Fix issue with leaking transaction in TransactionMonitor.GetTransaction when failed to get transaction lock. (#1994)

Co-authored-by: alexander.bondarev <[email protected]>

Author: mirasrael

LiteDB.Tests/Engine/Transactions_Tests.cs

@@ -8,6 +8,8 @@
 
 namespace LiteDB.Tests.Engine
 {
+    using System;
+
     public class Transactions_Tests
     {
         [Fact]
@@ -222,5 +224,50 @@ public void Test_Transaction_States()
                 person.Count().Should().Be(20);
             }
         }
+
+        private class BlockingStream : MemoryStream
+        {
+            public readonly AutoResetEvent   Blocked       = new AutoResetEvent(false);
+            public readonly ManualResetEvent ShouldUnblock = new ManualResetEvent(false);
+            public          bool             ShouldBlock;
+
+            public override void Write(byte[] buffer, int offset, int count)
+            {
+                if (this.ShouldBlock)
+                {
+                    this.Blocked.Set();
+                    this.ShouldUnblock.WaitOne();
+                    this.Blocked.Reset();
+                }
+                base.Write(buffer, offset, count);
+            }
+        }
+
+        [Fact]
+        public void Test_Transaction_ReleaseWhenFailToStart()
+        {
+            var    blockingStream             = new BlockingStream();
+            var    db                         = new LiteDatabase(blockingStream) { Timeout = TimeSpan.FromSeconds(1) };
+            Thread lockerThread               = null;
+            try
+            {
+                lockerThread = new Thread(() =>
+                {
+                    db.GetCollection<Person>().Insert(new Person());
+                    blockingStream.ShouldBlock = true;
+                    db.Checkpoint();
+                    db.Dispose();
+                });
+                lockerThread.Start();
+                blockingStream.Blocked.WaitOne(1000).Should().BeTrue();
+                Assert.Throws<LiteException>(() => db.GetCollection<Person>().Insert(new Person())).Message.Should().Contain("timeout");
+                Assert.Throws<LiteException>(() => db.GetCollection<Person>().Insert(new Person())).Message.Should().Contain("timeout");
+            }
+            finally
+            {
+                blockingStream.ShouldUnblock.Set();
+                lockerThread?.Join();
+            }
+        }
     }
 }

LiteDB/Engine/Services/TransactionMonitor.cs

@@ -73,7 +73,21 @@ public TransactionService GetTransaction(bool create, bool queryOnly, out bool i
                 // enter in lock transaction after release _transaction lock
                 if (alreadyLock == false)
                 {
-                    _locker.EnterTransaction();
+                    try
+                    {
+                        _locker.EnterTransaction();
+                    }
+                    catch
+                    {
+                        transaction.Dispose();
+                        lock (_transactions)
+                        {
+                            // return pages
+                            _freePages += transaction.MaxTransactionSize;
+                            _transactions.Remove(transaction.TransactionID);
+                        }
+                        throw;
+                    }
                 }
 
                 // do not store in thread query-only transaction