[MacPorts] #63085: clang-12: compiler-rt build sometimes fails parallel building

[MacPorts]

#63085: clang-12: compiler-rt build sometimes fails parallel building
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  Reporter:  blair     |      Owner:  kencu
      Type:  defect    |     Status:  assigned
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  clang-12  |
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Comment (by kencu):

 It appears that when running a custom_command, cmake does not use
 dependency tracking and collisions can happen.

 "Do not list the output in more than one independent target that may build
 in parallel or the two instances of the rule may conflict (instead use the
 add_custom_target() command to drive the command and make the other
 targets depend on that one)."

 https://cmake.org/cmake/help/v3.19/command/add_custom_command.html

 The workaround is to add_custom_target a dependency on the output of the
 custom command.

 https://gist.github.com/socantre/7ee63133a0a3a08f3990

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Ticket URL: <https://trac.macports.org/ticket/63085#comment:10>
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