[tor-bugs] #22141 [Metrics/metrics-lib]: Deprecate `DescriptorFile` and add relevant information to `Descriptor`

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#22141: Deprecate `DescriptorFile` and add relevant information to `Descriptor`
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 Reporter:  karsten              |          Owner:  karsten
     Type:  enhancement          |         Status:  needs_review
 Priority:  Medium               |      Milestone:  metrics-lib 1.9.0
Component:  Metrics/metrics-lib  |        Version:
 Severity:  Normal               |     Resolution:
 Keywords:                       |  Actual Points:
Parent ID:                       |         Points:
 Reviewer:                       |        Sponsor:
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Comment (by karsten):

 Replying to [comment:20 iwakeh]:
 > Oh, only the interface was added.
 > What could getAnnotations, getUnrecognizedLines return when called on an
 UnparseableDescriptor?

 Right now they don't return anything, that is, they return an empty list
 or `null`.

 But in theory we could return whatever annotations or unrecognized lines
 we found until running into a `DescriptorParseException`.  I have an idea
 how to build this with ''relatively'' little code changes, but I'll hold
 off until I hear whether that's even a good idea.

 Another solution that I don't like as much is that we introduce a new
 interface `ParsedDescriptor` that extends `Descriptor` and that other
 (parsed) descriptors extend.  That new interface could then contain these
 two methods, so they're not available in `UnparseableDescriptor`.  The
 part that I don't like there is that we'd have to deprecate the current
 methods in `Descriptor`, and applications depending on them would have to
 downcast their `Descriptor` instances to `ParsedDescriptor` instances just
 to get the annotations (or unrecognized lines).  Slightly eww.

 What do you think?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22141#comment:22>
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