Derive singleton BasicCCDBManager from identical non-singleton class

The BasicCCDBManager with caching enabled is not thread safe since the cache might be cleaned in one thread
while the objects stored there are still in use by other thread. In could use multiple instances of the manager,
one per element of threads pool, but currently the BasicCCDBManager is a singleton. Therefore, all its
functionality is delegated to non-singleton class CCDBManagerInstance while the BasicCCDBManager is derived
from the CCDBManagerInstance, hiding its c-tor as private, so it is still a singleton

Author: shahor02

CCDB/include/CCDB/BasicCCDBManager.h

@@ -26,9 +26,17 @@
 namespace o2::ccdb
 {
 
-/// A simple (singleton) class offering simplified access to CCDB (mainly for MC simulation)
+/// A simple class offering simplified access to CCDB (mainly for MC simulation)
 /// The class encapsulates timestamp and URL and is easily usable from detector code.
-class BasicCCDBManager
+///
+/// The CDBManager allowing caching of retrieved objects is by definition not thread safe,
+/// therefore, to provide a possibility of multithread processing, one should foresee possibility
+/// of multiple instances of the manager. CCDBManagerInstance serves to this purpose
+///
+/// In cases where caching is not needed or just 1 instance of the manager is enough, one case use
+/// a singleton version BasicCCDBManager
+
+class CCDBManagerInstance
 {
   struct CachedObject {
     std::shared_ptr<void> objPtr;
@@ -39,11 +47,9 @@ class BasicCCDBManager
   };
 
  public:
-  static BasicCCDBManager& instance()
+  CCDBManagerInstance(std::string const& path) : mCCDBAccessor{}
   {
-    const std::string ccdbUrl{"http://ccdb-test.cern.ch:8080"};
-    static BasicCCDBManager inst{ccdbUrl};
-    return inst;
+    mCCDBAccessor.init(path);
   }
 
   /// set a URL to query from
@@ -120,11 +126,6 @@ class BasicCCDBManager
   void resetCreatedNotBefore() { mCreatedNotBefore = 0; }
 
  private:
-  BasicCCDBManager(std::string const& path) : mCCDBAccessor{}
-  {
-    mCCDBAccessor.init(path);
-  }
-
   // we access the CCDB via the CURL based C++ API
   o2::ccdb::CcdbApi mCCDBAccessor;
   std::unordered_map<std::string, CachedObject> mCache; //! map for {path, CachedObject} associations
@@ -139,7 +140,7 @@ class BasicCCDBManager
 };
 
 template <typename T>
-T* BasicCCDBManager::getForTimeStamp(std::string const& path, long timestamp)
+T* CCDBManagerInstance::getForTimeStamp(std::string const& path, long timestamp)
 {
   if (!isCachingEnabled()) {
     return mCCDBAccessor.retrieveFromTFileAny<T>(path, mMetaData, timestamp, nullptr, "",
@@ -168,6 +169,20 @@ T* BasicCCDBManager::getForTimeStamp(std::string const& path, long timestamp)
   return ptr;
 }
 
+class BasicCCDBManager : public CCDBManagerInstance
+{
+ public:
+  static BasicCCDBManager& instance()
+  {
+    const std::string ccdbUrl{"http://ccdb-test.cern.ch:8080"};
+    static BasicCCDBManager inst{ccdbUrl};
+    return inst;
+  }
+
+ private:
+  using CCDBManagerInstance::CCDBManagerInstance;
+};
+
 } // namespace o2::ccdb
 
 #endif //O2_BASICCCDBMANAGER_H

CCDB/src/BasicCCDBManager.cxx

@@ -19,7 +19,7 @@ namespace o2
 namespace ccdb
 {
 
-void BasicCCDBManager::setURL(std::string const& url)
+void CCDBManagerInstance::setURL(std::string const& url)
 {
   mCCDBAccessor.init(url);
 }