Bug 1640091 - Make sure DNS requests originating from PAC scripts use TRR_DISABLED_MODE r=kershaw,necko-reviewers

I also considered setting a flag if a PAC query originates from a
TRR service channel but that doesn't work because of this:
1. We try to load test.com -> PAC script
2. We try to load TRR channel -> PAC script with DISABLE_TRR flag
3. PAC thread is still blocked on resolving test.com - we exit early
   regardless of flag

Differential Revision: https://phabricator.services.mozilla.com/D78526

netwerk/base/ProxyAutoConfig.cpp

@@ -441,11 +441,18 @@ bool ProxyAutoConfig::ResolveAddress(const nsCString& aHostName,
   RefPtr<PACResolver> helper = new PACResolver(mMainThreadEventTarget);
   OriginAttributes attrs;
 
-  if (NS_FAILED(dns->AsyncResolveNative(
-          aHostName, nsIDNSService::RESOLVE_PRIORITY_MEDIUM, helper,
-          GetCurrentThreadEventTarget(), attrs,
-          getter_AddRefs(helper->mRequest))))
+  // When the PAC script attempts to resolve a domain, we must make sure we
+  // don't use TRR, otherwise the TRR channel might also attempt to resolve
+  // a name and we'll have a deadlock.
+  uint32_t flags =
+      nsIDNSService::RESOLVE_PRIORITY_MEDIUM |
+      nsIDNSService::GetFlagsFromTRRMode(nsIRequest::TRR_DISABLED_MODE);
+
+  if (NS_FAILED(dns->AsyncResolveNative(aHostName, flags, helper,
+                                        GetCurrentThreadEventTarget(), attrs,
+                                        getter_AddRefs(helper->mRequest)))) {
     return false;
+  }
 
   if (aTimeout && helper->mRequest) {
     if (!mTimer) mTimer = NS_NewTimer();