cmake: fix protobuf handling

cmake/OpenCVFindProtobuf.cmake

@@ -6,9 +6,15 @@ if(NOT WITH_PROTOBUF)
   return()
 endif()
 
-ocv_option(BUILD_PROTOBUF "Force to build libprotobuf from sources" ON)
+ocv_option(BUILD_PROTOBUF "Force to build libprotobuf runtime from sources" ON)
 ocv_option(PROTOBUF_UPDATE_FILES "Force rebuilding .proto files (protoc should be available)" OFF)
 
+# BUILD_PROTOBUF=OFF: Custom manual protobuf configuration (see find_package(Protobuf) for details):
+# - Protobuf_INCLUDE_DIR
+# - Protobuf_LIBRARY
+# - Protobuf_PROTOC_EXECUTABLE
+
+
 function(get_protobuf_version version include)
   file(STRINGS "${include}/google/protobuf/stubs/common.h" ver REGEX "#define GOOGLE_PROTOBUF_VERSION [0-9]+")
   string(REGEX MATCHALL "[0-9]+" ver ${ver})
@@ -19,7 +25,9 @@ function(get_protobuf_version version include)
 endfunction()
 
 if(BUILD_PROTOBUF)
+  ocv_assert(NOT PROTOBUF_UPDATE_FILES)
   add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/protobuf")
+  set(Protobuf_LIBRARIES "libprotobuf")
   set(HAVE_PROTOBUF TRUE)
 else()
   unset(Protobuf_VERSION CACHE)
@@ -44,10 +52,7 @@ else()
 
   if(Protobuf_FOUND)
     if(TARGET protobuf::libprotobuf)
-      add_library(libprotobuf INTERFACE IMPORTED)
-      set_target_properties(libprotobuf PROPERTIES
-        INTERFACE_LINK_LIBRARIES protobuf::libprotobuf
-      )
+      set(Protobuf_LIBRARIES "protobuf::libprotobuf")
     else()
       add_library(libprotobuf UNKNOWN IMPORTED)
       set_target_properties(libprotobuf PROPERTIES
@@ -56,21 +61,31 @@ else()
         INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "${Protobuf_INCLUDE_DIR}"
       )
       get_protobuf_version(Protobuf_VERSION "${Protobuf_INCLUDE_DIR}")
+      set(Protobuf_LIBRARIES "libprotobuf")
     endif()
     set(HAVE_PROTOBUF TRUE)
   endif()
 endif()
 
 if(HAVE_PROTOBUF AND PROTOBUF_UPDATE_FILES AND NOT COMMAND PROTOBUF_GENERATE_CPP)
-  find_package(Protobuf QUIET)
-  if(NOT COMMAND PROTOBUF_GENERATE_CPP)
-    message(FATAL_ERROR "PROTOBUF_GENERATE_CPP command is not available")
-  endif()
+  message(FATAL_ERROR "Can't configure protobuf dependency (BUILD_PROTOBUF=${BUILD_PROTOBUF} PROTOBUF_UPDATE_FILES=${PROTOBUF_UPDATE_FILES})")
 endif()
 
 if(HAVE_PROTOBUF)
   list(APPEND CUSTOM_STATUS protobuf)
+  if(NOT BUILD_PROTOBUF)
+    if(TARGET "${Protobuf_LIBRARIES}")
+      get_target_property(__location "${Protobuf_LIBRARIES}" IMPORTED_LOCATION_RELEASE)
+      if(NOT __location)
+        get_target_property(__location "${Protobuf_LIBRARIES}" IMPORTED_LOCATION)
+      endif()
+    elseif(Protobuf_LIBRARY)
+      set(__location "${Protobuf_LIBRARY}")
+    else()
+      set(__location "${Protobuf_LIBRARIES}")
+    endif()
+  endif()
   list(APPEND CUSTOM_STATUS_protobuf "    Protobuf:"
     BUILD_PROTOBUF THEN "build (${Protobuf_VERSION})"
-    ELSE "${Protobuf_LIBRARY} (${Protobuf_VERSION})")
+    ELSE "${__location} (${Protobuf_VERSION})")
 endif()

modules/dnn/CMakeLists.txt

@@ -66,6 +66,9 @@ ocv_warnings_disable(CMAKE_CXX_FLAGS
     -Winvalid-offsetof                 # Apple Clang (attr_value.pb.cc)
 )
 
+set(include_dirs "")
+set(libs "")
+
 if(PROTOBUF_UPDATE_FILES)
   file(GLOB proto_files "${CMAKE_CURRENT_LIST_DIR}/src/tensorflow/*.proto" "${CMAKE_CURRENT_LIST_DIR}/src/caffe/opencv-caffe.proto" "${CMAKE_CURRENT_LIST_DIR}/src/onnx/opencv-onnx.proto")
   set(PROTOBUF_GENERATE_CPP_APPEND_PATH ON) # required for tensorflow
@@ -76,10 +79,15 @@ else()
   set(fw_inc "${CMAKE_CURRENT_LIST_DIR}/misc/caffe" "${CMAKE_CURRENT_LIST_DIR}/misc/tensorflow" "${CMAKE_CURRENT_LIST_DIR}/misc/onnx")
 endif()
 
-set(include_dirs ${fw_inc})
+list(APPEND include_dirs ${fw_inc})
+list(APPEND libs ${Protobuf_LIBRARIES})
+if(NOT BUILD_PROTOBUF)
+  list(APPEND include_dirs ${Protobuf_INCLUDE_DIRS})
+endif()
+
 set(sources_options "")
 
-set(libs libprotobuf ${LAPACK_LIBRARIES})
+list(APPEND libs ${LAPACK_LIBRARIES})
 if(OPENCV_DNN_OPENCL AND HAVE_OPENCL)
   list(APPEND include_dirs ${OPENCL_INCLUDE_DIRS})
 else()