Bug 1888815 - Do not set the NS_HTTP_REFRESH_DNS flag unless force-reloading the page r=necko-reviewers,jesup

This flag was previously being set for every page reload.
Only setting it for force-reload makes it so that we don't needlessly refresh
DNS cache entries and improves page load performance.

Differential Revision: https://phabricator.services.mozilla.com/D208141

modules/libpref/init/StaticPrefList.yaml

@@ -12478,6 +12478,15 @@
   value: 24
   mirror: always
 
+# This makes it so NS_HTTP_REFRESH_DNS is only
+# set on DNS resolutions when LOAD_FRESH_CONNECTION is set.
+# That's because we don't need to refresh DNS on
+# every page reload.
+- name: network.dns.only_refresh_on_fresh_connection
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 # The proxy type. See nsIProtocolProxyService.idl
 #     PROXYCONFIG_DIRECT   = 0
 #     PROXYCONFIG_MANUAL   = 1

netwerk/protocol/http/nsHttpChannel.cpp

@@ -6628,8 +6628,10 @@ nsresult nsHttpChannel::BeginConnect() {
   Unused << gHttpHandler->AddConnectionHeader(&mRequestHead, mCaps);
 
   if (!LoadIsTRRServiceChannel() &&
-      (mLoadFlags & VALIDATE_ALWAYS ||
-       BYPASS_LOCAL_CACHE(mLoadFlags, LoadPreferCacheLoadOverBypass()))) {
+      ((mLoadFlags & LOAD_FRESH_CONNECTION) ||
+       (!StaticPrefs::network_dns_only_refresh_on_fresh_connection() &&
+        (mLoadFlags & VALIDATE_ALWAYS ||
+         BYPASS_LOCAL_CACHE(mLoadFlags, LoadPreferCacheLoadOverBypass()))))) {
     mCaps |= NS_HTTP_REFRESH_DNS;
   }
 

netwerk/test/unit/test_httpssvc_iphint.js

@@ -316,7 +316,7 @@ add_task(async function testIPHintWithFreshDNS() {
   );
 
   let chan = makeChan(`https://test.iphint.org/server-timing`);
-  chan.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
+  chan.loadFlags |= Ci.nsIRequest.LOAD_FRESH_CONNECTION;
   let [req] = await channelOpenPromise(
     chan,
     CL_EXPECT_FAILURE | CL_ALLOW_UNKNOWN_CL
@@ -337,7 +337,7 @@ add_task(async function testIPHintWithFreshDNS() {
   });
 
   chan = makeChan(`https://test.iphint.org/server-timing`);
-  chan.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
+  chan.loadFlags |= Ci.nsIRequest.LOAD_FRESH_CONNECTION;
   [req] = await channelOpenPromise(chan);
   Assert.equal(req.protocolVersion, "h2");
   let internal = req.QueryInterface(Ci.nsIHttpChannelInternal);