Resume HTTP Post/upload with Indy

I'm trying to resume an upload using indy (HTTP Post), the code looks like this (using Delphi 2010, Indy 10.4736):

 IdHttp.Head('http://localhost/_tests/resume/large-file.bin');
 ByteRange           := IdHttp.Response.ContentLength + 1;

 // Attach the file to post/upload
 Stream              := TIdMultipartFormDataStream.Create;
 with Stream.AddFile('upload_file', 'D:\large-file.bin', 'application/octet-stream') do
 begin
      HeaderCharset  := 'utf-8';
      HeaderEncoding := '8';
 end;    // with

 with IdHTTP do
 begin
      IOHandler.LargeStream           := True;

      with Request do
      begin
           ContentRangeStart          := ByteRange;
           ContentRangeEnd            := (Stream.Size - ByteRange);
           ContentLength              := ContentRangeEnd;
           ContentRangeInstanceLength := ContentLength;
      end;    // with

      Post('http://localhost/_tests/resume/t1.php', Stream);
 end;    // with

but upload resume doesn't work :(

I looked into Indy's code, it seems that this function in IdIOHandler.pas

TIdIOHandler.Write()

always deal with complete streams/files (since the parameter ASize: TIdStreamSize seems to be always 0, which according to the code means sending the full file/stream).

This prevents indy from resuming the upload.

My question is: is it possible to avoid sending the full file?

Setting content range didn't change anything. I also tweaked indy's code (modified 3 lines) to make indy obey to the content range / stream position, but it's buggy and indy always end up hanging in IdStackWindows.pas because of an infinite timeout here:

TIdSocketListWindows.FDSelect()

As I told you in your earlier question, you have to post a TStream containing the remaining file data to upload. Don't use TIdMultipartFormDataStream.AddFile(), as that will send the entire file. Use the TStream overloaded version of TIdMultipartFormDataStream.AddFormField() instead.

And TIdHTTP is not designed to respect the ContentRange... properties. Most of the Request properties merely set the corresponding HTTP headers only, they do not influence the data. That is why your edits broke it.

Try this:

IdHttp.Head('http://localhost/_tests/resume/large-file.bin');
FileSize := IdHttp.Response.ContentLength;

FileStrm := TFileStream.Create('D:\large-file.bin', fmOpenRead or fmShareDenyWrite);
try
  if FileSize < FileStrm.Size then
  begin
    FileStrm.Position := FileSize;

    Stream := TIdMultipartFormDataStream.Create;
    try
      with Stream.AddFormField('upload_file', 'application/octet-stream', '', FileStrm, 'large-file.bin') do
      begin
        HeaderCharset  := 'utf-8';
        HeaderEncoding := '8';
      end;

      with IdHTTP do
      begin
        with Request do
        begin
          ContentRangeStart := FileSize + 1;
          ContentRangeEnd   := FileStrm.Size;
        end;

        Post('http://localhost/_tests/resume/t1.php', Stream);
      end;
    finally
      Stream.Free;
    end;
  end;
finally
  FileStrm.Free;
end;

With that said, this is a GROSS MISUSE of HTTP and multipart/form-data. For starters, the ContentRange values are in the wrong place. You are applying them to the entire Request as a whole, which is wrong. They would need to be applied at the FormField instead, but TIdMultipartFormDataStream does not currently support that. Second, multipart/form-data was not designed to be used like this anyway. It is fine for uploading a file from the beginning, but not for resuming a broken upload. You really should stop using TIdMultipartFormDataStream and just pass the file data directly to TIdHTTP.Post() like I suggested earlier, eg:

FileStrm := TFileStream.Create('D:\large-file.bin', fmOpenRead or fmShareDenyWrite);
try
  IdHTTP.Post('http://localhost/_tests/upload.php?name=large-file.bin', FileStrm);
finally
  FileStrm.Free;
end;

IdHTTP.Head('http://localhost/_tests/files/large-file.bin');
FileSize := IdHTTP.Response.ContentLength;

FileStrm := TFileStream.Create('D:\large-file.bin', fmOpenRead or fmShareDenyWrite);
try
  if FileSize < FileStrm.Size then
  begin
    FileStrm.Position := FileSize;
    IdHTTP.Post('http://localhost/_tests/resume.php?name=large-file.bin', FileStrm);
  end;
finally
  FileStrm.Free;
end;

I already explained earlier how to access the raw POST data in PHP.