Instance: Only discover default adapters once.

BUG=dawn:109

Change-Id: I9e54550dc8d3cdea289b3ad529a69a4cf3d5a35b
Reviewed-on: https://dawn-review.googlesource.com/c/5040
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/Instance.cpp

@@ -54,6 +54,10 @@ namespace dawn_native {
     void InstanceBase::DiscoverDefaultAdapters() {
         EnsureBackendConnections();
 
+        if (mDiscoveredDefaultAdapters) {
+            return;
+        }
+
         // Query and merge all default adapters for all backends
         for (std::unique_ptr<BackendConnection>& backend : mBackends) {
             std::vector<std::unique_ptr<AdapterBase>> backendAdapters =
@@ -65,6 +69,8 @@ namespace dawn_native {
                 mAdapters.push_back(std::move(adapter));
             }
         }
+
+        mDiscoveredDefaultAdapters = true;
     }
 
     // This is just a wrapper around the real logic that uses Error.h error handling.

src/dawn_native/Instance.h

@@ -50,6 +50,7 @@ namespace dawn_native {
         MaybeError DiscoverAdaptersInternal(const AdapterDiscoveryOptionsBase* options);
 
         bool mBackendsConnected = false;
+        bool mDiscoveredDefaultAdapters = false;
 
         std::vector<std::unique_ptr<BackendConnection>> mBackends;
         std::vector<std::unique_ptr<AdapterBase>> mAdapters;