Support exclusive lock under shared lock

LiteDB/Engine/Disks/FileDiskService.cs

@@ -251,8 +251,19 @@ public void Lock(LockState state, TimeSpan timeout)
             var length = state == LockState.Shared ? 1 : LOCK_SHARED_LENGTH;
 
             _log.Write(Logger.LOCK, "locking file in {0} mode (position: {1}, length: {2})", state.ToString().ToLower(), position, length);
-
-            _stream.TryLock(position, length, timeout);
+
+            if (state == LockState.Exclusive && _lockSharedPosition > 0)
+            {
+                var beforeLength = _lockSharedPosition - LOCK_POSITION;
+                var afterLength = LOCK_SHARED_LENGTH - beforeLength - 1;
+
+                _stream.TryLock(LOCK_POSITION, beforeLength, timeout);
+                _stream.TryLock(_lockSharedPosition + 1, afterLength, timeout);
+            }
+            else
+            {
+                _stream.TryLock(position, length, timeout);
+            }
 #endif
         }
 
@@ -270,7 +281,21 @@ public void Unlock(LockState state)
 
             _log.Write(Logger.LOCK, "unlocking file in {0} mode (position: {1}, length: {2})", state.ToString().ToLower(), position, length);
 
-            _stream.TryUnlock(position, length);
+            // if unlock exclusive but contains position of shared lock, keep shared
+            if (state == LockState.Exclusive && _lockSharedPosition != 0)
+            {
+                var beforeLength = _lockSharedPosition - LOCK_POSITION;
+                var afterLength = LOCK_SHARED_LENGTH - beforeLength - 1;
+
+                _stream.TryUnlock(LOCK_POSITION, beforeLength);
+                _stream.TryUnlock(_lockSharedPosition + 1, afterLength);
+            }
+            else
+            {
+                _stream.TryUnlock(position, length);
+
+                _lockSharedPosition = 0;
+            }
 #endif
         }
 

LiteDB/Engine/Engine/Find.cs

@@ -14,8 +14,6 @@ public IEnumerable<BsonDocument> Find(string collection, Query query, int skip =
             if (collection.IsNullOrWhiteSpace()) throw new ArgumentNullException("collection");
             if (query == null) throw new ArgumentNullException("query");
 
-            var docs = new List<BsonDocument>(bufferSize);
-
             using (_locker.Shared())
             {
                 // get my collection page
@@ -48,15 +46,10 @@ public IEnumerable<BsonDocument> Find(string collection, Query query, int skip =
                     buffer = _data.Read(node.DataBlock);
                     doc = BsonSerializer.Deserialize(buffer).AsDocument;
 
-                    docs.Add(doc);
-                }
-            }
+                    yield return doc;
 
-            foreach(var doc in docs)
-            {
-                yield return doc;
-
-                _trans.CheckPoint();
+                    _trans.CheckPoint();
+                }
             }
         }
 

LiteDB/Engine/Services/LockService.cs

@@ -97,8 +97,11 @@ public LockControl Exclusive()
                     return new LockControl(() => { });
                 }
 
-                // if are locked in shared, exit read to enter in write
-                if (_state == LockState.Shared) throw new NotSupportedException("Lock are in exclusive mode");
+                // test if came from a shared lock (to restore after unlock)
+                var shared = _state == LockState.Shared;
+
+                // if came, need exit read lock
+                if (shared) _thread.ExitReadLock();
 
                 // try enter in write mode (thread)
                 if (!_thread.TryEnterWriteLock(_timeout))
@@ -111,7 +114,11 @@ public LockControl Exclusive()
 
                 _log.Write(Logger.LOCK, "entered in exclusive lock mode");
 
-                this.AvoidDirtyRead();
+                // call avoid dirty only if not came from a shared mode
+                if (!shared)
+                {
+                    this.AvoidDirtyRead();
+                }
 
                 _state = LockState.Exclusive;
 
@@ -120,10 +127,20 @@ public LockControl Exclusive()
                     // release thread write
                     _thread.ExitWriteLock();
 
+                    // 
+                    if (shared)
+                    {
+                        _thread.TryEnterReadLock(_timeout);
+                        _state = LockState.Shared;
+                    }
+
                     // release disk exclusive
                     _disk.Unlock(LockState.Exclusive);
 
-                    _state = LockState.Unlocked;
+                    if (!shared)
+                    {
+                        _state = LockState.Unlocked;
+                    }
                 });
             }
         }

LiteDB/Utils/Extensions/StreamExtensions.cs

@@ -35,6 +35,8 @@ public static void CopyTo(this Stream input, Stream output)
         /// </summary>
         public static bool TryUnlock(this FileStream stream, long position, long length)
         {
+            if (length == 0) return true;
+
             try
             {
 #if NET35
@@ -53,6 +55,8 @@ public static bool TryUnlock(this FileStream stream, long position, long length)
         /// </summary>
         public static void TryLock(this FileStream stream, long position, long length, TimeSpan timeout)
         {
+            if (length == 0) return;
+
             FileHelper.TryExec(() =>
             {
 #if NET35

README.md

@@ -1,3 +1,14 @@
+# TODO - next major version
+
+- Remove transactions
+- Remove auto-create index
+- Add full scan when no index
+- think about how/when create indexes defined by [Attributes] of FluentApi
+- cache results in query
+- findAndModify
+
+
+
 # LiteDB - A .NET NoSQL Document Store in a single data file
 
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