[MacPorts] #16373: base should maintain a persistent working copy for all supported VCS fetches

[MacPorts]

#16373: base should maintain a persistent working copy for all supported VCS
fetches
---------------------------+-------------------------------
  Reporter:  ryandesign@…  |      Owner:  larryv@…
      Type:  enhancement   |     Status:  assigned
  Priority:  Normal        |  Milestone:  MacPorts Future
 Component:  base          |    Version:  1.7.0
Resolution:                |   Keywords:  performance fetch
      Port:                |
---------------------------+-------------------------------

Comment (by mojca@…):

 Can you please take a look at the attached `Portfile` for `xchm`? (Never
 mind the fact that it leads to a build error later on.)

 I copy-pasted some code from `portutil.tcl` and `portfetch.tcl`. This is
 the relevant part:
 {{{
 checksums           rmd160  ... \
                     sha256  ...

 use_xz              yes

 pre-fetch {
     if {![file exists ${distpath}/${distname}${extract.suffix}]} {
         set git_dir ${workpath}/git

         # clone the git repository
         set options "-q"
         set cmdstring "${git.cmd} clone $options ${git.url} ${git_dir}
 2>&1"
         ui_debug "Executing: $cmdstring"
         if {[catch {system $cmdstring} result]} {
             return -code error [msgcat::mc "Git clone failed"]
         }

         # create a tarball
         set xz [findBinary xz ${portutil::autoconf::xz_path}]
         set cmdstring "${git.cmd} archive ${git.branch}
 --prefix=${distname}/ | ${xz} > ${distpath}/${distname}${extract.suffix}"
         ui_debug "Executing: $cmdstring"
         if {[catch {system -W ${git_dir} ${cmdstring}} result]} {
             return -code error [msgcat::mc "Git archive failed"]
         }
     }
 }
 }}}

 It works like explained months ago:
   * In case the sources are missing, it will clone the repository and make
 a tarball out of it and store it to `${distpath}`.
   * (In extract phase the sources are extracted from the tarball, they are
 not taken from the git repository.)
   * Next time when the sources are needed, it will simply extract
 everything from the tarball, no need for a new clone and for consumption
 of a precious bandwidth.
   * I assume that the buildbots / other servers would then also
 automatically keep a mirror of these tarballs, so it would no longer be a
 problem if the git repository goes offline.

 However, this code should go to a portgroup (or possibly to core,
 depending on where the other code resides) and I'm not too comfortable
 writing the code for that yet.

 Can someone please provide some feedback about this approach and in case
 that the approach sounds reasonable, possibly help me rewrite the code?

-- 
Ticket URL: <https://trac.macports.org/ticket/16373#comment:32>
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