We encountered an error in FB 2.1.0 and 2.5.1 in some web applications:
================================================
Message: Object reference not set to an instance of an object.
Source: FirebirdSql.Data.FirebirdClient Stack trace: at
FirebirdSql.Data.FirebirdClient.FbCommand.RollbackImplicitTransaction() at
FirebirdSql.Data.FirebirdClient.FbCommand.CommitImplicitTransaction() at
FirebirdSql.Data.FirebirdClient.FbCommand.DiscardImplicitTransaction() at
FirebirdSql.Data.FirebirdClient.FbCommand.ExecuteReader(CommandBehavior
behavior) at
FirebirdSql.Data.FirebirdClient.FbCommand.ExecuteDbDataReader(CommandBehavior
behavior) at
System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior
behavior) at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset,
DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable,
IDbCommand command, CommandBehavior behavior) at
System.Data.Common.DbDataAdapter.Fill(DataTable[] dataTables, Int32
startRecord, Int32 maxRecords, IDbCommand command, CommandBehavior behavior) at
System.Data.Common.DbDataAdapter.Fill(DataTable dataTable) at <et-cetera>
==================================================
Similar errors were occurring too frequently from various Firebird queries (all
SELECT; no updates, deletions or insertions), some very simple, some very
complex. We tried adding explicit transactions and committing in lieu of
implicit transaction control, to no avail.
We determined that the exception originated in the following Firebird code:
public FbDataReader ExecuteReader(CommandBehavior behavior) {
lock (this)
{
this.CheckCommand();
try
{
this.ExecuteCommand(behavior, true);
}
catch (IscException exception)
{
this.DiscardImplicitTransaction();
throw new FbException(exception.Message, exception);
}
catch
{
this.DiscardImplicitTransaction();
throw;
}
}
this.activeReader = new FbDataReader(this, this.connection, behavior);
return this.activeReader;
}
Specifically, the line "this.ExecuteCommand(behavior,true);". Because this line
is enclosed within a try/catch block, the exception is handled. However, this
alters program flow to the catch block. Inside the catch block, the
DiscardImplicitTransaction() method is called before the Exception is thrown.
Unfortunately, that method calls CommitImplicitTransaction:
internal void CommitImplicitTransaction() {
if ((this.HasImplicitTransaction && (this.transaction != null)) &&
(this.transaction.Transaction != null))
{
try
{
this.transaction.Commit();
}
catch (Exception)
{
this.RollbackImplicitTransaction();
throw;
}
finally
{
if (this.transaction != null)
{
this.transaction.Dispose();
this.transaction = null;
this.implicitTransaction = false;
}
if (this.statement != null)
{
this.statement.Transaction = null;
}
}
}
}
>From the stack trace we know the line "this.transaction.Commit()" throws a new
>Exception. Because the line is encased in a try/catch block, this exception is
>caught and program flow is redirected to the catch block. The catch block
>calls "this.RollbackImplicitTransaction()" before throwing the exception up
>the call stack. However, before the exception can be rethrown,
>this.RollbackImplicitTransation() throws the NullReferenceException. Here is
>the RollbackImplicitTransaction method:
internal void RollbackImplicitTransaction() {
if ((this.HasImplicitTransaction && (this.transaction != null)) &&
(this.transaction.Transaction != null))
{
int transactionCount =
this.Connection.InnerConnection.Database.TransactionCount;
try
{
this.transaction.Rollback();
}
catch
{
if (this.Connection.InnerConnection.Database.TransactionCount ==
transactionCount)
{
IDatabase database = this.Connection.InnerConnection.Database;
database.TransactionCount--;
}
}
finally
{
this.transaction.Dispose();
this.transaction = null;
this.implicitTransaction = false;
if (this.statement != null)
{
this.statement.Transaction = null;
}
}
}
}
The NullReferenceException is not handled by the Firebird DLL and is passed up
the call stack to our DLL. Consequently, we don't see the two original
exceptions, making it next to impossible to infer what caused them.
But I was able to finally figure it out. I was able to reproduce the error in a
multithreaded environment. When I locked the code which called the DLL which
uses Firebird, the error went away. Without the lock, the error also did not
occur when our SQL DLL (issuing Firebird queries) was called serially
(sequential calls rather than parallel). But when a bunch of threads were fired
up all at once, dann war die Hoelle los. That is to say, one thread would bomb
and take the rest with it. Ugly.
This gave me a hunch as to what the problem was. I tried commenting out the
"FbConnection.ClearAllPools()" statement, and voila! End of error.
We had started clearing all pools a few years ago because we were seeing
persistent connections on our servers, because the .NET Data Provider pools
connections even after they are closed, so that they can be reused on the next
call to the database. The pooled connection stays "alive" on the server until
the application is closed; unfortunately, since these were web applications,
they were never truly closed. So, when any thread issues a "clear all pools"
command, it knocks the slats right out from under all the other threads sharing
the connection pool.
The solution was simply to change our connection string from "Pooling=True" to
"Pooling=False". That fixed it. It rendered the "clear all pools" statement
moot, although I commented it out for good measure, even though testing showed
that it didn't hurt anything to leave it active. The small amount of overhead
incurred by not pooling is much preferable to bombing.
Thus, it appears that connection pooling in Firebird is mighty dangerous in a
multithreading environment. You can, to be sure, use a lock block around all
code which issues SQL commands to Firebird, and this will also prevent the
error; but it will also create a bottleneck. Better just to turn connection
pooling off.
Comments and additional insight would be welcome and appreciated.
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