TStreamAdapter.Seek Broken For Large Files

EDIT I was completely wrong in my analysis and this bug was indeed fixed in XE4. I'm leaving the original (slightly edited) post below - firstly to remind myself to thoroughly test everything before posting and secondly to help some poor programmer who will run into the same problem with Delphi XE, XE2, XE3 ... in the future.

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Yesterday a colleague informed me that he added a workaround for a VCL bug (which we confirmed to exist in XE2, XE4 and XE5) to our codebase and asked me if I could “inform the authorities”, i.e. enter it into QualityCentral. Imagine my surprise when I not only found the bug already entered but marked as fixed!

The problematic code is posted below, problem marked with a yellow color as it is hard to spot.

function TStreamAdapter.Seek(dlibMove: Largeint; dwOrigin: Longint;
  out libNewPosition: Largeint): HResult;
var
  NewPos: LargeInt;
begin
  try
    if (dwOrigin < STREAM_SEEK_SET) or (dwOrigin > STREAM_SEEK_END) then
    begin
      Result := STG_E_INVALIDFUNCTION;
      Exit;
    end;
    NewPos := FStream.Seek(dlibMove, dwOrigin);
    if @libNewPosition <> nil then libNewPosition := NewPos;
    Result := S_OK;
  except
    Result := STG_E_INVALIDPOINTER;
  end;
end;

TStream implements two Seek methods (below). The former is here for backwards compatibility and allows only seeking in 32-bit-addressable (i.e. smaller than 4 GB) files. The latter supports files larger than 4 GB.

function Seek(Offset: Longint; Origin: Word): Longint; overload; virtual;
function Seek(const Offset: Int64; Origin: TSeekOrigin): Int64; overload; virtual;

If you Ctrl+Click on ‘Seek’ in the TStreamAdapter.Seek implementation, IDE will jump to the latter Seek – which is incorrect as in reality the former (32-bit) seek is called. That makes the problem harder to spot.

Embarcadero people – can somebody please reopen the original bug report? This is something that surely needs to be fixed.

Our temporary (we’ll see for how many future Delphi versions) fix is the same as in the QC #105985 – make a copy of the TStreamAdapter.Seek and cast the dwOrigin parameter into TSeekOrigin enumeration so that the correct TStream.Seek method is called.

type
  TStreamAdapter = class(Classes.TStreamAdapter)
  public
    function Seek(dlibMove: Int64; dwOrigin: Integer; out libNewPosition: Int64): HRESULT; override; stdcall;
  end;

function TStreamAdapter.Seek(dlibMove: Int64; dwOrigin: Integer; out libNewPosition: Int64): HRESULT;
var
  NewPos: LargeInt;
begin
  try
    if (dwOrigin < STREAM_SEEK_SET) or (dwOrigin > STREAM_SEEK_END) then
    begin
      Result := STG_E_INVALIDFUNCTION;
      Exit;
    end;
    NewPos := Stream.Seek(dlibMove, TSeekOrigin(dwOrigin));
    if @libNewPosition <> nil then
      libNewPosition := NewPos;
    Result := S_OK;
  except
    Result := STG_E_INVALIDPOINTER;
  end;
end;