[GitHub] [arrow] westonpace commented on pull request #12457: ARROW-15709: [C++] Compilation of ARROW_ENGINE fails if doing an "inline" build

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westonpace commented on pull request #12457:
URL: https://github.com/apache/arrow/pull/12457#issuecomment-1043485424


   @kou @pitrou We could use input on this approach.  The Substrait integration is a little unique because the `.proto` files themselves are not owned by us.  We could fork a copy of the proto files but that would make keeping up with updated versions a bit tricky.
   
   Protobuf is also a bit tricky because the version of the code generation tool (protoc) must match the version of the linked protobuf library exactly (this is different from flatbuffers which doesn't require a runtime dependency).
   
   Previously we were downloading the proto files using git and running protoc within the engine directory.  This had some negative side effects (https://github.com/apache/arrow/pull/12454) mainly due to our use of git.  Even beyond git however it was rather strange that this was resulting in a call to `externalproject_add` outside of `ThirdPartyToolchain.cmake` so @jvanstraten and I agreed it might be clear to treat Substrait as a "build-from-source only" third party dependency.  This PR is @jvanstraten 's attempt to make that work but it is also odd as it requires a call to `add_arrow_lib` inside of `ThirdPartyToolchain.cmake`.
   
   Any suggestions on a better approach or feedback on our current approach would be very much welcomed.


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