Passing an admitted permit -- i.e. one with count resources on it -- to
the multishard reader, will possibly result in a deadlock, because the
permit of the multishard reader is destroyed after the permits of its
child readers. Therefore its semaphore resources won't be automatically
released until children acquire their own resources.
This creates a dependency (an edge in the "resource allocation graph"),
where the semaphore used by the multishard reader depends on the
semaphores used by children. When such dependencies create a cycle, and
permits are acquired by different reads in just the right order, a
deadlock will happen.
Users of the multishard reader have to be aware of this gotcha -- and of
course they aren't. This is small wonder, considering that not even the
documentation on the multishard reader mentions this problem.
To work around this, the user has to call
`reader_permit::release_base_resources()` on the permit, before passing
it to the multishard reader.
On multiple occasions, developers (including the very author of the
multishard reader), forgot or didn't know about this and this resulted
in deadlocks down the line.
This is a design-flaw of the multishard reader, which is addressed in
this patch, after which, it is safe to pass admitted or not admitted
permits to the multishard reader, it will handle the call to
`release_base_resources()` if needed.
(cherry picked from commit 218ea449a5)
41a44ddc12