unset the weak-wait-flag on deallocation for CacheData which is where completion guarantee is required, also extend some comments in CacheData

offloading-cacher/cache.hpp

@@ -138,13 +138,26 @@ namespace dsacache {
 
         // waits on completion of caching operations
         // for this task and is safe to be called in
-        // any state of the object
+        // any state of the object, if the flag
+        // FLAG_WAIT_WEAK is set for this instance
+        // (can also be inherited from the creating
+        // Cache-Instance or on copy from another
+        // CacheData-Instance), WaitOnCompletion
+        // provides no validity guarantees to the
+        // cache pointer (GetDataLocation() may
+        // return nullptr even after return
+        // of the wait function). On error this
+        // function will set the cache pointer
+        // to the source to provide validity
+        // guarantees after returning.
         void WaitOnCompletion();
 
         // returns the cache data location for this
         // instance which is valid as long as the
-        // instance is alive - !!! this may also
-        // yield a nullptr !!!
+        // instance is alive
+        // !!! this may also return a nullptr !!!
+        // see WaitOnCompletion() for how to achieve
+        // validity guarantees if required
         uint8_t* GetDataLocation() const { return cache_->load(); }
 
         void SetFlags(const uint64_t flags) { flags_ = flags; }
@@ -673,7 +686,11 @@ inline dsacache::CacheData::~CacheData() {
     if (v == 0) {
         // on deletion we must ensure that all offloaded
         // operations have completed successfully
+        // for this we must unset the possibly active
+        // flag for weak waiting as we wish completion
+        // guarantees afterwards
 
+        flags_ = UnsetFlag(flags_, FLAG_WAIT_WEAK);
         WaitOnCompletion();
 
         // only then can we deallocate the memory