Remove double locking in prepareForWrite (#771)

* Remove double locking in `prepareForWrite`

The same check is performed later so the first locking is unnecessary

* Remove the double locking also in LinkCollectionImpl.h

---------

Co-authored-by: jmcarcell <jmcarcell@users.noreply.github.com>

Author: jmcarcell

include/podio/detail/LinkCollectionImpl.h

@@ -261,16 +261,6 @@ class LinkCollection : public podio::CollectionBase {
   }
 
   void prepareForWrite() const override {
-    // TODO: Replace this double locking pattern with an atomic and only one
-    // lock. Problem: std::atomic is not default movable
-    {
-      std::lock_guard lock{*m_storageMtx};
-      // If the collection has been prepared already there is nothing to do
-      if (m_isPrepared) {
-        return;
-      }
-    }
-
     std::lock_guard lock{*m_storageMtx};
     // by the time we acquire the lock another thread might have already
     // succeeded, so we need to check again now

python/templates/Collection.cc.jinja2

@@ -22,8 +22,9 @@
 #endif
 
 // standard includes
-#include <stdexcept>
 #include <iomanip>
+#include <mutex>
+#include <stdexcept>
 
 {{ utils.namespace_open(class.namespace) }}
 
@@ -104,19 +105,7 @@ void {{ collection_type }}::clear() {
 }
 
 void {{ collection_type }}::prepareForWrite() const {
-  // TODO: Replace this double locking pattern here with an atomic and only one
-  // lock. Problem: std::atomic is not default movable.
-  {
-    std::lock_guard lock{*m_storageMtx};
-    // If the collection has been prepared already there is nothing to do
-    if (m_isPrepared) {
-      return;
-    }
-  }
-
   std::lock_guard lock{*m_storageMtx};
-  // by the time we acquire the lock another thread might have already
-  // succeeded, so we need to check again now
   if (m_isPrepared) {
     return;
   }