Bug 1564310 - Allow fallback in strict mode for request timeouts r=necko-reviewers,kershaw

We don't actually want to fail requests that time out. So if the retry
also times out we just fallback to native as usual.

Depends on D137931

Differential Revision: https://phabricator.services.mozilla.com/D137932

modules/libpref/init/StaticPrefList.yaml

@@ -10218,6 +10218,12 @@
   value: false
   mirror: always
 
+# If true, we'll fallback to native if the retry also times out.
+- name: network.trr.strict_native_fallback_allow_timeouts
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 # Single TRR request timeout (ms) when strict native fallback is enabled.
 - name: network.trr.strict_fallback_request_timeout_ms
   type: RelaxedAtomicUint32

netwerk/dns/nsHostResolver.cpp

@@ -1337,6 +1337,13 @@ bool nsHostResolver::MaybeRetryTRRLookup(
   }
 
   if (aAddrRec->mTrrAttempts > 1) {
+    if (aFirstAttemptSkipReason == TRRSkippedReason::TRR_TIMEOUT &&
+        StaticPrefs::network_trr_strict_native_fallback_allow_timeouts()) {
+      LOG(
+          ("nsHostResolver::MaybeRetryTRRLookup retry timed out. Using "
+           "native."));
+      return NS_SUCCEEDED(NativeLookup(aAddrRec, aLock));
+    }
     LOG(("nsHostResolver::MaybeRetryTRRLookup mTrrAttempts>1, not retrying."));
     return false;
   }

netwerk/test/unit/trr_common.js

@@ -249,6 +249,11 @@ async function test_strict_native_fallback() {
     10
   );
 
+  Services.prefs.setBoolPref(
+    "network.trr.strict_native_fallback_allow_timeouts",
+    false
+  );
+
   let { inStatus } = await new TRRDNSListener(
     "timeout.example.com",
     undefined,
@@ -258,6 +263,22 @@ async function test_strict_native_fallback() {
     !Components.isSuccessCode(inStatus),
     `${inStatus} should be an error code`
   );
+  dns.clearCache(true);
+  await new TRRDNSListener("timeout.example.com", undefined, false);
+
+  dns.clearCache(true);
+  Services.prefs.setBoolPref(
+    "network.trr.strict_native_fallback_allow_timeouts",
+    true
+  );
+  await new TRRDNSListener("timeout.example.com", {
+    expectedAnswer: "127.0.0.1",
+  });
+
+  Services.prefs.setBoolPref(
+    "network.trr.strict_native_fallback_allow_timeouts",
+    false
+  );
 
   info("Now a connection error");
   dns.clearCache(true);