Serving files with IdHTTPServer when the files are being written

Question

I'm working with a TIdHTTPServer to serve files to clients, using the ResponseInfo->ServeFile function. This works fine for files that are "static": not being written by some other process. As far as I can see from the code, the ServeFile function internally uses a TIdReadFileExclusiveStream, which disallows me from reading a file being written, but I need to be able to send also files that are being written by some other process.

So, I moved to create a FileStream myself and use the ContentStream property to return it to the client, but I get a 0 bytes file in the client (for any file, being written or not), and I can't see what I'm missing or doing wrong. Here is the code I'm using on the OnCommandGet event handler:

AResponseInfo->ContentStream = new TFileStream(path, fmOpenRead | fmShareDenyNone);
AResponseInfo->ContentStream->Position = 0;
AResponseInfo->ContentLength = AResponseInfo->ContentStream->Size;
AResponseInfo->ResponseNo = 200;
AResponseInfo->WriteHeader();
AResponseInfo->WriteContent();

The ContentLength property at this point has a valid value (i.e., the file size when calling ContentStream->Size), and that's what I would like to send to the client, even if the file changes in between.

I have tried removing the WriteContent() function, the WriteHeader(), but the results are the same. I searched for some examples but the few I found are more or less the same than this code, so I don't know what's wrong. Most examples don't include the WriteContent() call, that's why I have tried removing them, but there doesn't seem to be any difference.

As a side note: the files being written take 24 hours to finish writing, but that's to be expected from the client side: I just need the bytes already written at the time of the request (even somewhat less is valid). The files will never get deleted: they will just keep getting bigger.

Any ideas?

Update

Using Fiddler, I get some warnings on protocol violations, that would be related to this. I get, for instance:

Content-Length mismatch: Response Header indicated 111,628,288 bytes, but server sent 41 bytes

The content length is correct, it's the file size, but I don't know what I'm doing wrong that makes the app sent just 41 bytes.

Answer 1

The final working code is:

    std::unique_ptr< TFileStream >fs(new TFileStream(path, fmOpenRead | fmShareDenyNone));

    fs->Position = 0;
    __int64 size = fs->Size;

    AResponseInfo->ContentLength = size;
    AResponseInfo->ResponseNo    = 200;

    AResponseInfo->WriteHeader();

    AContext->Connection->IOHandler->Write(fs.get(), size);

This allows the client to receive up to size bytes of the original file, even if the file is being written to at the same time.

For some reason passing the ContentStream did not return any content to the client, but doing the IOHandler->Write directly (which is what the ServeFile ends doing internally) works fine.