Fix multi callback issues: easy.pause(), easy.close(), multi.close()

[fsbs]

This ensures callbacks invoked by some easy-related functions have
access to the corect thread state.

Fixes callbacks invoked from:

• do_curl_perform
• do_curl_pause
• util_curl_xdecref

---

This is the fix I referred to in #592 (comment), a continuation of #594. The latter only fixed callbacks invoked by multi handles. This one fixes them when they're invoked by easy handles as well.

Practical example:

1. set socket and timer callback
2. add easy handle
3. perform a chunk of the transfer
4. pause the easy handle (do_curl_pause)

At this point callbacks will be invoked to let you know that you should stop polling for sockets associated with that easy handle. But only easy->state is set by PYCURL_BEGIN_ALLOW_THREADS, so the multi handle in multi_socket_callback() won't have access to that state and will fail to acquire thread.

Thus we need to set a reference to easy->multi_stack->state as well. For that purpose I've added PYCURL_BEGIN_ALLOW_THREADS_EASY and placed it at those points where easy handles can invoke callbacks.

The practical consequence of this fix is that event-driven transfers (such as asyncio) now work properly when easy handles are paused, unpaused or closed mid-transfer.

Previously multi_socket_callback() would silently fail, while your event loop would go on polling sockets that are not active anymore - or even worse (when easy handle is unpaused), the event loop would never learn about the sockets that it should start polling.

[fsbs]

The above build failed because of the assert on line 22:

pycurl/src/threadsupport.c

Lines 17 to 33 in 23afe62

	if (self->state != NULL)
	{
	/* inside perform() */
	assert(self->handle != NULL);
	if (self->multi_stack != NULL) {
	assert(self->multi_stack->state == NULL);
	}
	return self->state;
	}
	if (self->multi_stack != NULL && self->multi_stack->state != NULL)
	{
	/* inside multi_perform() */
	assert(self->handle != NULL);
	assert(self->multi_stack->multi_handle != NULL);
	assert(self->state == NULL);
	return self->multi_stack->state;
	}

The assert logic is OK: it verifies that easy.perform() is not called in a different thread at the same time as multi functions - not just multi.perform() though as the comment on line 28 might imply.

Simply setting both easy->state and easy->multi_stack->state at the same time was a really stupid way of fixing easy's callback invocations :) It worked fine for me because my callback tests were using asyncio instead of threads, while pytest uses threads.

I've now changed PYCURL_BEGIN_ALLOW_THREADS_EASY to this:

• if easy->multi_stack == NULL, then easy->state = state
• else easy->multi_stack->state = state

For easy.perform() I've also reverted back to using PYCURL_BEGIN_ALLOW_THREADS because even though call graph might show it can invoke multi's callbacks, that's not the correct way to use libcurl anyway: you have to remove an easy handle from multi before calling easy.perform() - and multi.remove_handle() has already been fixed by #594. So this fix now concerns only easy.pause() and easy.close().

pytest passes on my end now so I'll push the fix.

[fsbs]

I've written tests for SOCKETFUNCTION and TIMERFUNCTION. The tests cover issues that #594 and this patch fixes.

Good news is that they caught a bug which happens when you close multi handle mid-transfer without removing easy handles from it first.

I'm adding the fix and tests to this PR since it's related.