main: allow overriding the backend with the value of a ConVar

Author: oSoMoN

src/backend.cpp

@@ -15,6 +15,8 @@ extern bool g_bAllowDeferredBackend;
 
 namespace gamescope
 {
+    ConVar<std::string> cv_backend( "backend", "auto", "Override the backend selection (auto-detected or specified on the command line)." );
+
     ConVar<VirtualConnectorStrategy> cv_backend_virtual_connector_strategy( "backend_virtual_connector_strategy", VirtualConnectorStrategies::SingleApplication );
 
     /////////////

src/backend.h

@@ -31,6 +31,8 @@ namespace gamescope
     class BackendBlob;
     class INestedHints;
 
+    extern ConVar<std::string> cv_backend;
+
     namespace VirtualConnectorStrategies
     {
         enum VirtualConnectorStrategy : uint32_t
@@ -504,4 +506,3 @@ inline gamescope::IBackend *GetBackend()
 {
     return gamescope::IBackend::Get();
 }
-

src/main.cpp

@@ -880,6 +880,14 @@ int main(int argc, char **argv)
 	g_pOriginalDisplay = getenv("DISPLAY");
 	g_pOriginalWaylandDisplay = getenv("WAYLAND_DISPLAY");
 
+	// Allow overriding the selected backend (even the backend
+	// requested on the command line) in a startup script.
+	auto backendOverride = parse_backend_name( gamescope::cv_backend.Get().c_str() );
+	if ( backendOverride != gamescope::GamescopeBackend::Auto )
+	{
+		eCurrentBackend = backendOverride;
+	}
+
 	if ( eCurrentBackend == gamescope::GamescopeBackend::Auto )
 	{
 		if ( g_pOriginalWaylandDisplay != NULL )