SimpleXMLRPCDispatcher._dispatch mangles tracebacks when invoking RPC calls through _dispatch

[petr-motejlek]

BPO	29615
Nosy	@loewis, @serhiy-storchaka, @petr-motejlek
PRs	bpo-29615: SimpleXMLRPCDispatcher no longer chains KeyError (or any other exception) to exception(s) raised in the dispatched methods. #260 [3.6] bpo-29615: SimpleXMLRPCDispatcher no longer chains KeyError (or any other exception) to exception(s) raised in the dispatched methods. #478 [3.5] bpo-29615: SimpleXMLRPCDispatcher no longer chains KeyError (or any other exception) to exception(s) raised in the dispatched methods. #479 [Do Not Merge] Sample of CPython life with blurb. #703 [Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-03-17.21:13:55.521>
created_at = <Date 2017-02-21.20:30:25.353>
labels = ['3.7', 'type-bug', 'library']
title = 'SimpleXMLRPCDispatcher._dispatch mangles tracebacks when invoking RPC calls through _dispatch'
updated_at = <Date 2017-03-31.16:36:09.952>
user = 'https://github.com/petr-motejlek'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:09.952>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2017-03-17.21:13:55.521>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2017-02-21.20:30:25.353>
creator = 'petr@motejlek.net'
dependencies = []
files = []
hgrepos = []
issue_num = 29615
keywords = []
message_count = 4.0
messages = ['288319', '290297', '290298', '290351']
nosy_count = 3.0
nosy_names = ['loewis', 'serhiy.storchaka', 'petr@motejlek.net']
pr_nums = ['260', '478', '479', '703', '552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29615'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[petr-motejlek]

Hello,

We discovered that SimpleXMLRPCDispatcher mangles tracebacks printed from within the invoked RPC methods when an object that has _dispatch(method, params) defined has been registered with it

Steps to reproduce

• use https://docs.python.org/3.4/library/xmlrpc.server.html#xmlrpc.server.SimpleXMLRPCServer.register_instance and register within SimpleXMLRPCDispatcher an object with the _dispatch(method, params) method defined
• invoke any RPC method that prints a traceback (perhaps raise an exception in it, catch it and use logging.exception to log it) --> the traceback will display a very strange looking KeyError (its value being the name of the invoked method) and then the actual exception that was fed into logging.exception

The reason for this is that SimpleXMLRPCDispatcher._dispatch first attempts to acquire the function to invoke by reading from the functions registered with the dispatcher. When this attempt fails, KeyError is raised. During its handling different approaches are taken to acquire the function and for all but one when the function is acquired, handling of the KeyError ends and the function is called properly, outside of the except block

However, in the case of objects that define the _dispatch(method, params) function, the _dispatch(method, params) function is called straight away, still within the except block handling the KeyError, and this leads to the mangled traceback

This behavior is neither documented, nor expected (we believe) and thus the code should be changed

Will submit a PR (incl. tests) for this later on

[serhiy-storchaka]

New changeset c6e199f by Serhiy Storchaka (Petr Motejlek) in branch '3.5':
bpo-29615: backport to 3.5 (#479)
c6e199f

[serhiy-storchaka]

New changeset 3405792 by Serhiy Storchaka (Petr Motejlek) in branch '3.6':
bpo-29615: backport to 3.6 (#478)
3405792

[serhiy-storchaka]

New changeset 3c6314c by Serhiy Storchaka (Petr Motejlek) in branch 'master':
bpo-29615: SimpleXMLRPCDispatcher no longer chains KeyError (#260)
3c6314c