Exception while using TStreamReader with TZipFile

Tuesday, May 13th, 2014 | Dennis D. Spreen | Delphi Programming, Delphi XE6

If you’re using the TStreamReader with a TZDecompressionStream – the stream output by TZipFile.Read(const FileName: string; out Stream: TStream; out LocalHeader: TZipHeader) – you’re likely to experience the exception: EZDecompressionError with the message 'Invalid ZStream operation!'.

TZDecompressionStream allows only three seek methods:

Read forward from beginning (Offset = 0) and (Origin = soBeginning)
Read forward from current position (Offset >= 0) and (Origin = soCurrent)
Read end (Offset = 0) and (Origin = soEnd)

But TStreamReader adjusts the read buffer position with AdjustEndOfBuffer in its FillBuffer method like this:

FStream.Position := FStream.Position - Rewind;

Even if Rewind is 0 the setting of the stream position invokes a seek method call with Offset > 0 and Origin = soBeginning thus lead to the above exception.

“Solution”
Allow TZDecompressionStream.Seek seeking to the current stream position (if not originating from soEnd):

Change the file System.ZLib.pas to:

function TZDecompressionStream.Seek(const Offset: Int64; Origin: TSeekOrigin): Int64;

[...]

begin

if (Offset = 0) and (Origin = soBeginning) then

[...]

else if ((Offset >= 0) and (Origin = soCurrent)) then

[...]

else if (Offset = 0) and (Origin = soEnd) then

[...]

else

if (Origin = soEnd) or (Offset <> Position) then // Add this line to allow

raise EZDecompressionError.Create(SZInvalid); // seeking to actual position

result := FZStream.total_out;

end;

This won’t work if your TStreamReader read buffer needs to be rewinded to be sure you have a valid encoding in there. If you’re using ANSI/ASCII files this won’t happen (Rewind is always 0), if you’re using anything else (like utf-8) you have two possibilities: decompress the whole file to a memory stream first or write your own intermediate stream class which allows rewinding to a certain position but doesn’t keep the whole stream in memory like TMemoryStream does.

Tags: Delphi Programming