build: search protobuf using package config

protobuf v22.0 is packaged by in modern distros, but the FindProtobuf
module shipped along with CMake fails to add the new dependency of
abseil libraries to the linkage. so Seastar fail to link if compiled
with protobuf v22.0 and up.

so, in this change, we try to find protobuf using its CMake package
configuration first, and fall back to the Findprotobuf module. because
find_package() runs in module mode, and fall back to the config mode,
and the protobuf's CMake configuration adds the abseil libraries
dependency correctly, we need to use config mode first.

Refs scylladb#2113
Signed-off-by: Kefu Chai <[email protected]>

cmake/SeastarDependencies.cmake

@@ -95,7 +95,7 @@ macro (seastar_find_dependencies)
  GnuTLS
  LibUring
  LinuxMembarrier
- Protobuf
+ # Protobuf is searched manually.
  Sanitizers
  SourceLocation
  StdAtomic
@@ -130,8 +130,6 @@ macro (seastar_find_dependencies)
  VERSION 1.7.3)
  seastar_set_dep_args (GnuTLS REQUIRED
  VERSION 3.3.26)
- seastar_set_dep_args (Protobuf REQUIRED
- VERSION 2.5.0)
  seastar_set_dep_args (LibUring
  VERSION 2.0
  OPTION ${Seastar_IO_URING})
@@ -152,4 +150,23 @@ macro (seastar_find_dependencies)
  find_package ("${third_party}" ${_seastar_dep_args_${third_party}})
  endif ()
  endforeach ()
+
+ # workaround for https://gitlab.kitware.com/cmake/cmake/-/issues/25079
+ # since protobuf v22.0, it started using abseil, see
+ # https://github.com/protocolbuffers/protobuf/releases/tag/v22.0 .
+ # but due to https://gitlab.kitware.com/cmake/cmake/-/issues/25079,
+ # CMake's FindProtobuf does add this linkage yet. fortunately,
+ # ProtobufConfig.cmake provided by protobuf defines this linkage. so we try
+ # the CMake package configuration file first, and fall back to CMake's
+ # FindProtobuf module.
+ find_package (Protobuf 2.5.0
+ QUIET CONFIG)
+ if (Protobuf_FOUND)
+ # do it again, so the message is printed when the package is found
+ find_package(Protobuf 2.5.0
+ CONFIG REQUIRED)
+ else ()
+ find_package(Protobuf 2.5.0
+ REQUIRED)
+ endif ()
 endmacro ()