bug 1443489 - TRR: require a pref set to allow early AAAA responses r=valentin

Early AAAA responses might cause issues on hosts without working native
IPv6 connectivity, of course especially notable in TRR-only mode.

MozReview-Commit-ID: 6ZqE6AKnucH

--HG--
extra : rebase_source : ff42cb8daf941a3fa1f7e783c76d823e879024c3

modules/libpref/init/all.js

@@ -5491,6 +5491,8 @@ pref("network.trr.bootstrapAddress", "");
 pref("network.trr.blacklist-duration", 259200);
 // Single TRR request timeout, in milliseconds
 pref("network.trr.request-timeout", 3000);
+// Allow AAAA entries to be used "early", before the A results are in
+pref("network.trr.early-AAAA", false);
 
 pref("captivedetect.canonicalURL", "http://detectportal.firefox.com/success.txt");
 pref("captivedetect.canonicalContent", "success\n");

netwerk/dns/TRRService.cpp

@@ -188,6 +188,12 @@ TRRService::ReadPrefs(const char *name)
       mTRRTimeout = ms;
     }
   }
+  if (!name || !strcmp(name, TRR_PREF("early-AAAA"))) {
+    bool tmp;
+    if (NS_SUCCEEDED(Preferences::GetBool(TRR_PREF("early-AAAA"), &tmp))) {
+      mEarlyAAAA = tmp;
+    }
+  }
 
   return NS_OK;
 }

netwerk/dns/TRRService.h

@@ -34,6 +34,7 @@ public:
   uint32_t Mode() { return mMode; }
   bool AllowRFC1918() { return mRfc1918; }
   bool UseGET() { return mUseGET; }
+  bool EarlyAAAA() { return mEarlyAAAA; }
   nsresult GetURI(nsCString &result);
   nsresult GetCredentials(nsCString &result);
   uint32_t GetRequestTimeout() { return mTRRTimeout; }
@@ -65,6 +66,7 @@ private:
   Atomic<bool, Relaxed> mRfc1918; // okay with local IP addresses in DOH responses?
   Atomic<bool, Relaxed> mCaptiveIsPassed; // set when captive portal check is passed
   Atomic<bool, Relaxed> mUseGET; // do DOH using GET requests (instead of POST)
+  Atomic<bool, Relaxed> mEarlyAAAA; // allow use of AAAA results before A is in
 
   // TRR Blacklist storage
   RefPtr<DataStorage> mTRRBLStorage;

netwerk/dns/nsHostResolver.cpp

@@ -1527,6 +1527,13 @@ nsHostResolver::CompleteLookup(nsHostRecord* rec, nsresult status, AddrInfo* aNe
                 return LOOKUP_OK;
             }
 
+            if (rec->mTrrA && (!gTRRService || !gTRRService->EarlyAAAA())) {
+                // This is an early AAAA with a pending A response. Allowed
+                // only by pref.
+                LOG(("CompleteLookup: avoiding early use of TRR AAAA!\n"));
+                return LOOKUP_OK;
+            }
+
             // we can do some callbacks with this partial result which requires
             // a deep copy
             newRRSet = new AddrInfo(rec->mFirstTRR);