twistd sometimes errors with "reactor already installed"

New submission from ivank <[email protected]>:

When using twistd -r <anything-but-select> <plugin>, and dropin.cache is not generated yet, and <plugin> imports the reactor (or imports anything that imports the reactor), twistd fails with:

"/usr/local/bin/twistd: The specified reactor cannot be used, failed with error: reactor already installed."

This only happens if the reactor is imported when the plugin file is loaded (imported?). It does not happen if the reactor is imported in makeService.

I don't believe this is documented anywhere, the error is uninformative, and the failure is hard to track down.

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Type     : defect
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3785