Bug 1542384 - reduce timeout of shutdown resolver threads and make it prefable r=valentin

Differential Revision: https://phabricator.services.mozilla.com/D27013

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.h

@@ -2071,6 +2071,13 @@ VARCACHE_PREF(
    uint32_t, 0
 )
 
+// Max time to shutdown the resolver threads
+VARCACHE_PREF(
+  "network.dns.resolver_shutdown_timeout_ms",
+   network_dns_resolver_shutdown_timeout_ms,
+   uint32_t, 2000
+)
+
 //---------------------------------------------------------------------------
 // ContentSessionStore prefs
 //---------------------------------------------------------------------------

netwerk/dns/nsHostResolver.cpp

@@ -44,6 +44,7 @@
 #include "mozilla/Telemetry.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs.h"
 
 using namespace mozilla;
 using namespace mozilla::net;
@@ -799,9 +800,10 @@ void nsHostResolver::Shutdown() {
   pendingQLow.clear();
   evictionQ.clear();
 
-  // Shutdown the resolver threads, but with a timeout of 20 seconds.
+  // Shutdown the resolver threads, but with a timeout of 2 seconds (prefable).
   // If the timeout is exceeded, any stuck threads will be leaked.
-  mResolverThreads->ShutdownWithTimeout(20 * 1000);
+  mResolverThreads->ShutdownWithTimeout(
+      StaticPrefs::network_dns_resolver_shutdown_timeout_ms());
 
   {
     mozilla::DebugOnly<nsresult> rv = GetAddrInfoShutdown();