[tahoe-lafs-trac-stream] [tahoe-lafs] #1770: always include a useful message in calls to log.err

[tahoe-lafs]

#1770: always include a useful message in calls to log.err
----------------------------+--------------------
     Reporter:  davidsarah  |      Owner:
         Type:  defect      |     Status:  new
     Priority:  normal      |  Milestone:  1.10.0
    Component:  code        |    Version:  1.9.1
   Resolution:              |   Keywords:  error
Launchpad Bug:              |
----------------------------+--------------------

Comment (by warner):

 Hm, maybe. It'd have to look something like this:

 {{{
     d = ...
     d.addCallback(...)
     util.terminateDeferredChain(d)

 def terminateDeferredChain(d):
     stack = traceback.format_stack(1)
     d.addErrback(log.err, why="call stack was %s" % (stack,))
 }}}

 (since {{{log.err}}} is usually added to the end of a Deferred
 chain, by the time it's called, the stack no longer contains any
 information about what caused the {{{log.err}}} to be added)

 My problem with that is twofold:

 * tracebacks are expensive to create, and
   {{{terminateDeferredChain()}}} gets called every time, not just
   in the case where {{{log.err()}}} gets invoked
 * I've rarely found a whole stacktrace to be significantly more
   useful than a UMID, at least for async errors. There just aren't
   very many different ways to reach the same function: the top
   stack frame is enough to guess the rest, and a UMID is enough to
   deduce the top frame.

 Also, the "util.terminateDeferredChain(d)" at the end of a series
 of "d.addCallback" lines would look kind of out-of-place, but maybe
 I could get used to it.

 It might be worth testing how expensive it is to extract just a
 single stack frame, instead of the whole traceback. If
 terminateDeferredChain() could grab just the file+lineno+name of
 the calling function, that'd contain as much information as a UMID
 but tolerate cut-and-paste coding a lot better. OTOH, a static UMID
 is super-fast.

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1770#comment:2>
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