cmSystemTools: Restore GetRealPathResolvingWindowsSubst

We need it to restore behavior on Windows.  Revert commit 16af27fd42
(cmSystemTools: Drop GetRealPathResolvingWindowsSubst, 2024-11-04,
v4.0.0-rc1~521^2), but with a cleaned up implementation.

Issue: #26750
Issue: #18033

Source/cmSystemTools.cxx

@@ -1305,6 +1305,37 @@ bool FileModeGuard::HasErrors() const
   return filepath_.empty();
 }
 
+std::string cmSystemTools::GetRealPathResolvingWindowsSubst(
+  std::string const& path, std::string* errorMessage)
+{
+#ifdef _WIN32
+  // uv_fs_realpath uses Windows Vista API so fallback to kwsys if not found
+  std::string resolved_path;
+  uv_fs_t req;
+  int err = uv_fs_realpath(nullptr, &req, path.c_str(), nullptr);
+  if (!err) {
+    resolved_path = std::string((char*)req.ptr);
+    cmSystemTools::ConvertToUnixSlashes(resolved_path);
+  } else if (err == UV_ENOSYS) {
+    resolved_path = cmsys::SystemTools::GetRealPath(path, errorMessage);
+  } else if (errorMessage) {
+    cmsys::Status status =
+      cmsys::Status::Windows(uv_fs_get_system_error(&req));
+    *errorMessage = status.GetString();
+    resolved_path.clear();
+  } else {
+    resolved_path = path;
+  }
+  // Normalize to upper-case drive letter as cm::PathResolver does.
+  if (resolved_path.size() > 1 && resolved_path[1] == ':') {
+    resolved_path[0] = toupper(resolved_path[0]);
+  }
+  return resolved_path;
+#else
+  return cmsys::SystemTools::GetRealPath(path, errorMessage);
+#endif
+}
+
 std::string cmSystemTools::GetRealPath(std::string const& path,
                                        std::string* errorMessage)
 {

Source/cmSystemTools.h

@@ -651,6 +651,12 @@ public:
   static std::string GetComspec();
 #endif
 
+  /** Get the real path for a given path, removing all symlinks.
+      This variant of GetRealPath also works on Windows but will
+      resolve subst drives too.  */
+  static std::string GetRealPathResolvingWindowsSubst(
+    std::string const& path, std::string* errorMessage = nullptr);
+
   /** Get the real path for a given path, removing all symlinks.  */
   static std::string GetRealPath(std::string const& path,
                                  std::string* errorMessage = nullptr);

Source/cmTimestamp.cxx

@@ -62,7 +62,8 @@ std::string cmTimestamp::FileModificationTime(char const* path,
                                               std::string const& formatString,
                                               bool utcFlag) const
 {
-  std::string real_path = cmSystemTools::GetRealPath(path);
+  std::string real_path =
+    cmSystemTools::GetRealPathResolvingWindowsSubst(path);
 
   if (!cmsys::SystemTools::FileExists(real_path)) {
     return std::string();