[MacPorts] #13509: Support .dmg disk images as distfiles with new use_dmg option

[MacPorts]

#13509: Support .dmg disk images as distfiles with new use_dmg option
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  Reporter:  ryandesign at macports.org  |       Owner:  ryandesign at macports.org   
      Type:  enhancement              |      Status:  new                       
  Priority:  Normal                   |   Milestone:  MacPorts base enhancements
 Component:  base                     |     Version:  1.6.0                     
Resolution:                           |    Keywords:                            
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Comment (by ryandesign at macports.org):

 Bear in mind that my knowledge of Tcl and MacPorts base is still very
 limited, so I don't know how to wrap what I've done in a Mac OS X platform
 test. Is there existing code in other parts of base that I could look at?

 I think it would be ok to commit the functionality now, and add support
 for non-Mac platforms--or beautify the error messages on those platforms--
 later. I wouldn't expect dmg-based extraction to be used on platforms
 other than Mac OS X anyway, since dmg is a Mac OS X format. Or rather, I
 wouldn't expect developers to distribute things on a dmg unless they
 intended it to be used exclusively on Mac OS X. To that end, I would
 expect any port using dmg-based extraction to declare only "`platforms
 macosx`", and I would further expect MacPorts to prevent installation of
 the port on platforms that are not macosx.

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Ticket URL: <https://trac.macosforge.org/projects/macports/ticket/13509#comment:2>
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