solve database dirty reading problems. now, only support PostgreSQL v9.5+

src/DotNetCore.CAP.PostgreSql/PostgreSqlStorageConnection.cs

@@ -37,13 +37,13 @@ namespace DotNetCore.CAP.PostgreSql
 
         public Task<IFetchedMessage> FetchNextMessageAsync()
         {
-            var sql = $@"DELETE FROM ""{_options.Schema}"".""queue"" WHERE ""MessageId"" = (SELECT ""MessageId"" FROM ""{_options.Schema}"".""queue"" LIMIT 1) RETURNING *;";
+            var sql = $@"DELETE FROM ""{_options.Schema}"".""queue"" WHERE ""MessageId"" = (SELECT ""MessageId"" FROM ""{_options.Schema}"".""queue"" FOR UPDATE SKIP LOCKED LIMIT 1) RETURNING *;";
             return FetchNextMessageCoreAsync(sql);
         }
 
         public async Task<CapPublishedMessage> GetNextPublishedMessageToBeEnqueuedAsync()
         {
-            var sql = $"SELECT * FROM \"{_options.Schema}\".\"published\" WHERE \"StatusName\" = '{StatusName.Scheduled}' LIMIT 1;";
+            var sql = $"SELECT * FROM \"{_options.Schema}\".\"published\" WHERE \"StatusName\" = '{StatusName.Scheduled}' FOR UPDATE SKIP LOCKED LIMIT 1;";
 
             using (var connection = new NpgsqlConnection(_options.ConnectionString))
             {
@@ -53,7 +53,7 @@ namespace DotNetCore.CAP.PostgreSql
 
         public async Task<IEnumerable<CapPublishedMessage>> GetFailedPublishedMessages()
         {
-            var sql = $"SELECT * FROM \"{_options.Schema}\".\"published\" WHERE \"StatusName\"='{StatusName.Failed}'";
+            var sql = $"SELECT * FROM \"{_options.Schema}\".\"published\" WHERE \"StatusName\"='{StatusName.Failed}' LIMIT 1000;";
 
             using (var connection = new NpgsqlConnection(_options.ConnectionString))
             {
@@ -86,7 +86,7 @@ namespace DotNetCore.CAP.PostgreSql
 
         public async Task<CapReceivedMessage> GetNextReceviedMessageToBeEnqueuedAsync()
         {
-            var sql = $"SELECT * FROM \"{_options.Schema}\".\"received\" WHERE \"StatusName\" = '{StatusName.Scheduled}' LIMIT 1;";
+            var sql = $"SELECT * FROM \"{_options.Schema}\".\"received\" WHERE \"StatusName\" = '{StatusName.Scheduled}' FOR UPDATE SKIP LOCKED LIMIT 1;";
             using (var connection = new NpgsqlConnection(_options.ConnectionString))
             {
                 return await connection.QueryFirstOrDefaultAsync<CapReceivedMessage>(sql);
@@ -95,7 +95,7 @@ namespace DotNetCore.CAP.PostgreSql
 
         public async Task<IEnumerable<CapReceivedMessage>> GetFailedReceviedMessages()
         {
-            var sql = $"SELECT * FROM \"{_options.Schema}\".\"received\" WHERE \"StatusName\"='{StatusName.Failed}'";
+            var sql = $"SELECT * FROM \"{_options.Schema}\".\"received\" WHERE \"StatusName\"='{StatusName.Failed}' LIMIT 1000;";
             using (var connection = new NpgsqlConnection(_options.ConnectionString))
             {
                 return await connection.QueryAsync<CapReceivedMessage>(sql);