`allowance` is computed before the body arrives, so a chunk that completed
while this one was still streaming was not counted in it. Two overlapping
0.75MB chunks under a 1MB cap were therefore both accepted, leaving 1.5MB on
disk; enough concurrent streams could go well past the cap before anyone asked
to complete the upload.
The buffered version got this right for free, because it only ever checked
after reading the whole body. The streaming version keeps its pre-read check —
that is what makes a too-large request cheap — and asks again with the current
aggregate before renaming the staging file into place.
Relates to issue 1403
Paul Nothaft
0435840a0f