An upload that landed let go of its blob before the tree had its node. A
write open in between took the base it sampled at its start, the revision
the upload replaced, found none of its bytes cached, and queued an
incomplete revision; the next write open failed with EIO.
The drain keeps the landing node aside until the tree has it, and a write
open with no queued revision takes its base from that node, or from the
tree as it is when the handle goes in. The fake Drive can hold the drain
in that window (`hold_next_landing`).
nvrl
2502af6e3c