Because we can abort a read_page completion, this means a page can be in
the cache but be unloaded and unlocked. However, if we do not evict that
page from the page cache, we will return an unloaded page later which
will trigger assertions later on. This is worsened by the fact that page
cache is not per `Statement`, so you can abort a completion in one
Statement, and trigger some error in the next one if we don't evict the
page in these circumstances.
Also, to propagate IO errors we need to return the Error from
IOCompletions on step.
Closes#2785
Commit ebe6aa0d28 ("adjust cfg for unix
and linux IO") adjusted the I/O conditional compilation, but forgot that
Android and iOS are also part of Unix target family.
Fixes#2500
The completion callback can be invoked only once via `OnceLock`, let's not
crash if we e.g. call `Completion::abort()` on an already finished completion.
Closes#2673
Problem
There are several problems with our current statically allocated
`BufferPool`.
1. You cannot open two databases in the same process with different
page sizes, because the `BufferPool`'s `Arena`s will be locked forever
into the page size of the first database. This is the case regardless
of whether the two `Database`s are open at the same time, or if the first
is closed before the second is opened.
2. It is impossible to even write Rust tests for different page sizes because
of this, assuming the test uses a single process.
Solution
Make `Database` own `BufferPool` instead of it being statically allocated, so this
problem goes away.
Note that I didn't touch the still statically-allocated `TEMP_BUFFER_CACHE`, because
it should continue to work regardless of this change. It should only be a problem if
the user has two or more databases with different page sizes open simultaneously, because
`TEMP_BUFFER_CACHE` will only support one pool of a given page size at a time, so the rest
of the allocations will go through the global allocator instead.
Notes
I extracted this change out from #2569, because I didn't want it to be smuggled in without
being reviewed as an individual piece.
