Add an option for Caffe2 to link with local protobuf. (#2306)

CMakeLists.txt

@@ -29,6 +29,9 @@ option(BUILD_DOCS "Build documentation" OFF)
 option(BUILD_CUSTOM_PROTOBUF "If set, build Caffe2's own protobuf under third_party" OFF)
 option(BUILD_PYTHON "Build Python binaries" ON)
 option(BUILD_SHARED_LIBS "Build libcaffe2.so" ON)
+cmake_dependent_option(
+    CAFFE2_LINK_LOCAL_PROTOBUF "If set, build protobuf inside libcaffe2.so." ON
+    "BUILD_SHARED_LIBS AND BUILD_CUSTOM_PROTOBUF" OFF)
 cmake_dependent_option(
     CAFFE2_USE_MSVC_STATIC_RUNTIME "Using MSVC static runtime libraries" ON
     "NOT BUILD_SHARED_LIBS" OFF)

caffe2/CMakeLists.txt

@@ -76,12 +76,38 @@ install(FILES ${PROJECT_BINARY_DIR}/caffe2/core/macros.h
 
 
 # ---[ List of libraries to link with
-
 add_library(caffe2_protos $<TARGET_OBJECTS:Caffe_PROTO> $<TARGET_OBJECTS:Caffe2_PROTO>)
 add_dependencies(caffe2_protos Caffe_PROTO Caffe2_PROTO)
 # TODO: enable using lite protobuf.
-target_link_libraries(caffe2_protos PUBLIC protobuf::libprotobuf)
+# If we are going to link protobuf locally inside caffe2 libraries, what we will do is
+# to create a helper static library that always contains libprotobuf source files, and
+# link the caffe2 related dependent libraries to it.
 target_include_directories(caffe2_protos INTERFACE $<INSTALL_INTERFACE:include>)
+# Reason for this public dependency is as follows:
+# (1) Strictly speaking, we should not expose any Protobuf related functions. We should
+#     only use function interfaces wrapped with our own public API, and link protobuf
+#     locally.
+# (2) However, currently across the Caffe2 codebase, we have extensive use of protobuf
+#     functionalities. For example, not only libcaffe2.so uses it, but also other
+#     binaries such as python extensions etc. As a result, we will have to have a
+#     transitive dependency to libprotobuf.
+#
+# Good thing is that, if we specify CAFFE2_LINK_LOCAL_PROTOBUF, then we do not need to
+# separately deploy protobuf binaries - libcaffe2.so will contain all functionalities
+# one needs. One can verify this via ldd.
+#
+# Todo item in the future includes:
+# (1) Properly define public API that do not directly depend on protobuf itself.
+# (2) expose the libprotobuf.a file for dependent libraries to link to.
+# (3) build libprotobuf with -fvisibility=hidden to avoid version conflicts.
+#
+# What it means for users/developers?
+# (1) Users: nothing affecting the users, other than the fact that CAFFE2_LINK_LOCAL_PROTOBUF
+#     avoids the need to deploy protobuf.
+# (2) Developers: if one simply uses core caffe2 functionality without using protobuf,
+#     nothing changes. If one has a dependent library that uses protobuf, then one needs to
+#     have the right protobuf version as well as linking to libprotobuf.a.
+target_link_libraries(caffe2_protos PUBLIC protobuf::libprotobuf)
 install(TARGETS caffe2_protos EXPORT Caffe2Targets DESTINATION lib)
 
 # Compile exposed libraries.

cmake/ProtoBuf.cmake

@@ -9,16 +9,33 @@ macro(custom_protobuf_find)
     # so we turn it off here.
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-deprecated-declarations")
   endif()
-  # If we are building Caffe2 as shared libs, we will also build protobuf as
-  # shared libs.
-  set(protobuf_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
+  if (${CAFFE2_LINK_LOCAL_PROTOBUF})
+    # If we are going to link protobuf locally, we will need to turn off
+    # shared libs build for protobuf.
+    set(protobuf_BUILD_SHARED_LIBS OFF)
+  else()
+    # If we are building Caffe2 as shared libs, we will also build protobuf as
+    # shared libs.
+    set(protobuf_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
+  endif()
   # We will make sure that protobuf and caffe2 uses the same msvc runtime.
   set(protobuf_MSVC_STATIC_RUNTIME ${CAFFE2_USE_MSVC_STATIC_RUNTIME})
   if (MSVC AND BUILD_SHARED_LIBS)
     add_definitions(-DPROTOBUF_USE_DLLS)
   endif()
+
+  if (${CAFFE2_LINK_LOCAL_PROTOBUF})
+    # We will need to build protobuf with -fPIC.
+    set(__caffe2_protobuf_cmake_fpic ${CMAKE_POSITION_INDEPENDENT_CODE})
+    set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+  endif()
+
   add_subdirectory(${PROJECT_SOURCE_DIR}/third_party/protobuf/cmake)
 
+  if (${CAFFE2_LINK_LOCAL_PROTOBUF})
+    set(CMAKE_POSITION_INDEPENDENT_CODE ${__caffe2_protobuf_cmake_fpic})
+  endif()
+
   # Protobuf "namespaced" target is only added post protobuf 3.5.1. As a
   # result, for older versions, we will manually add alias.
   if (NOT TARGET protobuf::libprotobuf)
@@ -32,9 +49,13 @@ endmacro()
 # coded BUILD_CUSTOM_PROTOBUF, we will hard code the use of custom protobuf
 # in the submodule.
 if (ANDROID OR IOS)
-  message(STATUS
-      "For Android and iOS cross compilation, I am automatically using "
-      "custom protobuf under third party.")
+  if (NOT ${BUILD_CUSTOM_PROTOBUF})
+    message(WARNING
+        "For Android and iOS cross compilation, I am automatically using "
+        "custom protobuf under third party. Note that this behavior may "
+        "change in the future, and you will need to specify "
+        "-DBUILD_CUSTOM_PROTOBUF=ON explicitly.")
+  endif()
   custom_protobuf_find()
   # Unfortunately, new protobuf does not support libprotoc and protoc
   # cross-compilation so we will need to exclude it.

cmake/Summary.cmake

@@ -20,6 +20,10 @@ function (caffe2_print_configuration_summary)
   message(STATUS "")
 
   message(STATUS "  BUILD_BINARY          : ${BUILD_BINARY}")
+  message(STATUS "  BUILD_CUSTOM_PROTOBUF : ${BUILD_CUSTOM_PROTOBUF}")
+  if (${CAFFE2_LINK_LOCAL_PROTOBUF})
+    message(STATUS "  CAFFE2_LINK_LOCAL_PROTOBUF : ${CAFFE2_LINK_LOCAL_PROTOBUF}")
+  endif()
   message(STATUS "  BUILD_DOCS            : ${BUILD_DOCS}")
   message(STATUS "  BUILD_PYTHON          : ${BUILD_PYTHON}")
   if (${BUILD_PYTHON})