cmake: EXTERNAL_MPGEN cleanups #147

1. 
   ryanofsky commented at 6:01 pm on February 3, 2025: collaborator

   Suggested by hebasto in https://github.com/chaincodelabs/libmultiprocess/pull/142#pullrequestreview-2588097796
2. cmake: EXTERNAL_MPGEN cleanups

   Suggested by Hennadii Stepanov <32963518+hebasto@users.noreply.github.com> in
   https://github.com/chaincodelabs/libmultiprocess/pull/142#pullrequestreview-2588097796

   21b92b69c9
3. ryanofsky merged this on Feb 3, 2025
4. ryanofsky closed this on Feb 3, 2025

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5. in CMakeLists.txt:27 in 21b92b69c9

   23@@ -24,7 +24,7 @@ if(Libmultiprocess_ENABLE_CLANG_TIDY)
   24   set(CMAKE_CXX_CLANG_TIDY "${CLANG_TIDY_EXECUTABLE}")
   25 endif()
   26 
   27-set(EXTERNAL_MPGEN "" CACHE STRING "Use the supplied mpgen binary rather than the one built internally")
   

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   theuni commented at 6:12 pm on February 3, 2025:

   Hmm, I purposefully avoided namespacing this because I was assuming that parent projects would always want to expose the same option and have it be automatically inherited here. Now they’ll have to map it.

   Not a big deal to me either way, I guess.

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   ryanofsky commented at 8:07 pm on February 5, 2025:

   For reference, code which sets this in depends is implemented in https://github.com/bitcoin/bitcoin/commit/99b7b857d3b8b9ce9bd7501e2480583369740c55 from https://github.com/bitcoin/bitcoin/pull/31741 and seems to work reasonably well

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   theuni commented at 8:34 pm on February 5, 2025:

   For depends, sure, it doesn’t matter. But when doing a non-depends cross-build, ccmake shows:

   0CAPNPC_CXX_EXECUTABLE
   1CAPNP_EXECUTABLE
   2...
   3Libmultiprocess_EXTERNAL_MPGEN
   

   which is rather inconsistent and non-obvious to builders imo. Seems to me Capnp has it right. For a Core, I wouldn’t expect to see that namespace.

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   ryanofsky commented at 8:55 pm on February 5, 2025:

   I see. This suggestion just came from https://github.com/chaincodelabs/libmultiprocess/pull/142#discussion_r1938116078, and I’m happy to rename this to something else or make a different change. Given the other names maybe MPGEN_EXECUTABLE would be a good name for this.

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   ryanofsky commented at 8:55 pm on February 5, 2025:

   @hebasto might have more ideas too.

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   theuni commented at 3:15 pm on February 6, 2025:

   I see. This suggestion just came from #142 (comment), and I’m happy to rename this to something else or make a different change. Given the other names maybe MPGEN_EXECUTABLE would be a good name for this.

   Fwiw, I originally picked EXTERNAL_CAPNP to mirror (what I thought was) the way Capnp handles it. But I see now that Capnp’s is just a bool and CAPNP_EXECUTABLE is the actual path variable. So ACK to MPGEN_EXECUTABLE instead if we go down this path.

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   hebasto commented at 3:20 pm on February 6, 2025:

   So ACK to MPGEN_EXECUTABLE instead if we go down this path.

   So do I.

6. in cmake/TargetCapnpSources.cmake:70 in 21b92b69c9

   68     if(NOT EXISTS "${MPGEN_BINARY}")
   69-      message(FATAL_ERROR "EXTERNAL_MPGEN: \"${MPGEN_BINARY}\" does not exist.")
   70+      message(FATAL_ERROR "Libmultiprocess_EXTERNAL_MPGEN: \"${MPGEN_BINARY}\" does not exist.")
   71     endif()
   72   elseif(TARGET Libmultiprocess::multiprocess)
   73-    set(MPGEN_BINARY $<TARGET_FILE:Libmultiprocess::mpgen>)
   

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   theuni commented at 6:15 pm on February 3, 2025:

   I attempted this a bunch of different ways, and with some approach this was necessary, but I forget why. I assume this is fine, thanks for simplifying.
7. 
   theuni commented at 6:16 pm on February 3, 2025: contributor

   Thank you for the cleanups!
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