GenEx/LINK_LIBRARY: Add NEEDED_LIBRARY support on Linux

Help/release/dev/link-library-needed-library-linux.rst

@@ -0,0 +1,6 @@
+link-library-needed-library-linux
+----------------------------------
+
+* The :genex:`$<LINK_LIBRARY:NEEDED_LIBRARY,...>` generator expression feature
+  is now supported on Linux with GNU-compatible linkers (GNU ld, gold, LLD,
+  MOLD).

Help/variable/include/LINK_LIBRARY_PREDEFINED_FEATURES.rst

@@ -101,6 +101,19 @@
     Uses the ``-needed_library`` or ``-needed-l`` option as appropriate,
     with the same linker constraints as ``NEEDED_FRAMEWORK``.
 
+  Linux
+    .. versionadded:: 4.5
+
+    Wraps the library with ``--no-as-needed`` / ``--as-needed`` to force
+    an entry in the binary's ``DT_NEEDED`` list regardless of whether any
+    symbols from the library are directly referenced.  When the linker
+    supports ``--push-state`` / ``--pop-state`` (GNU ld >= 2.25, gold,
+    LLD, MOLD), the state-stacking form is used instead to correctly
+    restore the pre-existing ``--as-needed`` state rather than
+    unconditionally enabling it after the library.  This feature is only
+    meaningful for shared libraries; applying it to a static library
+    produces a developer warning and has no effect.
+
 ``REEXPORT_LIBRARY``
   This is similar to the ``REEXPORT_FRAMEWORK`` feature,  except it is for use
   with non-framework targets or libraries (Apple platforms only).

Modules/Platform/Linker/GNU.cmake

@@ -57,6 +57,31 @@ function(__cmake_set_whole_archive_feature __linker __linker_options)
   set(CMAKE_${__lang}LINKER_PUSHPOP_STATE_SUPPORTED "${CMAKE_${__lang}LINKER_PUSHPOP_STATE_SUPPORTED}" PARENT_SCOPE)
 endfunction()
 
+# NEEDED_LIBRARY Feature for LINK_LIBRARY generator expression
+## Force a shared library into DT_NEEDED even when no symbols are referenced
+function(__cmake_set_needed_library_feature __linker __linker_options)
+  unset(__lang)
+  if(ARGC EQUAL "3")
+    set(__lang "${ARGV2}_")
+  endif()
+
+  __cmake_check_linker_pushpop_state("${__linker}" "${__linker_options}" ${ARGV2})
+
+  ## NEEDED_LIBRARY: Force DT_NEEDED entry for a shared library
+  if(CMAKE_${__lang}LINKER_PUSHPOP_STATE_SUPPORTED)
+    set(CMAKE_${__lang}LINK_LIBRARY_USING_NEEDED_LIBRARY "LINKER:--push-state,--no-as-needed"
+                                                         "<LINK_ITEM>"
+                                                         "LINKER:--pop-state" PARENT_SCOPE)
+  else()
+    set(CMAKE_${__lang}LINK_LIBRARY_USING_NEEDED_LIBRARY "LINKER:--no-as-needed"
+                                                         "<LINK_ITEM>"
+                                                         "LINKER:--as-needed" PARENT_SCOPE)
+  endif()
+  set(CMAKE_${__lang}LINK_LIBRARY_USING_NEEDED_LIBRARY_SUPPORTED TRUE PARENT_SCOPE)
+  set(CMAKE_${__lang}LINK_LIBRARY_NEEDED_LIBRARY_ATTRIBUTES LIBRARY_TYPE=SHARED DEDUPLICATION=DEFAULT OVERRIDE=DEFAULT PARENT_SCOPE)
+  set(CMAKE_${__lang}LINKER_PUSHPOP_STATE_SUPPORTED "${CMAKE_${__lang}LINKER_PUSHPOP_STATE_SUPPORTED}" PARENT_SCOPE)
+endfunction()
 
 ## Configure system linker
 __cmake_set_whole_archive_feature("${CMAKE_LINKER}" "")
+__cmake_set_needed_library_feature("${CMAKE_LINKER}" "")

Modules/Platform/Linker/Linux-GNU.cmake

@@ -13,5 +13,8 @@ macro(__linux_linker_gnu lang)
     __cmake_set_whole_archive_feature("${CMAKE_${lang}_COMPILER_LINKER}"
                                       "${CMAKE_${lang}_COMPILER_LINKER_ARCHITECTURE_FLAGS}"
                                       "${lang}")
+    __cmake_set_needed_library_feature("${CMAKE_${lang}_COMPILER_LINKER}"
+                                       "${CMAKE_${lang}_COMPILER_LINKER_ARCHITECTURE_FLAGS}"
+                                       "${lang}")
   endif()
 endmacro()

Tests/RunCMake/target_link_libraries-LINK_LIBRARY/RunCMakeTest.cmake

@@ -123,6 +123,11 @@ if(APPLE AND (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID MATCHES
   if (CMAKE_C_COMPILER_ID STREQUAL "AppleClang" AND CMAKE_C_COMPILER_VERSION GREATER_EQUAL "12")
     run_cmake_target(apple_library needed_library main-needed_library)
   endif()
+elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang"))
+  run_cmake(linux_library_external)
+  run_cmake_target(linux_library_external build external)
+  run_cmake_with_options(linux_library "-DRunCMake_BINARY_DIR=${RunCMake_BINARY_DIR}")
+  run_cmake_target(linux_library needed_library main-needed_library)
 endif()
 
 # WHOLE_ARCHIVE feature

Tests/RunCMake/target_link_libraries-LINK_LIBRARY/linux_library.cmake

@@ -0,0 +1,10 @@
+
+enable_language(C)
+
+add_library(lib SHARED base.c lib.c)
+
+# feature NEEDED_LIBRARY
+add_executable(main-needed_library main.c)
+target_link_directories(main-needed_library PRIVATE "${RunCMake_BINARY_DIR}/linux_library_external-build"
+                                                     "${RunCMake_BINARY_DIR}/linux_library_external-build/$<CONFIG>")
+target_link_libraries(main-needed_library PRIVATE "$<LINK_LIBRARY:NEEDED_LIBRARY,lib,external>")

Tests/RunCMake/target_link_libraries-LINK_LIBRARY/linux_library_external.cmake

@@ -0,0 +1,4 @@
+
+enable_language(C)
+
+add_library(external SHARED unref.c)