[ET Trac] [Einstein Toolkit] #1667: ExternalLibraries/MPI should report error messages

[Einstein Toolkit]

#1667: ExternalLibraries/MPI should report error messages
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  Reporter:  hinder  |       Owner:                     
      Type:  defect  |      Status:  new                
  Priority:  minor   |   Milestone:                     
 Component:  Cactus  |     Version:  development version
Resolution:          |    Keywords:  MPI                
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Comment (by eschnett):

 These are the five cases that we need to cover:

 1. use an installed library at a specified location (set MPI_DIR to point
 to the library)
 2. always build it (set MPI_DIR to BUILD)
 3. do nothing, e.g. for Cray (set MPI_DIR to NONE)
 4. search for an installed library, build it not found (MPI_DIR is empty,
 i.e. this is the default)
 5. search for an installed library, fail if not found (MPI_DIR is
 NO_BUILD)

 I think this covers all interesting cases. The case NONE could also be
 handled by setting MPI_DIR and MPI_LIBS to a "fantasy" directory and
 "fantasy" library, but that's slightly inelegant.

 Note that the user's option settings are ignored in all cases except 1.

 I notice we're overloading the meaning of MPI_DIR. We could instead use a
 setting for MPI, which would be less confusing.

 We could also rename NO_BUILD to
 SEARCH_AND_IGNORE_USER_SETTINGS_AND_NEVER_BUILD. That options is a bit of
 an outlier, because we know in this case that we need to use a system
 library (so it's presumably a strange system), but we also expect Cactus
 to find this system library.

 We should probably abort with an error if user options are set and are
 ignored.

-- 
Ticket URL: <https://trac.einsteintoolkit.org/ticket/1667#comment:11>
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