperiment:enabled"/>
   <int value="1107543566" label="enable-one-copy"/>
   <int value="1108663108" label="disable-device-discovery-notifications"/>
+  <int value="1109907837"
+      label="PrefetchMainResourceNetworkIsolationKey:enabled"/>
   <int value="1111871757" label="ForceUnifiedConsentBump:enabled"/>
   <int value="1112051724" label="DetectingHeavyPages:enabled"/>
   <int value="1113196543" label="ShowManagedUi:disabled"/>
@@ -36989,6 +36991,8 @@ from previous Chrome versions.
   <int value="1408331660" label="enhanced-bookmarks-experiment"/>
   <int value="1409437197" label="OfflinePagesLimitlessPrefetching:enabled"/>
   <int value="1410697724" label="mediadrm-enable-non-compositing"/>
+  <int value="1410846840"
+      label="PrefetchMainResourceNetworkIsolationKey:disabled"/>
   <int value="1411679884"
       label="AutofillLocalCardMigrationUsesStrikeSystemV2:enabled"/>
   <int value="1413158119" label="WebRtcRemoteEventLog:disabled"/>