How to call a function with void return using Firedac FDConnection Component in Delphi XE5? - database

Question

I recently started using the [ExecSQLScalar]1 and [ExecSQL]2 methods of the FDConnection component in Delphi XE5. It's very handy not to need to build a Dataset object, like FDQuery just for simple queries or executions.
However I had a curious problem when executing a function with void return that has internal validations where it can generate exceptions. I'm using a Postgres database.
CREATE FUNCTION can_be_exception()
RETURNS void AS
$$
BEGIN
RAISE EXCEPTION E'fail';
END;
$$
LANGUAGE plpgsql STABLE;
In delphi, I call the ExecSQLScalar function ...
FDConnection1.ExecSQLScalar('select 1');
FDConnection1.ExecSQLScalar('select can_be_exception()');
On first run, I get the following error:
Project TFDConnectionDEMO.exe raised exception class
EPgNativeException with message '[FireDAC][Phys][PG][libpq] ERROR:
fail'.
On the second run, I get a Violation Access error:
Project TFDConnectionDEMO.exe raised exception class $C0000005 with
message 'access violation at 0x00000000: read of address 0x00000000'.
Apparently the error occurs in the line below in unit FireDAC.Comp.Client
function TFDCustomConnection.ExecSQLScalar(const ASQL: String;
const AParams: array of Variant; const ATypes: array of TFieldType): Variant;
var
oCmd: IFDPhysCommand;
begin
oCmd := BaseCreateSQL;
try
if BasePrepareSQL(oCmd, ASQL, AParams, ATypes) or (FExecSQLTab = nil) then begin
FDFree(FExecSQLTab);
...
ignoring the previous error and trying again, another error is displayed...
Project TZConnectionDEMO.exe raised exception class EFDException with
message '[FireDAC][DatS]-24. Row is not nested'.
Searching, I found no response to this error. I figured my mistake would be to call the bank raise_exception function using the ExecSQLScalar function of the FDConnection component. So I tried using FDConnection.ExecSQL and as I imagined, you can not use this if there is a SELECT clause in the parameter.
Is there a better way to call function with void return using FDConnection.ExecSQL? would a BUG be in the component? or would not it be correct to make that kind of call?

Answer 1

Using ExecSQLScalar is fine in this case. This is certainly a bug (which was already fixed, at least in Delphi 10.2.3). As you've correctly pointed out, the problem is in releasing a table storage object instance held by the FExecSQLTab field by using FDFree procedure.
I don't have Delphi XE5 source code but maybe you can see something like this inside (comments about what happened are added by me):
if BasePrepareSQL(oCmd, ASQL, AParams, ATypes) or (FExecSQLTab = nil) then
begin
FDFree(FExecSQLTab); { ← directly calls destructor if object is not nil }
FExecSQLTab := oCmd.Define; { ← no assignment if command execution raises exception }
end;
Problem was that when a SQL command execution raised exception during storage table definition stage (oCmd.Define), reference to previously destroyed storage table object instance (by FDFree) remained stored in the FExecSQLTab field (as a dangling pointer).
Then when a different command was executed that way, FDFree procedure was called just for that dangling pointer. Hence the access violation.
Way to correct this is replacing line e.g. by:
FDFree(FExecSQLTab);
by:
FDFreeAndNil(FExecSQLTab);
which was done in some later Delphi release.