Update CMake configuration to add a dependency on Abseil

.bazelignore

@@ -1,4 +1,5 @@
 # These are fetched as external repositories.
+third_party/abseil-cpp
 third_party/benchmark
 third_party/googletest
 _build/

.gitmodules

@@ -5,3 +5,7 @@
 	path = third_party/googletest
 	url = https://github.com/google/googletest.git
 	ignore = dirty
+[submodule "third_party/abseil-cpp"]
+	path = third_party/abseil-cpp
+	url = https://github.com/abseil/abseil-cpp.git
+	branch = lts_2021_11_02

CMakeLists.txt

@@ -162,6 +162,15 @@ file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/cmaketest.map)
 
 find_package(Threads REQUIRED)
 
+# We can install dependencies from submodules if we're running
+# CMake v3.13 or newer.
+if(CMAKE_VERSION VERSION_LESS 3.13)
+  set(_protobuf_INSTALL_SUPPORTED_FROM_MODULE OFF)
+else()
+  set(_protobuf_INSTALL_SUPPORTED_FROM_MODULE ON)
+endif()
+
+
 set(_protobuf_FIND_ZLIB)
 if (protobuf_WITH_ZLIB)
   find_package(ZLIB)
@@ -301,6 +310,10 @@ if (protobuf_UNICODE)
   add_definitions(-DUNICODE -D_UNICODE)
 endif (protobuf_UNICODE)
 
+set(protobuf_ABSL_PROVIDER "module" CACHE STRING "Provider of absl library")
+set_property(CACHE protobuf_ABSL_PROVIDER PROPERTY STRINGS "module" "package")
+
+include(${protobuf_SOURCE_DIR}/cmake/abseil-cpp.cmake)
 include(${protobuf_SOURCE_DIR}/cmake/libprotobuf-lite.cmake)
 include(${protobuf_SOURCE_DIR}/cmake/libprotobuf.cmake)
 if (protobuf_BUILD_LIBPROTOC)

Makefile.am

@@ -1403,6 +1403,7 @@ EXTRA_DIST = $(@DIST_LANG@_EXTRA_DIST)              \
   BUILD                                             \
   WORKSPACE                                         \
   CMakeLists.txt                                    \
+  cmake/abseil-cpp.cmake                           \
   cmake/CMakeLists.txt                              \
   cmake/README.md                                   \
   cmake/conformance.cmake                           \

cmake/abseil-cpp.cmake

@@ -0,0 +1,28 @@
+if(protobuf_ABSL_PROVIDER STREQUAL "module")
+  if(NOT ABSL_ROOT_DIR)
+    set(ABSL_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/abseil-cpp)
+  endif()
+  if(EXISTS "${ABSL_ROOT_DIR}/CMakeLists.txt")
+    if(protobuf_INSTALL)
+      # When protobuf_INSTALL is enabled and Abseil will be built as a module,
+      # Abseil will be installed along with protobuf for convenience.
+      set(ABSL_ENABLE_INSTALL ON)
+    endif()
+    add_subdirectory(${ABSL_ROOT_DIR} third_party/abseil-cpp)
+  else()
+    message(WARNING "protobuf_ABSL_PROVIDER is \"module\" but ABSL_ROOT_DIR is wrong")
+  endif()
+  if(protobuf_INSTALL AND NOT _protobuf_INSTALL_SUPPORTED_FROM_MODULE)
+    message(WARNING "protobuf_INSTALL will be forced to FALSE because protobuf_ABSL_PROVIDER is \"module\" and CMake version (${CMAKE_VERSION}) is less than 3.13.")
+    set(protobuf_INSTALL FALSE)
+  endif()
+elseif(protobuf_ABSL_PROVIDER STREQUAL "package")
+  # Use "CONFIG" as there is no built-in cmake module for absl.
+  find_package(absl REQUIRED CONFIG)
+endif()
+set(_protobuf_FIND_ABSL "if(NOT TARGET absl::strings)\n  find_package(absl CONFIG)\nendif()")
+
+set(protobuf_ABSL_USED_TARGETS
+  absl::strings
+  absl::strings_internal
+)

