Problems getting the JSON data from DLL using SuperObject and OmniThreadLibrary - delphi

Question

I'm using Delphi XE, I have the following code for my program and DLL:
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, superobject,
OtlCommon, OtlCollections, OtlParallel;
type
TForm1 = class(TForm)
btnStart: TButton;
btnStop: TButton;
procedure btnStartClick(Sender: TObject);
procedure btnStopClick(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
procedure FormCreate(Sender: TObject);
private
FLogger : IOmniBackgroundWorker;
FPipeline: IOmniPipeline;
FLogFile: TextFile;
strict protected
procedure Async_Log(const workItem: IOmniWorkItem);
procedure Async_Files(const input, output: IOmniBlockingCollection);
procedure Async_Parse(const input: TOmniValue; var output: TOmniValue);
procedure Async_JSON(const input, output: IOmniBlockingCollection);
end;
var
Form1: TForm1;
function GetJSON(AData: PChar): ISuperObject; stdcall; external 'my.dll';
implementation
uses OtlTask, IOUtils;
{$R *.dfm}
function GetJSON_local(AData: PChar): ISuperObject;
var
a: ISuperObject;
sl: TStringList;
begin
sl := TStringList.Create;
try
sl.Text := StrPas(AData);
Result := SO();
Result.O['array'] := SA([]);
a := SO;
a.S['item1'] := sl[14];
Result.A['array'].Add(a);
a := nil;
a := SO;
a.S['item2'] := sl[15];
Result.A['array'].Add(a);
finally
sl.Free;
end;
end;
procedure TForm1.FormCreate(Sender: TObject);
var
s: string;
begin
// log
s := ExtractFilePath(Application.ExeName) + 'Logs';
if not TDirectory.Exists(s) then TDirectory.CreateDirectory(s);
s := Format(s+'\%s.txt', [FormatDateTime('yyyy-mm-dd_hh-nn-ss', Now)]);
AssignFile(FLogFile, s);
Rewrite(FLogFile);
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
CloseFile(FLogFile);
end;
procedure TForm1.Async_Log(const workItem: IOmniWorkItem);
begin
WriteLn(FLogFile, workItem.Data.AsString);
end;
procedure TForm1.Async_Files(const input, output: IOmniBlockingCollection);
var
f: string;
begin
while not input.IsCompleted do begin
for f in TDirectory.GetFiles(ExtractFilePath(Application.ExeName), '*.txt') do
output.TryAdd(f); // output as FileName
Sleep(1000);
end;
end;
procedure TForm1.Async_Parse(const input: TOmniValue; var output: TOmniValue);
var
sl: TStringList;
begin
sl := TStringList.Create;
try
sl.LoadFromFile(input.AsString);
// output := GetJSON_local(PChar(sl.Text)); // output as ISuperObject --- local function
output := GetJSON(PChar(sl.Text)); // output as ISuperObject --- DLL function
finally
sl.Free;
end;
FLogger.Schedule(FLogger.CreateWorkItem(Format('%s - File processed: %s', [DateTimeToStr(Now), input.AsString])));
end;
procedure TForm1.Async_JSON(const input, output: IOmniBlockingCollection);
var
value: TOmniValue;
JSON: ISuperObject;
begin
for value in input do begin
if value.IsException then begin
FLogger.Schedule(FLogger.CreateWorkItem(value.AsException.Message));
value.AsException.Free;
end
else begin
JSON := value.AsInterface as ISuperObject;
FLogger.Schedule(FLogger.CreateWorkItem(JSON.AsString));
end;
end;
end;
//
procedure TForm1.btnStartClick(Sender: TObject);
begin
btnStart.Enabled := False;
FLogger := Parallel.BackgroundWorker.NumTasks(1).Execute(Async_Log);
FPipeline := Parallel.Pipeline
.Stage(Async_Files)
.Stage(Async_Parse)
.Stage(Async_JSON)
.Run;
end;
procedure TForm1.btnStopClick(Sender: TObject);
begin
if Assigned(FPipeline) and Assigned(FLogger) then begin
FPipeline.Input.CompleteAdding;
FPipeline := nil;
FLogger.Terminate(INFINITE);
FLogger := nil;
end;
btnStart.Enabled := True;
end;
end.
// DLL code
library my;
uses
SysUtils,
Classes, superobject;
function GetJSON(AData: PChar): ISuperObject; stdcall;
var
a: ISuperObject;
sl: TStringList;
begin
sl := TStringList.Create;
try
sl.Text := StrPas(AData);
Result := SO();
Result.O['array'] := SA([]);
a := SO;
a.S['item1'] := sl[14];
Result.A['array'].Add(a);
a := nil;
a := SO;
a.S['item2'] := sl[15];
Result.A['array'].Add(a);
finally
sl.Free;
end;
end;
exports
GetJSON;
begin
end.
When I try to run with debugging my code, after a few calls of the dll GetJSON function i get the following error:
"Project test_OTL_SO.exe raised exception class EAccessViolation with message 'Access violation at address 005A2F8A in module 'my.dll'. Write of address 00610754'."
However, this issue does not occur when I use the same local function GetJSON_local.
Could anyone suggest what am I doing wrong here?
EDIT: (solution)
I write this code for my DLL:
procedure GetJSON_(const AData: PChar; out Output: WideString); stdcall;
var
json, a: ISuperObject;
sl: TStringList;
begin
sl := TStringList.Create;
try
sl.Text := AData;
json := SO();
json.O['array'] := SA([]);
a := SO;
a.S['item1'] := sl[14];
json.A['array'].Add(a);
a := nil;
a := SO;
a.S['item2'] := sl[15];
json.A['array'].Add(a);
Output := json.AsString;
finally
sl.Free;
end;
end;
and changed the code of Async_Parse procedure:
procedure TForm1.Async_Parse(const input: TOmniValue; var output: TOmniValue);
var
sl: TStringList;
ws: WideString;
begin
sl := TStringList.Create;
try
sl.LoadFromFile(input.AsString);
GetJSON_(PChar(sl.Text), ws); // DLL procedure
output := SO(ws); // output as ISuperObject
finally
sl.Free;
end;
FLogger.Schedule(FLogger.CreateWorkItem(Format('%s - File processed: %s', [DateTimeToStr(Now), input.AsString])));
end;

Answer 1

The problem is your passing of ISuperObject interfaces across a module boundary. Although interfaces can be safely used that way, the methods of the interface are not safe. Some of the methods of the interface accept, or return, strings, objects, etc. That is, types that are not safe for interop.
Some examples of methods that are not safe:
function GetEnumerator: TSuperEnumerator; // TSuperEnumerator is a class
function GetS(const path: SOString): SOString; // returns a Delphi string
function SaveTo(stream: TStream; indent: boolean = false;
escape: boolean = true): integer; overload; // TStream is a class
function AsArray: TSuperArray; // TSuperArray is a class
// etc.
You should serialize the JSON to text, and pass that text between your modules.