[tor-bugs] #17134 [Tor]: Add a way to say "Use this option only if supported" in torrc

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#17134: Add a way to say "Use this option only if supported" in torrc
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 Reporter:  nickm        |          Owner:
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:  Tor: 0.2.8.x-final
Component:  Tor          |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 It would be nice to have a torrc that sets some option _when Tor supports
 that option_, but not otherwise (eg when older versions of Tor read it).

 There are two ways to do this I can think of:

 1. '''Future-compatible only'''

 We could have a new syntax, something like `?Option Value`.  Meaning: if
 the option is prefixed with a question mark, it does not cause an error
 when unrecognized.  (It _should_ cause a warning.)

 2. '''Backward-compatible too, kinda ugly'''

 We could have a syntax that old Tor versions interpret as a comment, but
 which new Tor versions interpret as above.  My first thought is to use
 some magic string like `#$%^option value`.  This could cause problems if
 anybody is actually writing comments like that.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17134>
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