Modernizing the gflags dependency in cmake.

Summary:
Historically, for interface dependent libraries (glog, gflags and protobuf), exposing them in Caffe2Config.cmake is usually difficult.

New versions of glog and gflags ship with new-style cmake targets, so one does not need to use variables. New-style targets also make it easier for people to depend on them in installed config files.

This diff modernizes the gflags library, and still provides a fallback path if the installed gflags does not have cmake config files coming with it.

It does change one behavior of the build process though - when one specifies -DUSE_GFLAGS=ON but gflags cannot be found, the old script automatically turns it off but the new script crashes, forcing the user to specify USE_GFLAGS=OFF.
Closes https://github.com/caffe2/caffe2/pull/1819

Differential Revision: D6826604

Pulled By: Yangqing

fbshipit-source-id: 210f3926f291c8bfeb24eb9671e5adfcbf8cf7fe

CMakeLists.txt

@@ -193,7 +193,7 @@ if (USE_GLOG)
   list(APPEND CAFFE2_INTERFACE_LIBS ${GLOG_LIBRARIES})
 endif()
 if (USE_GFLAGS)
-  list(APPEND CAFFE2_INTERFACE_LIBS ${GFLAGS_LIBRARIES})
+  list(APPEND CAFFE2_INTERFACE_LIBS gflags)
 endif()
 if (NOT CAFFE2_USE_CUSTOM_PROTOBUF)
   list(APPEND CAFFE2_INTERFACE_LIBS ${PROTOBUF_LIBRARIES})
@@ -232,6 +232,10 @@ if (BUILD_SHARED_LIBS)
       ${PROJECT_BINARY_DIR}/Caffe2Config.cmake
       DESTINATION share/cmake/Caffe2
       COMPONENT dev)
+  install(FILES
+      ${PROJECT_SOURCE_DIR}/cmake/public/gflags.cmake
+      DESTINATION share/cmake/Caffe2/public
+      COMPONENT dev)
   install(EXPORT Caffe2Targets DESTINATION share/cmake/Caffe2
       FILE Caffe2Targets.cmake
       COMPONENT dev)

cmake/Caffe2Config.cmake.in

@@ -10,6 +10,19 @@ set(CAFFE2_VERSION_MINOR @CAFFE2_VERSION_MINOR@)
 set(CAFFE2_VERSION_PATCH @CAFFE2_VERSION_PATCH@)
 set(CAFFE2_VERSION "@CAFFE2_VERSION@")
 
+# Depending on whether Caffe2 uses gflags during compile time or
+# not, invoke gflags.
+if (@USE_GFLAGS@)
+  include("${CMAKE_CURRENT_LIST_DIR}/public/gflags.cmake")
+  if (NOT TARGET gflags)
+    message(FATAL_ERROR
+        "Your installed Caffe2 version uses gflags but the gflags library "
+        "cannot be found. Did you accidentally remove it, or have you set "
+        "the right CMAKE_PREFIX_PATH and/or GFLAGS_ROOT_DIR? If you do not "
+        "have gflags, you will need to install gflags and set the library "
+        "path accordingly.")
+  endif()
+endif()
 
 # import targets
 include ("${CMAKE_CURRENT_LIST_DIR}/Caffe2Targets.cmake")

cmake/Dependencies.cmake

@@ -84,15 +84,14 @@ endif()
 
 # ---[ gflags
 if(USE_GFLAGS)
-  find_package(GFlags)
-  if(GFLAGS_FOUND)
+  include(cmake/public/gflags.cmake)
+  if (TARGET gflags)
     set(CAFFE2_USE_GFLAGS 1)
-    caffe2_include_directories(${GFLAGS_INCLUDE_DIRS})
-    list(APPEND Caffe2_DEPENDENCY_LIBS ${GFLAGS_LIBRARIES})
+    list(APPEND Caffe2_DEPENDENCY_LIBS gflags)
   else()
     message(WARNING
-        "gflags is not found. Caffe2 will build without gflags support but it "
-        "is strongly recommended that you install gflags. Suppress this "
+        "gflags is not found. Caffe2 will build without gflags support but "
+        "it is strongly recommended that you install gflags. Suppress this "
         "warning with -DUSE_GFLAGS=OFF")
     set(USE_GFLAGS OFF)
   endif()

