feat(3p/nix): Add protobuf & gRPC dependencies and generate sources

Adds dependencies on the gRPC & protobuf libraries, and implements Nix
code to generate the C++ sources from the included proto definitions.

This is theoretically supported via CMake, but practically doesn't
work and I don't care to debug why.

Doing it like this lets us instead add a CMake library target for our
proto definitions based on the sources generated by Nix.

Pros:
  * no need to deal with the gRPC CMake mess
  * it works!

Cons: * iteration requires nix-shell restart
Change-Id: Ie1fe9807fc96c49cb8f7161ba59d093456062b15
Reviewed-on: https://cl.tvl.fyi/c/depot/+/927
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/CMakeLists.txt

@@ -5,6 +5,7 @@
 # this location and this setup mimics that (with the exception of the
 # various Nix libraries).
 
+add_subdirectory(proto)
 add_subdirectory(libutil)
 add_subdirectory(libstore)
 add_subdirectory(libmain)

third_party/nix/src/proto/CMakeLists.txt

@@ -0,0 +1,37 @@
+# -*- mode: cmake; -*-
+#
+# The proto generation happens outside of CMake and the path to the
+# generated files is passed in via the environment variable
+# $NIX_PROTO_SRCS.
+#
+# This configuration defines a library target that compiles these
+# sources and makes the headers available.
+
+add_library(nixproto SHARED)
+set_property(TARGET nixproto PROPERTY CXX_STANDARD 17)
+
+set(HEADER_FILES
+  $ENV{NIX_PROTO_SRCS}/libproto/worker.grpc.pb.h
+  $ENV{NIX_PROTO_SRCS}/libproto/worker.pb.h
+)
+
+target_sources(nixproto
+  PUBLIC
+    ${HEADER_FILES}
+
+  PRIVATE
+    $ENV{NIX_PROTO_SRCS}/libproto/worker.grpc.pb.cc
+    $ENV{NIX_PROTO_SRCS}/libproto/worker.pb.cc
+)
+
+target_link_libraries(nixproto
+  gRPC::grpc++_reflection
+  protobuf::libprotobuf
+)
+
+target_include_directories(nixproto
+  INTERFACE $ENV{NIX_PROTO_SRCS}
+)
+
+INSTALL(FILES ${HEADER_FILES} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/nix/libproto)
+INSTALL(TARGETS nixproto DESTINATION ${CMAKE_INSTALL_LIBDIR})