[Buildbot] #3513: Provide helpers for doStepIf on properties

[Buildbot trac]

#3513: Provide helpers for doStepIf on properties
------------------------+-----------------------
Reporter:  gracinet     |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  minor        |  Milestone:  undecided
 Version:  0.8.12       |   Keywords:
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 If I understood correctly, on Nine, ``doStepIf`` can't access properties
 directly anymore, they have to access db, use a deferred etc.

 It would be nice for adoption to provide helpers for that. I can share
 something that works like this, provided one tells me where to put it.

 {{{
       @conditions.props
       def is_python_34(props):  # will be readily usable as a doStepIf
           return props.get('py_version'[0]) == 3.4
 }}}

 The `conditions.props` decorator takes care of calling the DB API,
 `inlineCallbacks` etc.

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Ticket URL: <http://trac.buildbot.net/ticket/3513>
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