Add a mechanism for viewing the state and history of a deferred's callbacks as an optional debugging mode

[radix]

Terry Jones implemented a nice class for tracking deferred history to help debugging strange Deferred problems. It'd be nice to test this code and integrate it into the core of Twisted, with something like defer.setHistoryTracking(True) to turn it on.

The original mailing list post is here:

​http://www.twistedmatrix.com/pipermail/twisted-python/2008-October/018535.html

[Glyph]

Feel free :).

[terrycojones]

It would be nice to add simple timing information to the history too (when was a callback func added, when was it called, when did it return). That would perhaps be of some assistance to people trying to understand how much time was spent on what during the execution of a deferred. The history function (that gets inserted in the callback chain) could then print the timing info.

[Jean-Paul Calderone]

This is a duplicate of #1402.