cmake/Modules/FindGFlags.cmake

@@ -1,38 +0,0 @@
-# - Try to find GFLAGS
-#
-# The following variables are optionally searched for defaults
-#  GFLAGS_ROOT_DIR:            Base directory where all GFLAGS components are found
-#
-# The following are set after configuration is done:
-#  GFLAGS_FOUND
-#  GFLAGS_INCLUDE_DIRS
-#  GFLAGS_LIBRARIES
-#  GFLAGS_LIBRARYRARY_DIRS
-
-include(FindPackageHandleStandardArgs)
-
-set(GFLAGS_ROOT_DIR "" CACHE PATH "Folder contains Gflags")
-
-# We are testing only a couple of files in the include directories
-if(NOT WIN32)
-    find_path(GFLAGS_INCLUDE_DIR gflags/gflags.h
-        PATHS ${GFLAGS_ROOT_DIR})
-endif()
-
-if(MSVC)
-    find_package(gflags NO_MODULE)
-    set(GFLAGS_LIBRARY ${gflags_LIBRARIES})
-else()
-    find_library(GFLAGS_LIBRARY gflags)
-endif()
-
-find_package_handle_standard_args(GFlags DEFAULT_MSG GFLAGS_INCLUDE_DIR GFLAGS_LIBRARY)
-
-
-if(GFLAGS_FOUND)
-    set(GFLAGS_INCLUDE_DIRS ${GFLAGS_INCLUDE_DIR})
-    set(GFLAGS_LIBRARIES ${GFLAGS_LIBRARY})
-    message(STATUS "Found gflags  (include: ${GFLAGS_INCLUDE_DIR}, library: ${GFLAGS_LIBRARY})")
-    mark_as_advanced(GFLAGS_LIBRARY_DEBUG GFLAGS_LIBRARY_RELEASE
-                     GFLAGS_LIBRARY GFLAGS_INCLUDE_DIR GFLAGS_ROOT_DIR)
-endif()

cmake/public/gflags.cmake

@@ -0,0 +1,67 @@
+# ---[ gflags
+find_package(gflags)
+
+if (TARGET gflags)
+  message(STATUS "Found gflags with new-style gflags target.")
+elseif(GFLAGS_FOUND)
+  message(STATUS "Found gflags with old-style gflag starget.")
+  add_library(gflags UNKNOWN IMPORTED)
+  set_property(
+      TARGET gflags PROPERTY IMPORTED_LOCATION ${GFLAGS_LIBRARY})
+  set_property(
+      TARGET gflags PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+      ${GFLAGS_INCLUDE_DIR})
+else()
+  message(STATUS "Cannot find gflags with config files. Using legacy find.")
+
+  # - Try to find GFLAGS in the legacy way.
+  #
+  # The following variables are optionally searched for defaults
+  #  GFLAGS_ROOT_DIR:            Base directory where all GFLAGS components are found
+  #
+  # The following are set after configuration is done:
+  #  GFLAGS_FOUND
+  #  GFLAGS_INCLUDE_DIRS
+  #  GFLAGS_LIBRARIES
+  #  GFLAGS_LIBRARYRARY_DIRS
+  include(FindPackageHandleStandardArgs)
+  set(GFLAGS_ROOT_DIR "" CACHE PATH "Folder contains Gflags")
+
+  # We are testing only a couple of files in the include directories
+  if(NOT WIN32)
+      find_path(GFLAGS_INCLUDE_DIR gflags/gflags.h
+          PATHS ${GFLAGS_ROOT_DIR})
+  endif()
+
+  if(MSVC)
+      find_package(gflags NO_MODULE)
+      set(GFLAGS_LIBRARY ${gflags_LIBRARIES})
+  else()
+      find_library(GFLAGS_LIBRARY gflags)
+  endif()
+
+  find_package_handle_standard_args(
+      gflags DEFAULT_MSG GFLAGS_INCLUDE_DIR GFLAGS_LIBRARY)
+
+  if(GFLAGS_FOUND)
+    message(
+        STATUS
+        "Found gflags  (include: ${GFLAGS_INCLUDE_DIR}, "
+        "library: ${GFLAGS_LIBRARY})")
+    add_library(gflags UNKNOWN IMPORTED)
+    set_property(
+        TARGET gflags PROPERTY IMPORTED_LOCATION ${GFLAGS_LIBRARY})
+    set_property(
+        TARGET gflags PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+        ${GFLAGS_INCLUDE_DIR})
+  endif()
+endif()
+
+# After above, we should have the gflags target now.
+if (NOT TARGET gflags)
+  message(WARNING
+      "gflags cannot be found. Depending on whether you are building Caffe2 "
+      "or a Caffe2 dependent library, the next warning / error will give you "
+      "more info.")
+endif()
+

scripts/build_android.sh

@@ -64,6 +64,7 @@ CMAKE_ARGS+=("-DANDROID_TOOLCHAIN=gcc")
 
 # Disable unused dependencies
 CMAKE_ARGS+=("-DUSE_CUDA=OFF")
+CMAKE_ARGS+=("-DUSE_GFLAGS=OFF")
 CMAKE_ARGS+=("-DUSE_OPENCV=OFF")
 CMAKE_ARGS+=("-DUSE_LMDB=OFF")
 CMAKE_ARGS+=("-DUSE_LEVELDB=OFF")

scripts/build_ios.sh

@@ -53,6 +53,7 @@ CMAKE_ARGS+=("-DBUILD_PYTHON=OFF")
 
 # Disable unused dependencies
 CMAKE_ARGS+=("-DUSE_CUDA=OFF")
+CMAKE_ARGS+=("-DUSE_GFLAGS=OFF")
 CMAKE_ARGS+=("-DUSE_OPENCV=OFF")
 CMAKE_ARGS+=("-DUSE_LMDB=OFF")
 CMAKE_ARGS+=("-DUSE_LEVELDB=OFF")