FileH is a handle representing snapshot of a file. If, for a pgoffset,
fileh already has loaded page, but we know the content of the file has
changed externally after loading has been done, we need to propagate to
fileh that such-and-such page should be invalidated (and reloaded on
next access).

This patch introduces

    fileh_invalidate_page(fileh, pgoffset)

to do just that.

In the next patch we'll use this facility to propagate invalidations of
ZBlk ZODB objects to virtmem subsystem.

NOTE

Since invalidation removes "dirtiness" from a page state, several
subsequent invalidations can make a fileh completely non-dirty
(invalidating all dirty page). Previously fileh->dirty was just a one
bit, so we needed to improve how we track dirtiness.

One way would be to have a dirty list for fileh pages and operate on
that. This has advantage to even optimize dirty pages processing like
fileh_dirty_writeout() where we currently scan through all fileh pages
just to write only PAGE_DIRTY ones.

Another simpler way is to make fileh->dirty a counter and maintain that.

Since we are going to move virtmem subsystem back into the kernel, here,
a simpler less-intrusive approach is used.

Previously we were limited to printing traceback starting down from just
storeblk() via explicit PyErr_PrintEx() - because pybuf was attached to
memory which could go away right after return from C function - so we
had to destroy that object for sure, not letting any traceback to hold a
reference to it.

This turned out to be too limiting and not showing full context where
errors happen.

So do the following trick: before returning, reattach pybuf to empty
region at NULL, and this way we don't need to worry about pybuf pointing
to memory which can go away -> thus instead of printing exception locally
- just return it the usual way it is done with C api in Python.

NOTE In contrast to PyMemoryViewObject, PyBufferObject definition is not
public, so to support Python2 - had to copy its definition to PY2 compat
header.

NOTE2 loadblk() is not touched - the loading is done from sighandler
context, which simulates as if it work in separate python thread, so it
is leaved as is for now.

At present several threads running can corrupt internal virtmem
datastructures (e.g. ram->lru_list, fileh->pagemap, etc).

This can happen even if we have zope instances only with 1 worker thread
- because there are other "system" thread, and python garbage collection
can trigger at any thread, so if a virtmem object, e.g. VMA or FileH was
there sitting at GC queue to be collected, their collection, and thus
e.g. vma_unmap() and fileh_close() will be called from
different-from-worker thread.

Because of that virtmem just has to be aware of threads not to allow
internal datastructure corruption.

On the other hand, the idea of introducing userspace virtual memory
manager turned out to be not so good from performance and complexity
point of view, and thus the plan is to try to move it back into the
kernel. This way it does not make sense to do a well-optimised locking
implementation for userspace version.

So we do just a simple single "protect-all" big lock for virtmem.

Of a particular note is interaction with Python's GIL - any long-lived
lock has to be taken with GIL released, because else it can deadlock:

    t1  t2

    G
    V   G
   !G   V
    G

so we introduce helpers to make sure the GIL is not taken, and to retake
it back if we were holding it initially.

Those helpers (py_gil_ensure_unlocked / py_gil_retake_if_waslocked) are
symmetrical opposites to what Python provides to make sure the GIL is
locked (via PyGILState_Ensure / PyGILState_Release).

Otherwise, the patch is more-or-less straightforward application for
one-big-lock to protect everything idea.

Mutex lock/unlock should not fail if mutex was correctly initialized/used.

We factored out SIGSEGV block/restore from fileh_dirty_writeout() to all
functions in cb7a7055 (bigfile/virtmem: Block/restore SIGSEGV in
non-pagefault-handling function). The restoration however just sets
whole thread sigmask.

It could be possible that between block/restore calls procmask for other
signals could be changed, and this way - setting procmask directly - we
will overwrite them.

So be careful, and when restoring SIGSEGV mask, touch mask bit for only
that signal.

( we need xsigismember helper to get this done, which is also introduced
  in this patch )

We'll need this for function which return error not in errno - e.g.
pthread_sigmask().

Exposes BigFile - this way users can define BigFile backend in Python.

Also exposed are BigFile handles, and VMA objects which are results of
mmaping.

Does similar things to what kernel does - users can mmap file parts into
address space and access them read/write. The manager will be getting
invoked by hardware/OS kernel for cases when there is no page loaded for
read, or when a previousle read-only page is being written to.

Additionally to features provided in kernel, it support to be used to
store back changes in transactional way (see fileh_dirty_writeout()) and
potentially use huge pages for mappings (though this is currently TODO)

Users can inherit from BigFile and provide custom ->loadblk() and
->storeblk() to load/store file blocks from a database or some other
storage. The system then could use such files to memory map them into
user address space (see next patch).

This thing allows to get aliasable RAM from OS kernel and to manage it.
Currently we get memory from a tmpfs mount, and hugetlbfs should also
work, but is TODO because hugetlbfs in the kernel needs to be improved.

We need aliasing because we'll need to be able to memory map the same
page into several places in address space, e.g. for taking two slices
overlapping slice of the same array at different times.

Comes with test programs that show we aliasing does not work for
anonymous memory.

This will be the core of virtual memory subsystem. For now we just
define a structure to describe pages of memory and add utility to
allocate address space from OS.

For BigFiles we'll needs to maintain `{} offset-in-file -> void *` mapping. A
hash or a binary tree could be used there, but since we know files are
most of the time accessed sequentially and locally in pages-batches, we
can also organize the mapping in batches of keys.

Specifically offset bits are so divided into parts, that every part
addresses 1 entry in a table of hardware-page in size. To get to the actual
value, the system lookups first table by first part of offset, then from
first table and next part from address - second table, etc.

To clients this looks like a dictionary with get/set/del & clear methods,
but lookups are O(1) time always, and in contrast to hashes values are
stored with locality (= adjacent lookups almost always access the same tables).

Like taking an exact integer log2, upcasting pointers for C-style
inheritance done in a Plan9 way, and wrappers to functions which should
never fail.

Modelled by ones used in Linux kernel.