fix: better support for cross-compilation

[coryan]

Allow overrides for the protobuf::protoc and gRPC::grpc_cpp_plugin targets. On cross-compilation builds the targets point to the binaries for the target, but we need binaries that can run on the host build.

Fixes #11779

---

This change is 

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0c7f077) 93.79% compared to head (bac7581) 93.79%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11782   +/-   ##
=======================================
  Coverage   93.79%   93.79%           
=======================================
  Files        1834     1834           
  Lines      165867   165867           
=======================================
+ Hits       155574   155576    +2     
+ Misses      10293    10291    -2     

see 8 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[h-vetinari]

It should be safe even for cross-compilation to locate grpc-cpp wherever Protobuf_PROTOC_EXECUTABLE is. This is what we ended up doing for open-telemetry, for example.

That would save you a magical, private variable, but to be clear: that's a perfectly fine solution as well.

[coryan]

That seems intriguing. If I am following the suggestion one would install protoc and grpc_cpp_plugin in $PREFIX/bin or something similar? Unfortunately vcpkg installs protoc in $PREFIX/tools/protobuf/protoc and the gRPC plugins in $PREFIX/tools/grpc/*.

I can give that a try once I get something working across all our builds.

[h-vetinari]

If I am following the suggestion one would install protoc and grpc_cpp_plugin in $PREFIX/bin or something similar?

To be honest I don't know how exactly CMake does it, but the idea is to search grpc_cpp_plugin relative to protoc. For cross-compilation (in conda-forge), the latter will be in $BUILD_PREFIX/bin, which will similarly be correct for grpc, while for native compilation, protoc will be in $PREFIX/bin and again that suits grpc too.

[h-vetinari]

Unfortunately vcpkg installs protoc in $PREFIX/tools/protobuf/protoc and the gRPC plugins in $PREFIX/tools/grpc/*.

Yeah, that might mean it's not a directly suitable idea for you here. 😑

I don't know how the path traversal in find_program(<something> HINTS ${PATH} ...) would look like. Perhaps it's still salvageable, but we'd have to try to find out...

[coryan]

I don't know how the path traversal in find_program(<something> HINTS ${PATH} ...) would look like. Perhaps it's still salvageable, but we'd have to try to find out...

Seems like we are getting to diminishing returns pretty quickly. The default works for most builds, and a CMake flag should work in all (I hope) other cases. Seems like each package manager would require a different HINT entry and at the point we might as well keep the setting in the package manager recipe.

[h-vetinari]

My guess was that CMake will exhaustively search the entire folder hierarchy under HINT, and then the hint of "start with protoc" would also have been correct for vcpkg. But I don't care about this very much, it was just a thought to save you some complexity