Add a with_truncate_on_close to DmaStreamWriterBuilder

[tontinton]

What does this PR do?

Adds a with_truncate_on_close to DmaStreamWriterBuilder, to remove any padding that might have been added by glommio.

Motivation

I wanted to get the file size of a file by reading its metadata using file_size(), instead of saving it to another file for example, and it was important to me to calculate how many items I have written to the file by a simple calculation instead of parsing it.

Any padding added would have ruined the calculation.

[vlovich]

I'd love to see a unit test that shows the motivating example. I don't understand the impact this flag has because close consumes the DmaStreamWriterState. And if the flag is true, then we avoid the logic in must_truncate to force a truncation, but truncation should only have any effect if and only if flushed pos > aligned pos (i.e. must_truncate would be returning true anyway).

[tontinton]

There's a check though in must_truncate:

        if let FileStatus::Closed = self.file_status {
            return false;
        }

Which is true when you call close on the file.

I can maybe write a test later, currently a bit busy.

[vlovich]

I see. If that's the case, I would posit that there's a higher level logical bug where that state is being set prematurely and truncation isn't getting applied correctly (assuming this is happening in a very simple happy path).

[tontinton]

Closing as fixed in #611