[tor-bugs] #17000 [metrics-lib]: Allow to distinguish between relay and bridge descriptors

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#17000: Allow to distinguish between relay and bridge descriptors
-------------------------+-------------------------
 Reporter:  karsten      |          Owner:  karsten
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:
Component:  metrics-lib  |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 So far, we did not distinguish between relay and bridge descriptors in the
 case of server descriptors and extra-info descriptors.  This works just
 fine, because we tried hard to re-use existing keywords in relay
 descriptors for sanitized contents in bridge descriptors to facilitate
 parsing.

 However, some applications process both relay and bridge descriptors and
 need to add workarounds for distinguishing between the two.  For example,
 they couldn't just read the contents of CollecTor's recent/ directory,
 because they wouldn't be able to know whether a ServerDescriptor instance
 was published by a relay or bridge.  Or even worse, if an application
 expects a given directory to contain relay descriptors and that directory
 suddenly contains bridge descriptors, that application wouldn't notice.

 I pushed [https://gitweb.torproject.org/user/karsten/metrics-
 lib.git/log/?h=relay-bridge-descs branch relay-bridge-descs to my public
 repository] which attempts to fix this.  Please review.

 This change adds new interfaces to distinguish between relay and bridge
 descriptors.  It still supports the existing two interfaces that don't
 make this distinction.  Those two interfaces are not deprecated, because
 it's okay if an application does not care whether a descriptor was
 published by a relay or bridge.

 This change is in parts based on a discussion with atagar.  Thanks!

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17000>
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