From 0a5efe84894e92668a1d34742ccbe358c2ce0971 Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Mon, 10 Mar 2025 14:07:46 -0400
Subject: [PATCH] cmSystemTools: Fix GetRealPath implementation on Windows

In commit 823e1df54c (cmSystemTools: Implement GetRealPath on Windows,
2024-11-04, v4.0.0-rc1~521^2~1) we implemented the POSIX behavior that
resolves symlinks followed by '..' components.  However, Windows just
removes them lexically.  Also, we were not handling all junction types.

Instead, use `GetFinalPathNameByHandleW` via `uv_fs_realpath`.

Note that we previously attempted this in commit 640709e7db
(cmSystemTools: Implement GetRealPath on Windows, 2017-10-02,
v3.11.0-rc1~445^2~1) but reverted it in commit 83630d4918
(cmSystemTools: Revert GetRealPath implementation on Windows,
2018-05-29, v3.11.3~3^2) due to resolving `subst` drives.  This time,
add code to re-`subst`itute the drive in the resolved path.

Fixes: #26750
Issue: #17206
---
 Source/cmSystemTools.cxx | 45 ++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index a0150e6a64..c2c5cd77cf 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -1340,19 +1340,42 @@ std::string cmSystemTools::GetRealPath(std::string const& path,
                                        std::string* errorMessage)
 {
 #ifdef _WIN32
-  std::string resolved_path;
-  using namespace cm::PathResolver;
-  // IWYU pragma: no_forward_declare cm::PathResolver::Policies::RealPath
-  static Resolver<Policies::RealPath> const resolver(RealOS);
-  cmsys::Status status = resolver.Resolve(path, resolved_path);
-  if (!status) {
-    if (errorMessage) {
-      *errorMessage = status.GetString();
-      resolved_path.clear();
-    } else {
-      resolved_path = path;
+  std::string resolved_path =
+    cmSystemTools::GetRealPathResolvingWindowsSubst(path, errorMessage);
+
+  // If the original path used a subst drive and the real path starts
+  // with the substitution, restore the subst drive prefix.  This may
+  // incorrectly restore a subst drive if the underlying drive was
+  // encountered via an absolute symlink, but this is an acceptable
+  // limitation to otherwise preserve susbt drives.
+  if (resolved_path.size() >= 2 && resolved_path[1] == ':' &&
+      path.size() >= 2 && path[1] == ':' &&
+      toupper(resolved_path[0]) != toupper(path[0])) {
+    // FIXME: Add thread_local or mutex if we use threads.
+    static std::map<char, std::string> substMap;
+    char const drive = static_cast<char>(toupper(path[0]));
+    std::string maybe_subst = cmStrCat(drive, ":/");
+    auto smi = substMap.find(drive);
+    if (smi == substMap.end()) {
+      smi = substMap
+              .emplace(
+                drive,
+                cmSystemTools::GetRealPathResolvingWindowsSubst(maybe_subst))
+              .first;
+    }
+    std::string const& resolved_subst = smi->second;
+    std::string::size_type const ns = resolved_subst.size();
+    if (ns > 0) {
+      std::string::size_type const np = resolved_path.size();
+      if (ns == np && resolved_path == resolved_subst) {
+        resolved_path = maybe_subst;
+      } else if (ns > 0 && ns < np && resolved_path[ns] == '/' &&
+                 resolved_path.compare(0, ns, resolved_subst) == 0) {
+        resolved_path.replace(0, ns + 1, maybe_subst);
+      }
     }
   }
+
   return resolved_path;
 #else
   return cmsys::SystemTools::GetRealPath(path, errorMessage);