cmake: Fix test.capnp.{h,c++} file generation race

c++/src/capnp/CMakeLists.txt

@@ -204,6 +204,13 @@ if(BUILD_TESTING)
 
   capnp_generate_cpp(test_capnp_cpp_files test_capnp_h_files ${test_capnp_files})
 
+  # TODO(cleanup): capnp-tests and capnp-heavy-tests both depend on the test.capnp output files. In
+  #   a parallel Makefile-based build (maybe others?), they can race and cause the custom capnp
+  #   command in capnp_generate_cpp() to run twice. To get around this I'm using a custom target to
+  #   force CMake to generate race-free Makefiles. Remove this garbage when we move to a
+  #   target-based capnp_generate() command, as that will make CMake do the right thing by default.
+  add_custom_target(test_capnp DEPENDS ${test_capnp_cpp_files} ${test_capnp_h_files})
+
   if(CAPNP_LITE)
     set(test_libraries capnp kj-test kj)
   else()
@@ -229,6 +236,7 @@ if(BUILD_TESTING)
     ${test_capnp_h_files}
   )
   target_link_libraries(capnp-tests ${test_libraries})
+  add_dependencies(capnp-tests test_capnp)
   add_dependencies(check capnp-tests)
   add_test(NAME capnp-tests-run COMMAND capnp-tests)
 
@@ -261,6 +269,7 @@ if(BUILD_TESTING)
       )
     endif()
 
+    add_dependencies(capnp-heavy-tests test_capnp)
     add_dependencies(check capnp-heavy-tests)
     add_test(NAME capnp-heavy-tests-run COMMAND capnp-heavy-tests)