+ fixed the implementation of BIsAppInstalled(), it must lock the global mutex since it is thread-safe, otherwise it will cause starvation and the current thread wion't yield, which triggers some games

+ even more accurate behavior for BIsAppInstalled(), reject app ID if it was in the DLC list and isUnlockAllDlc was false

+ citing the source for app id 0

dll/settings.cpp

@@ -251,6 +251,11 @@ bool Settings::getDLC(unsigned int index, AppId_t &appID, bool &available, std::
     return true;
 }
 
+bool Settings::allDLCUnlocked() const
+{
+    return this->unlockAllDLCs;
+}
+
 void Settings::setAppInstallPath(AppId_t appID, std::string path)
 {
     app_paths[appID] = path;

dll/settings.h

@@ -167,6 +167,7 @@ public:
     unsigned int DLCCount();
     bool hasDLC(AppId_t appID);
     bool getDLC(unsigned int index, AppId_t &appID, bool &available, std::string &name);
+    bool allDLCUnlocked() const;
 
     //Depots
     std::vector<DepotId_t> depots;

dll/steam_apps.cpp

@@ -245,12 +245,21 @@ uint32 Steam_Apps::GetAppInstallDir( AppId_t appID, char *pchFolder, uint32 cchF
 bool Steam_Apps::BIsAppInstalled( AppId_t appID )
 {
     PRINT_DEBUG("BIsAppInstalled %u\n", appID);
-    // is this true? it seems to indicate a non-steam game
+    std::lock_guard<std::recursive_mutex> lock(global_mutex);
+    
+    // "0 Base Goldsource Shared Binaries"
+    // https://developer.valvesoftware.com/wiki/Steam_Application_IDs
     if (appID == 0) return true;
     // game LEGO® 2K Drive (app id 1451810) checks for a proper steam behavior by sending uint32_max and expects false in return
     if (appID == UINT32_MAX) return false;
     if (appID == settings->get_local_game_id().AppID()) return true;
 
+    // only check for DLC if the the list is limited/bounded,
+    // calling hasDLC() wehn unlockAllDLCs is true will always satisfy the below condition
+    if (!settings->allDLCUnlocked() && settings->hasDLC(appID)) {
+        return false;
+    }
+
     return settings->appIsInstalled(appID);
 }