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

@@ -6,3 +6,7 @@
 	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
+	branch = lts_2021_11_02

CMakeLists.txt

@@ -155,6 +155,14 @@ 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)
@@ -290,6 +298,11 @@ 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(${protobuf_SOURCE_DIR}/cmake/abseil-cpp.cmake)
+
 if (protobuf_BUILD_PROTOBUF_BINARIES)
   include(${protobuf_SOURCE_DIR}/cmake/libprotobuf-lite.cmake)
   if (NOT DEFINED protobuf_LIB_PROTOBUF_LITE)

cmake/abseil-cpp.cmake

@@ -0,0 +1,32 @@
+# Setup our dependency on Abseil.
+
+set(ABSL_PROPAGATE_CXX_STD ON)
+
+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/conformance.cmake

@@ -48,8 +48,11 @@ target_include_directories(
   conformance_cpp
   PUBLIC ${protobuf_SOURCE_DIR})
 
+target_include_directories(conformance_cpp PRIVATE ${ABSL_ROOT_DIR})
+
 target_link_libraries(conformance_test_runner ${protobuf_LIB_PROTOBUF})
 target_link_libraries(conformance_cpp ${protobuf_LIB_PROTOBUF})
+target_link_libraries(conformance_cpp ${protobuf_ABSL_USED_TARGETS})
 
 add_test(NAME conformance_cpp_test
   COMMAND ${CMAKE_CURRENT_BINARY_DIR}/conformance_test_runner

cmake/libprotobuf-lite.cmake

@@ -23,6 +23,7 @@ if(${CMAKE_SYSTEM_NAME} STREQUAL "Android")
   target_link_libraries(libprotobuf-lite PRIVATE log)
 endif()
 target_include_directories(libprotobuf-lite PUBLIC ${protobuf_SOURCE_DIR}/src)
+target_include_directories(libprotobuf-lite PRIVATE ${ABSL_ROOT_DIR})
 if(protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotobuf-lite
     PUBLIC  PROTOBUF_USE_DLLS

cmake/libprotobuf.cmake

@@ -26,6 +26,7 @@ if(${CMAKE_SYSTEM_NAME} STREQUAL "Android")
   target_link_libraries(libprotobuf PRIVATE log)
 endif()
 target_include_directories(libprotobuf PUBLIC ${protobuf_SOURCE_DIR}/src)
+target_include_directories(libprotobuf PRIVATE ${ABSL_ROOT_DIR})
 if(protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotobuf
     PUBLIC  PROTOBUF_USE_DLLS

cmake/libprotoc.cmake

@@ -16,6 +16,7 @@ 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

@@ -6,6 +6,7 @@ add_executable(protoc ${protoc_files} ${protobuf_version_rc_file})
 target_link_libraries(protoc
   libprotoc
   libprotobuf
+  ${protobuf_ABSL_USED_TARGETS}
 )
 add_executable(protobuf::protoc ALIAS protoc)
 

cmake/tests.cmake

@@ -78,7 +78,9 @@ endforeach(proto_file)
 
 add_library(protobuf-lite-test-common STATIC
   ${lite_test_util_srcs} ${lite_test_proto_files})
-target_link_libraries(protobuf-lite-test-common ${protobuf_LIB_PROTOBUF_LITE} GTest::gmock)
+target_include_directories(protobuf-lite-test-common PRIVATE ${ABSL_ROOT_DIR})
+target_link_libraries(protobuf-lite-test-common
+  ${protobuf_LIB_PROTOBUF_LITE} ${protobuf_ABSL_USED_TARGETS} GTest::gmock)
 
 set(common_test_files
   ${test_util_hdrs}
@@ -89,7 +91,9 @@ set(common_test_files
 
 add_library(protobuf-test-common STATIC
   ${common_test_files} ${tests_proto_files})
-target_link_libraries(protobuf-test-common ${protobuf_LIB_PROTOBUF} GTest::gmock)
+target_include_directories(protobuf-test-common PRIVATE ${ABSL_ROOT_DIR})
+target_link_libraries(protobuf-test-common
+  ${protobuf_LIB_PROTOBUF} ${protobuf_ABSL_USED_TARGETS} GTest::gmock)
 
 set(tests_files
   ${protobuf_test_files}