OQueue Panics when Cursor Underflows

[ioeddk]

OQueue panics in the following scenario:

1. During the boot, there weren't enough BioCompletionStat entries in the BioCompletionOQueue.
2. However, the LinnOS policy always attempts to peak four elements in the BioCompletionOQueue.
3. While there are fewer than four elements in BioCompletionOQueue, the Cursor with usize as its type has self.0 - rhs less than 0, which causes panic immediately.

pub struct Cursor(usize);

impl Sub<usize> for Cursor {
    type Output = Cursor;

    fn sub(self, rhs: usize) -> Self::Output {
        Cursor(self.0 - rhs)
    }
}

#228 is a temporary solution to bypass the panic. But the first few OQueue reads might not return reliable values.

@arthurp Any thoughts?

[arthurp]

I think the correct semantics is to have the "negative" Cursor point to an always missing value. I'll comment more on the fix to discuss the correct way to make this happen.