find_library: Skip 'lib => lib<arch>' searches if one symlinks the other

The `FIND_LIBRARY_USE_LIB<arch>_PATHS` global properties ask
`find_library` to look in `lib<arch>` directories automatically before
corresponding `lib` directories.  However, if `lib<arch>` is just a
symlink to `lib` (or vice-versa) then we should skip adding the
`lib<arch>` path.  Such symlinks typically only exist to satisfy
software that expects the `lib<arch>` path to be available.

Fixes: #16687

Source/cmFindLibraryCommand.cxx

@@ -72,6 +72,18 @@ void cmFindLibraryCommand::AddArchitecturePaths(const char* suffix)
   }
 }
 
+static bool cmLibDirsLinked(std::string const& l, std::string const& r)
+{
+  // Compare the real paths of the two directories.
+  // Since our caller only changed the trailing component of each
+  // directory, the real paths can be the same only if at least one of
+  // the trailing components is a symlink.  Use this as an optimization
+  // to avoid excessive realpath calls.
+  return (cmSystemTools::FileIsSymlink(l) ||
+          cmSystemTools::FileIsSymlink(r)) &&
+    cmSystemTools::GetRealPath(l) == cmSystemTools::GetRealPath(r);
+}
+
 void cmFindLibraryCommand::AddArchitecturePath(
   std::string const& dir, std::string::size_type start_pos, const char* suffix,
   bool fresh)
@@ -87,6 +99,11 @@ void cmFindLibraryCommand::AddArchitecturePath(
     std::string libX = lib + suffix;
     bool use_libX = cmSystemTools::FileIsDirectory(libX);
 
+    // Avoid copies of the same directory due to symlinks.
+    if (use_libX && use_lib && cmLibDirsLinked(libX, lib)) {
+      use_libX = false;
+    }
+
     if (use_libX) {
       libX += dir.substr(pos + 3);
       std::string::size_type libX_pos = pos + 3 + strlen(suffix) + 1;
@@ -106,6 +123,11 @@ void cmFindLibraryCommand::AddArchitecturePath(
     std::string dirX = dir + suffix;
     bool use_dirX = cmSystemTools::FileIsDirectory(dirX);
 
+    // Avoid copies of the same directory due to symlinks.
+    if (use_dirX && use_dir && cmLibDirsLinked(dirX, dir)) {
+      use_dirX = false;
+    }
+
     if (use_dirX) {
       dirX += "/";
       this->SearchPaths.push_back(dirX);

Tests/RunCMake/find_library/LibArchLink-stderr.txt

+TOP_LIBRARY='[^']*/Tests/RunCMake/find_library/LibArchLink-build/lib/libtop.a'
+SUB_LIBRARY='[^']*/Tests/RunCMake/find_library/LibArchLink-build/lib/sub/libsub.a'

Tests/RunCMake/find_library/LibArchLink.cmake

+set(CMAKE_SIZEOF_VOID_P 4)
+set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS ON)
+list(APPEND CMAKE_FIND_LIBRARY_PREFIXES lib)
+list(APPEND CMAKE_FIND_LIBRARY_SUFFIXES .a)
+
+file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/lib)
+execute_process(COMMAND ${CMAKE_COMMAND} -E create_symlink lib ${CMAKE_CURRENT_BINARY_DIR}/lib32)
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/lib/libtop.a" "top")
+find_library(TOP_LIBRARY
+  NAMES top
+  PATHS ${CMAKE_CURRENT_BINARY_DIR}/lib
+  NO_DEFAULT_PATH
+  )
+message("TOP_LIBRARY='${TOP_LIBRARY}'")
+
+file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/lib/sub)
+execute_process(COMMAND ${CMAKE_COMMAND} -E create_symlink . ${CMAKE_CURRENT_BINARY_DIR}/lib/sub/32)
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/lib/sub/libsub.a" "sub")
+find_library(SUB_LIBRARY
+  NAMES sub
+  PATHS ${CMAKE_CURRENT_BINARY_DIR}/lib/sub
+  NO_DEFAULT_PATH
+  )
+message("SUB_LIBRARY='${SUB_LIBRARY}'")

Tests/RunCMake/find_library/RunCMakeTest.cmake

 include(RunCMake)
 
 run_cmake(Created)
+if(CMAKE_HOST_UNIX)
+  run_cmake(LibArchLink)
+endif()
 if(WIN32 OR CYGWIN)
   run_cmake(PrefixInPATH)
 endif()