Logical Race Fix

Big thanks to @danp for finding and fixing a race

toctou race between:

• check: if a write is too large to fit in the remaining space in a buffer
• use: write to the buffer, capping the write at the size of the gap

Since we blocked on a zero-sized gap before doing the write, it's possible that the gap could have grown larger than the write, causing us to slice the write past it's boundaries!