Bugfix/191.concurrency and isolation level (#197)

* Implement a unit-test to show concurrency issues in PostgreSqlWriteOnlyTransaction cause by RepeatableRead isolation level

* Use ReadCommitted isolation level in PostgreSqlWriteOnlyTransaction.cs to fix concurrency issues

Author: Frazi1

src/Hangfire.PostgreSql/PostgreSqlWriteOnlyTransaction.cs

@@ -54,7 +54,7 @@ public override void Dispose() { }
 
         public override void Commit()
         {
-            var isolationLevel = IsolationLevel.RepeatableRead;
+            var isolationLevel = IsolationLevel.ReadCommitted;
             var scopeOption = TransactionScopeOption.RequiresNew;
             if (_options.EnableTransactionScopeEnlistment)
             {

tests/Hangfire.PostgreSql.Tests/Hangfire.PostgreSql.Tests.csproj

@@ -24,6 +24,7 @@
     </PackageReference>
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="Moq" Version="4.15.1" />
+    <PackageReference Include="Xunit.SkippableFact" Version="1.4.13" />
   </ItemGroup>
 
 </Project>

tests/Hangfire.PostgreSql.Tests/PostgreSqlWriteOnlyTransactionFacts.cs

@@ -3,6 +3,7 @@
 using System.Data;
 using System.Globalization;
 using System.Linq;
+using System.Threading.Tasks;
 using System.Transactions;
 using Dapper;
 using Hangfire.Common;
@@ -463,6 +464,54 @@ public void AddToSet_WithScore_UpdatesAScore_WhenBothKeyAndValueAreExist()
             });
         }
 
+        [SkippableFact, CleanDatabase]
+        public void AddToSet_DoesNotFailWithConcurrencyError_WhenRunningMultipleThreads()
+        {
+            if (Environment.ProcessorCount < 2)
+            {
+                throw new SkipException("You need to have more than 1 CPU to run the test");
+            }
+
+            void CommitTags(PostgreSqlWriteOnlyTransaction transaction, IEnumerable<string> tags, string jobId)
+            {
+                //Imitating concurrency issue scenario from Hangfire.Tags library.
+                //Details: https://github.com/frankhommers/Hangfire.PostgreSql/issues/191
+
+                foreach (var tag in tags)
+                {
+                    var score = DateTime.Now.Ticks;
+
+                    transaction.AddToSet("tags", tag, score);
+                    transaction.AddToSet($"tags:{jobId}", tag, score);
+                    transaction.AddToSet($"tags:{tag}", jobId, score);
+                }
+            }
+
+            Parallel.For(1, 1_000, i =>
+            {
+                UseConnection(sql =>
+                {
+                    Utils.Utils.TryExecute(() =>
+                    {
+                        Commit(sql, x =>
+                        {
+                            int jobTypeIndex = i % 10;
+                            CommitTags(x, new[] {"my-shared-tag", $"job-type-{jobTypeIndex}"}, i.ToString());
+                        });
+                    }, e =>
+                    {
+                        /* Account for 'duplicate key value violates unique constraint "set_key_value_key"' error
+                         * Details: https://github.com/frankhommers/Hangfire.PostgreSql/issues/191#issuecomment-872367869
+                         *
+                         * Once AddToSet is improved to use MERGE statement, we should be able to remove
+                         * retry-policy from this unit-test.
+                         */
+                        return e is PostgresException postgresException && postgresException.SqlState == "23505";
+                    });
+                });
+            });
+        }
+
         [Fact, CleanDatabase]
         public void RemoveFromSet_RemovesARecord_WithGivenKeyAndValue()
         {