So, it turns out that if you have a first `Deferred` `D1`, you can add a
callback which returns another `Deferred` `D2`, and `D2` must then complete
before any further callbacks on `D1` will execute (and later callbacks on `D1`
get the *result* of `D2` rather than `D2` itself).

So, `D1` might have `called=True` (as in, it has started running its
callbacks), but any new callbacks added to `D1` won't get run until `D2`
completes - so if you `yield D1` in an `inlineCallbacks` function, your `yield`
will 'block'.

In conclusion: some of our assumptions in `logcontext` were invalid. We need to
make sure that we don't optimise out the logcontext juggling when this
situation happens. Fortunately, it is easy to detect by checking `D1.paused`.

* add some doc about wtf this thing does
* pin Twisted to < 18.4
* add explicit dep on six (fixes #3089)

* When creating a new event, cap its depth to 2^63 - 1
* When receiving events, reject any without a sensible depth

As per https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI

fixes #3135

Signed-off-by: Will Hunt <will@half-shot.uk>

add guard for None on purge_history api

remove duplicates from groups tables

Remove redundant call to preserve_fn

Trap exceptions thrown within run_in_background

run config tests on py3

Signed-off-by: Adrian Tschira <nota@notafile.com>

Make event properties raise AttributeError instead

make imports local

move httplib import to six

Move more xrange to six

Don't yield in list comprehensions

Open config file in non-bytes mode

Open certificate files as bytes

Replace stringIO imports with six

Add py3 tests to tox with folders that work

submit_event_for_as doesn't return a deferred anyway, so this is pointless.

more bytes strings

add missing comma

Use run_in_background in preference to preserve_fn

Construct HMAC as bytes on py3

Use six.moves.urlparse

Signed-off-by: Adrian Tschira <nota@notafile.com>

Signed-off-by: Adrian Tschira <nota@notafile.com>

plus a bonus next()

Signed-off-by: Adrian Tschira <nota@notafile.com>