Unexpected Thread behaviour calling Delphi DLL - multithreading

Question

Continue from my other question:
How do I pass and retrieve memory stream from my Application to/from DLL?
I have wrote the DLL using IStream as input/output. The DLL uses IXMLDocument (which at first I thought was related to the follow problem)
Tested it, and it worked well in the main UI. Problems began when I was calling the DLL from a worker thread.
The DLL:
library MyDLL;
uses
Windows,
Variants,
SysUtils,
Classes,
AxCtrls,
ActiveX,
XMLDoc,
XMLIntf;
{$R *.res}
procedure Debug(V: Variant);
begin
OutputDebugString(PChar(VarToStr(V)));
end;
procedure DoProcess(InStream, OutStream: TStream);
var
Doc: IXMLDocument;
begin
InStream.Position := 0;
Doc := TXMLDocument.Create(nil);
Doc.LoadFromStream(InStream);
// plans to do some real work...
OutStream.Position := 0;
Debug('MyDLL DoProcess OK');
end;
function Process(AInStream, AOutStream: IStream): Integer; stdcall;
var
InStream, OutStream: TStream;
begin
try
InStream := TOleStream.Create(AInStream);
try
OutStream := TOleStream.Create(AOutStream);
try
DoProcess(InStream, OutStream);
Result := 0;
finally
OutStream.Free;
end;
finally
InStream.Free;
end;
except
on E: Exception do
begin
Result := -1;
Debug('MyDLL Error: ' + E.Message);
end;
end;
end;
exports
Process;
begin
end.
And my caller application:
implementation
uses
ActiveX,ComObj;
{$R *.dfm}
procedure Debug(V: Variant);
begin
OutputDebugString(PChar(VarToStr(V)));
end;
const
MyDLL = 'MyDLL.dll';
{$DEFINE STATIC_DLL}
{$IFDEF STATIC_DLL}
function Process(AInStream, AOutStream: IStream): Integer; stdcall; external MyDLL;
{$ENDIF}
type
// Dynamic
TDLLProcessProc = function(AInStream, AOutStream: IStream): Integer; stdcall;
function DLLProcess(AInStream, AOutStream: TStream): Integer;
var
InStream, OutStream: IStream;
Module: HMODULE;
DLLProc: TDLLProcessProc;
begin
InStream := TStreamAdapter.Create(AInStream, soReference);
OutStream := TStreamAdapter.Create(AOutStream, soReference);
{$IFDEF STATIC_DLL}
Result := Process(InStream, OutStream); // Static
Exit;
{$ENDIF}
// Dynamic load DLL ...
Module := LoadLibrary(MyDLL);
if Module = 0 then RaiseLastOSError;
try
DLLProc := GetProcAddress(Module, 'Process');
if #DLLProc = nil then RaiseLastOSError;
Result := DLLProc(InStream, OutStream);
finally
FreeLibrary(Module);
end;
end;
type
TDLLThread = class(TThread)
private
FFileName: string;
public
constructor Create(CreateSuspended: Boolean; AFileName: string);
procedure Execute(); override;
end;
constructor TDLLThread.Create(CreateSuspended: Boolean; AFileName: string);
begin
FreeOnTerminate := True;
FFileName := AFileName;
inherited Create(CreateSuspended);
end;
procedure TDLLThread.Execute;
var
InStream, OutStream: TMemoryStream;
RetValue: Integer;
begin
try
//CoInitializeEx(nil, COINIT_APARTMENTTHREADED);
CoInitialize(nil);
try
InStream := TMemoryStream.Create;
try
InStream.LoadFromFile(FFileName);
OutStream := TMemoryStream.Create;
try
RetValue := DLLProcess(InStream, OutStream);
Sleep(0);
Debug('TDLLThread Result=> ' + IntToStr(RetValue));
if RetValue = 0 then
begin
Debug('TDLLThread OK');
end;
finally
OutStream.Free;
end;
finally
InStream.Free;
end;
finally
CoUninitialize;
end;
except
on E: Exception do
begin
Debug('TDLLThread Error: ' + E.Message);
end;
end;
end;
procedure TForm1.Button1Click(Sender: TObject); // Test
var
I: Integer;
begin
for I := 1 to 5 do
TDLLThread.Create(False, '1.xml');
end;
When running some tests I sometimes get Access Violations which even the exceptions blocks can't catch. And the program simply crashes with Runtime error 216 at xxxxxxx or Invalid pointer operation.
I have tried both static and dynamic DLL linking (figured maybe the dynamic linking has race condition in the LoadLibrary/FreeLibrary).
First I thought IXMLDocument was the main issue:
Doc := TXMLDocument.Create(nil);
Doc.LoadFromStream(InStream);
This sometimes randomly failed with no apparent reason with:
Invalid at the top level of the
document.
Or:
A name was started with an invalid character.
I thought maybe it used some shared resources. but even omitting these lines caused AVs!
So the DLL is practically doing nothing special.
I also Don't see anything special which could infect DLLMain.
I have no Idea what is going on... Can someone suggest how to handle this situation? (Can someone reproduce this behavior?)
EDIT: I just wanted to add a related question (with similar IsMultiThread solution):
Delphi DLL - thread safe
And some tips about IsMultiThread:
IsMultiThread Variable

Answer 1

The memory manager in Delphi has optimisations for single threaded use. These are enabled by default. If your code is multi-threaded then this optimisation needs to be disabled. Do that by setting IsMultiThread to True.
In a module that creates a Delphi thread, the framework sets IsMultiThread to True when a thread is created. In your program the threads are created by the host and so nothing in the library sets IsMultiThread to True. So you must do that explicitly in the DLL. In the main section of the library .dpr file write this:
begin
IsMultiThread := True;
end.