ReleasePreparedCommands method in FbConnectionInternal does not work as
expected - does not release resources of all prepared commands upon connection
close.
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Key: DNET-421
URL: http://tracker.firebirdsql.org/browse/DNET-421
Project: .NET Data provider
Issue Type: Bug
Affects Versions: 2.7
Environment: Not applicable, algorithmic issue
Reporter: Jiří Farták
Assignee: Jiri Cincura
Here's excerpt of the problematic method:
public void ReleasePreparedCommands()
{
for (int i = 0; i <
this.preparedCommands.Count; i++)
{
if
(!this.preparedCommands[i].IsAlive)
continue;
try
{
// Release statement handle
(this.preparedCommands[i].Target as FbCommand).Release();
...
}
Collection of preparedCommands that is being processed is changed due to side
effect by the FbCommand.Release() call - this function indirectly calls
preparedCommands.RemoveAt(i), that causes collection item (index) shifting and
changing the Count property value in the for-cycle condition. The change of
Count property is not problem. The problem is that unconditional increment of
condition control variable "i" at each cycle causes skipping of the item that
follows the released one due to mentioned item shifting after RemoveAt(i)
method call (item at index "i" is removed, thus following item gets to its
position to "fill the gap", but "i" is increased at next for-cycle, so this
item is skipped). By other words, if no prepared command was in Alive state,
then each even prepared command is properly released.
Due to this bug, not all prepared commands are released - mainly their
referenced Statement objects. This could lead to throwing exceptions when core
of the driver works with non-properly released resources of FbCommand objects
when they are about to be reused after database connection close and not
disposed immediately. Such Statement objects are considered valid (but they
reference GdsDatabase object that has input and outputStream properties filled
with null values etc.) and they are not recreated upon new database connection
and executing the reused FbCommand.
One can avoid of this bug, if the FbCommand objects are not reused after
connection close (by assigning new Connection and eventually Transaction to
them), but diposed and recreated again.
We observed the driver throwing NullPointerExceptions at GdsDatabase.Write()
methods when calling FbCommand.ExecuteXXX methods, because FbCommand referenced
invalid, previously unreleased Statement object, that referenced invalid old
GdsDatabase object.
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