I take most of this back. I just saw that in twisted.internet.threads there is now a
function that does this exactly. Last time I looked at the ticket that led to this, there were still some things being worked out. This look very nice though!
On 9/4/07, Brian Granger email@example.com wrote:
This is a usage case that we run into regularly. We have a lot of PB based servers, but sometimes people want to use a client to them from an interactive Python or IPython prompt. At that point you have to run the reactor in a different thread.
Not true, in fact. See twisted/conch/stdio.py, for example. I don't know what the Twisted/IPython integration code ended up looking like (or if it was ever taken to a satisfactory "completion" point) but I don't think it is necessary to have multiple threads for that use case, either.
I have, but this doesn't solve the problem as I recall. First, these users want to be able to use PB clients from vanilla python interpreters and IPython out of the box - not from within some other process doing the tricks that are in conch/stdio.py. Am I not correct that threads are needed in this case? Second, what this module does is actually make the terminal asynchronous, which is exactly the opposite of what these users want. They *want* a synchronous terminal and a blocking PB clients. I hope I am not coming across as angry about this - I'm not. To me the lesson is that as long as everything is really asynchronous anything is possible within the constraints of this asynchronous universe. It's the multiple universe things that makes life complicated (asyn + syn).
But, I have never seen a method of putting a blocking API on top of such a client that was _really_ thread safe. I have seen a few approaches that seem to work, but that are clearly non-thread safe. I would love to see a robust solution for this problem though - one that could be fully trusted.
Why isn't a solution based on Twisted's thread-safe event-posting API (reactor.callFromThread) and the thread-safe event-posting API of whatever other thing is being integrated with "really" thread-safe? Sure, you can write programs which will deadlock as a result of application-level bugs where one thread is blocked on a request and the other thread needs some information from it before the request can be satisfied, but this is inherent to wanting to drive an asynchronous API synchronously. Aside from it, I don't think there are any problems with what I described above.
It probably is, but I have never seen an implementation that actually does this. Do you know of one? The ones that I have seen - even those which use callFromThread have not filled me with thoughts of robustness, stability and thread safety. I should look at this again though.
Also, I should mention that in IPython this is not a show stopper for us, but it does mean that we currently don't use PB in all of the places we might otherwise.
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