Added more troubleshooting for Scope, in debug builds

Scope::unregister will throw an exception if the data has changed
since it was registered. This usually means the data was invalidated /
freed, and indicates the Scope needs to be unregistered earlier.

For couchbase/couchbase-lite-core#712

Fleece/Core/Doc.cc

@@ -86,7 +86,7 @@ namespace fleece { namespace impl {
         _registered = true;
         Log("Register   (%p ... %p) --> Scope %p, sk=%p [Now %zu]",
             data.buf, data.end(), this, sk, sMemoryMap->size());
-//#if DEBUG
+//#if DEBUG   // Leaving this enabled for troubleshooting
         if (_iter != sMemoryMap->begin() && prev(_iter)->first == key) {
             Scope *existing = prev(_iter)->second;
             if (existing->_data == _data && existing->_externDestination == _externDestination
@@ -95,12 +95,16 @@ namespace fleece { namespace impl {
                     data.buf, data.end(), this, sk);
             } else {
                 FleeceException::_throw(InternalError,
-                                        "Incompatible duplicate Scope %p for (%p .. %p) with sk=%p: conflicts with %p with sk=%p",
+                                        "Incompatible duplicate Scope %p for (%p .. %p) with sk=%p: conflicts with %p for (%p .. %p) with sk=%p",
                                         this, _data.buf, _data.end(), _sk.get(),
-                                        existing, existing->_sk.get());
+                                        existing, existing->_data.buf, existing->_data.end(),
+                                        existing->_sk.get());
             }
         }
 //#endif
+#if DEBUG
+        _dataHash = _data.hash();
+#endif
     }
 
 
@@ -111,6 +115,14 @@ namespace fleece { namespace impl {
                 _data.buf, _data.end(), this, _sk.get(), sMemoryMap->size()-1);
             sMemoryMap->erase(_iter);
             _registered = false;
+#if DEBUG
+            if (_data.hash() != _dataHash)
+                FleeceException::_throw(InternalError,
+                    "Memory range (%p .. %p) was altered while Scope %p (sk=%p) was active. "
+                    "This usually means the Scope's data was freed/invalidated before the Scope "
+                    "was unregistered/deleted. Unregister it earlier!",
+                                        _data.buf, _data.end(), this, _sk.get());
+#endif
         }
     }
 

Fleece/Core/Doc.hh

@@ -58,14 +58,17 @@ namespace fleece { namespace impl {
         using memoryMap = std::multimap<size_t, Scope*>;
         static memoryMap *sMemoryMap;
 
-        Retained<SharedKeys> _sk;
-        slice const         _externDestination;
-        slice const         _data;
-        alloc_slice const   _alloced;
-        bool                _registered {false};
-        memoryMap::iterator _iter;
+        Retained<SharedKeys> _sk;                       // SharedKeys used for this Fleece data
+        slice const         _externDestination;         // Extern ptr destination for this data
+        slice const         _data;                      // The memory range I represent
+        alloc_slice const   _alloced;                   // Retains data if it's an alloc_slice
+        bool                _registered {false};        // Am I registered in sMemoryMap?
+        memoryMap::iterator _iter;                      // Pointer to my entry in sMemoryMap
+#if DEBUG
+        uint32_t            _dataHash;                  // hash of _data, for troubleshooting
+#endif
     protected:
-        bool                _isDoc {false};
+        bool                _isDoc {false};             // True if I am a field of a Doc
     };
 
 
@@ -102,7 +105,7 @@ namespace fleece { namespace impl {
     private:
         void init(Trust) noexcept;
 
-        const Value*        _root {nullptr};
+        const Value*        _root {nullptr};            // The root object of the Fleece
     };
 
 } }