cmFindCommon: Fix cmFindCommon::RerootPaths to use real paths

When comparing if a candidate path is already a subdirectory of a root
path, we should compare the real paths, to ensure we canonicalize the
paths.

Fixes: #27493

Source/cmFindCommon.cxx

@@ -301,8 +301,9 @@ void cmFindCommon::RerootPaths(std::vector<std::string>& paths,
 
   auto isSameDirectoryOrSubDirectory = [](std::string const& l,
                                           std::string const& r) {
-    return (cmSystemTools::GetRealPath(l) == cmSystemTools::GetRealPath(r)) ||
-      cmSystemTools::IsSubDirectory(l, r);
+    std::string const lReal = cmSystemTools::GetRealPath(l);
+    std::string const rReal = cmSystemTools::GetRealPath(r);
+    return (lReal == rReal) || cmSystemTools::IsSubDirectory(lReal, rReal);
   };
 
   for (auto const& r : roots) {

Tests/RunCMake/find_package/FindRootPathAndPrefixPathWithCommonSubdir.cmake

@@ -0,0 +1,50 @@
+set(regular_root "${CMAKE_CURRENT_SOURCE_DIR}/FindRootPathAndPrefixPathAreEqual")
+set(regular_prefix "${regular_root}/lib/cmake")
+
+# 'emptydir' must be a real dir on the file system, otherwise CMake
+# won't canonicalize the path when getting resolving the real path.
+set(dotted_root "${regular_root}/emptydir/..")
+set(dotted_prefix "${dotted_root}/lib/cmake")
+
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE "ONLY")
+
+# Case with no '..' in the paths
+set(CMAKE_FIND_ROOT_PATH "${regular_root}")
+set(CMAKE_PREFIX_PATH "${regular_prefix}")
+message(STATUS "Looking for Foo without '..' in the paths")
+find_package(Foo
+             REQUIRED
+             CONFIG
+             NO_CMAKE_ENVIRONMENT_PATH
+             NO_SYSTEM_ENVIRONMENT_PATH
+             # Important because CMAKE_SYSTEM_PREFIX_PATH might contain "/" as a prefix
+             # And when "/" is rerooted onto the root above, the package is found even if
+             # CMAKE_PREFIX_PATH is empty. We want to ensure that we hit
+             # the CMAKE_FIND_ROOT_PATH == CMAKE_PREFIX_PATH code path.
+             NO_CMAKE_SYSTEM_PATH
+             )
+if(Foo_FOUND)
+    message(STATUS "Foo found for case without '..' in the paths")
+endif()
+
+# Unset the cache variable to find the package again.
+unset(Foo_DIR CACHE)
+
+# Case with '..' in the paths
+set(CMAKE_FIND_ROOT_PATH "${dotted_root}")
+set(CMAKE_PREFIX_PATH "${dotted_prefix}")
+message(STATUS "Looking for Foo with '..' in the paths")
+find_package(Foo
+             REQUIRED
+             CONFIG
+             NO_CMAKE_ENVIRONMENT_PATH
+             NO_SYSTEM_ENVIRONMENT_PATH
+             # Important because CMAKE_SYSTEM_PREFIX_PATH might contain "/" as a prefix
+             # And when "/" is rerooted onto the root above, the package is found even if
+             # CMAKE_PREFIX_PATH is empty. We want to ensure that we hit
+             # the CMAKE_FIND_ROOT_PATH == CMAKE_PREFIX_PATH code path.
+             NO_CMAKE_SYSTEM_PATH
+             )
+if(Foo_FOUND)
+    message(STATUS "Foo found for case with '..' in the paths")
+endif()

Tests/RunCMake/find_package/RunCMakeTest.cmake

@@ -48,6 +48,7 @@ run_cmake(RequiredOptionalKeywordsClash)
 run_cmake(RequiredVarOptional)
 run_cmake(RequiredVarNested)
 run_cmake(FindRootPathAndPrefixPathAreEqual)
+run_cmake(FindRootPathAndPrefixPathWithCommonSubdir)
 run_cmake(SetFoundFALSE)
 run_cmake(UnwindIncludeBlock)
 run_cmake(UnwindIncludeFunction)