[MacPorts] #54787: Don't install dependencies of packages that can't be installed

[MacPorts]

#54787: Don't install dependencies of packages that can't be installed
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  Reporter:  akimd        |      Owner:
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:  2.4.1
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 Yes that would be nice. I'd love for there to be a new subcommand I could
 run, e.g. `port installable pdftk`, which could tell you in advance if a
 port is expected to be installable on the current system or not. And
 MacPorts could use that same hypothetical capability internally as you
 suggest, to prevent installing dependencies for non-installable ports.

 But I don't know how we would accomplish that in base right now. The pdftk
 port has a `pre-fetch` block where the determination is made whether to
 attempt the install or bail. By the time that base is evaluating the `pre-
 fetch` block, dependencies have already been computed and installed. The
 introduction of a new phase, run before dependencies are computed, which
 ports could override with such checks, could be a possible solution.

 We've talked about this problem before. I think someome mentioned that
 solving this problem will be easier once we have a
 [wiki:SummerOfCode2015Libsolv new dependency resolution engine].

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Ticket URL: <https://trac.macports.org/ticket/54787#comment:1>
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