Entry `drop` function can take a long time

[wavenator]

The current drop implementation for Entry is as follows:

impl<'a, R: 'a + Read> Drop for Entry<'a, R> {
    fn drop(&mut self) {
        if self.position < self.length {
            // Consume the rest of the data in this entry.
            let mut remaining = self.reader.take(self.length - self.position);
            let _ = io::copy(&mut remaining, &mut io::sink());
        }
    }
}

When dealing with large entries, this function can take a very long time. Could there be a reason for this implementation? Do we really need to consume the reader?

[bjorn3]

This is necessary to seek the reader to the header of the next entry. There is no Seek bound, so reader.seek() can't be used.

[wavenator]

I'm confused. I'm not familiar with the code but why don't we use Reader::seek here?
Eventually, this drop implementation takes a lot of time when dealing with a lot of entries in a big AR file. Can't it be solved?

[bjorn3]

reader.seek() requires a Seek bound. It could be added, but because it currently isn't, .seek() can't be used without a breaking change. A breaking change may be acceptable. At least it would be fine with me speaking as user of rust-ar.

[wavenator]

@mdsteele WDYT about adding Seek bound to Reader?

[wavenator]

BTW what about calling consume?

[mdsteele]

Yes, it was written that way to avoid needing a Seek bound. At the time I was trying to stay similar to the API of the tar crate, which also does not require a Seek bound. Once nice feature of not requiring Seek is that you can easily extract a compressed archive by using a reader adapter that provides streaming decompression (for example, GzipReader, which doesn't implement Seek). It'd be nice not to lose that capability.

I poked around just now, though, and it looks like tar recently landed a (not yet published) change to provide an alternate Archive::entries_with_seek method to allow opting in to a more efficient implementation when the underlying reader implements Seek. So that might be a good approach.