[MacPorts] #15712: Add versions to platforms

[MacPorts]

#15712: Add versions to platforms
--------------------------+----------------------------
  Reporter:  raimue       |      Owner:  larryv
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.6.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
--------------------------+----------------------------

Comment (by raimue):

 You would have to use `platforms-append` like we do for any other option
 that comes from PortGroups. Why would we want to handle `platforms`
 differently than other options?

 {{{
 # PortGroup ...
 platforms {darwin > 10}

 # Portfile
 platforms-append {darwin < 10} {darwin 11}
 }}}

 Overall, we mostly care whether the current platform is supported or not.
 There is no need for complicated blacklists or whitelists. There is
 usually only a need to express a maximum or minimum version requirement
 for each platform.

 To find out if the current platform is compatible, we walk through this
 full list of requirements as explained [#comment:20 above]. We check each
 item of this list (`{darwin > 10} {darwin < 10} {darwin 11}`) against the
 current platform.

 Let me write some pseudo-code for the algorithm:

 {{{
 state = MAYBE
 foreach item in $platforms
     supported = check_if_supported(current_platform, current_version,
 item)
     if supported == YES
         state = YES
     else if supported == NO
         if state == MAYBE
             state = NO
         endif
     else
         // MAYBE is ignored
     endif
 endforeach

 define check_if_supported(current_platform, current_version, item)
     platform, operator, version = split(item)
     if current_platform != platform
         return MAYBE
     endif
     if current_version operator version    // think of: 1.5 < 2.0
         return YES
     else
         return NO
     endif
 enddefine
 }}}

 If we end with `state == NO`, the current platform is definitely not
 supported. If we end with `state == MAYBE`, this build might work and we
 warn the user, but continue. If we end with `state == yes`, we obviously
 just continue.

 The example list from above would always evaluate to `YES` on macOS except
 for `darwin 10` (or `darwin 10.0.0`, depending on how we compare the
 version).

 I do not see a use case when you would need to broaden the list of
 supported versions later. If a PortGroup says that it requires `darwin <
 10`, under what circumstances would the port need to say that an older
 version is actually supported? Then the requirement in the PortGroup was
 likely bogus and needs to be fixed. And even then, you can still override
 it in the Portfile by just using `platforms` (without `-append`) or delete
 the specific item with a `-delete` or `-replace`. It is just like any
 other option.

 I do not understand why you also bring dependencies into this... They can
 declare whatever they want and yes, it might stop us from installing
 dependents. But this has no influence on the way we specify `platforms` in
 this port. We will check whether the current platform is supported each
 time we want to install ports, so we automatically do it for the
 dependency. If it does not support it, we stop at this point and all the
 dependents cannot be installed. To give an appropriate error message we
 have to evaluate this for each port separately anway, so I do not see when
 we would have to merge anything.

-- 
Ticket URL: <https://trac.macports.org/ticket/15712#comment:30>
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