cmake/libprotobuf.cmake

@@ -116,6 +116,9 @@ if(protobuf_HAVE_LD_VERSION_SCRIPT)
     LINK_DEPENDS ${protobuf_SOURCE_DIR}/src/libprotobuf.map)
 endif()
 target_link_libraries(libprotobuf PRIVATE ${CMAKE_THREAD_LIBS_INIT})
+target_include_directories(libprotobuf
+ PRIVATE ${ABSL_ROOT_DIR}
+)
 if(protobuf_WITH_ZLIB)
   target_link_libraries(libprotobuf PRIVATE ${ZLIB_LIBRARIES})
 endif()

cmake/libprotoc.cmake

@@ -131,6 +131,9 @@ if(protobuf_HAVE_LD_VERSION_SCRIPT)
     LINK_DEPENDS ${protobuf_SOURCE_DIR}/src/libprotoc.map)
 endif()
 target_link_libraries(libprotoc PRIVATE libprotobuf)
+target_include_directories(libprotoc
+ PRIVATE ${ABSL_ROOT_DIR}
+)
 if(protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotoc
     PUBLIC  PROTOBUF_USE_DLLS

cmake/protobuf-config.cmake.in

@@ -3,6 +3,7 @@ include("${CMAKE_CURRENT_LIST_DIR}/protobuf-options.cmake")
 
 # Depend packages
 @_protobuf_FIND_ZLIB@
+@_protobuf_FIND_ABSL@
 
 # Imported targets
 include("${CMAKE_CURRENT_LIST_DIR}/protobuf-targets.cmake")

cmake/protoc.cmake

@@ -9,7 +9,11 @@ set(protoc_rc_files
 endif()
 
 add_executable(protoc ${protoc_files} ${protoc_rc_files})
-target_link_libraries(protoc libprotoc libprotobuf)
+target_link_libraries(protoc
+  libprotoc
+  libprotobuf
+  ${protobuf_ABSL_USED_TARGETS}
+)
 add_executable(protobuf::protoc ALIAS protoc)
 
 set_target_properties(protoc PROPERTIES

cmake/tests.cmake

@@ -21,6 +21,7 @@ else()
   set(googlemock_source_dir "${protobuf_SOURCE_DIR}/third_party/googletest/googlemock")
   set(googletest_source_dir "${protobuf_SOURCE_DIR}/third_party/googletest/googletest")
   include_directories(
+    ${ABSL_ROOT_DIR}
     ${googlemock_source_dir}
     ${googletest_source_dir}
     ${googletest_source_dir}/include
@@ -255,7 +256,14 @@ if (MSVC)
     /wd4146 # unary minus operator applied to unsigned type, result still unsigned
   )
 endif()
-target_link_libraries(tests protobuf-lite-test-common protobuf-test-common libprotoc libprotobuf GTest::gmock_main)
+target_link_libraries(tests
+  protobuf-lite-test-common
+  protobuf-test-common
+  libprotoc
+  libprotobuf
+  GTest::gmock_main
+  ${protobuf_ABSL_USED_TARGETS}
+)
 
 set(test_plugin_files
   ${protobuf_SOURCE_DIR}/src/google/protobuf/compiler/mock_code_generator.cc
@@ -265,7 +273,12 @@ set(test_plugin_files
 )
 
 add_executable(test_plugin ${test_plugin_files})
-target_link_libraries(test_plugin libprotoc libprotobuf GTest::gmock)
+target_link_libraries(test_plugin
+  libprotoc
+  libprotobuf
+  GTest::gmock
+  ${protobuf_ABSL_USED_TARGETS}
+)
 
 set(lite_test_files
   ${protobuf_SOURCE_DIR}/src/google/protobuf/lite_unittest.cc

src/google/protobuf/message.h

@@ -126,7 +126,6 @@
 #include <google/protobuf/map.h>  // TODO(b/211442718): cleanup
 #include <google/protobuf/message_lite.h>
 
-
 // Must be included last.
 #include <google/protobuf/port_def.inc>