[common/fs] Preserve Windows roots during path sanitization (#4437)

Commit 5b4c29b123 made SanitizePath rebuild paths from split components after resolving traversal. That reconstruction retained only one leading separator, collapsing UNC paths and breaking network-backed directories.

Parse the path root separately from normal components. Preserve UNC prefixes, drive-absolute and drive-relative forms, and extended UNC/device namespaces while normalizing separators and dot components.

Treat the UNC server/share portion as part of the root so parent traversal cannot escape into another share. Also preserve root-only results such as / and C:\\ instead of stripping their final separator.

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4437
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

2 files changed, 68 insertions, 14 deletions

diff --git a/src/common/fs/path_util.cpp b/src/common/fs/path_util.cpp
index b895aac54c..82ea8f6cb2 100644
--- a/src/common/fs/path_util.cpp
+++ b/src/common/fs/path_util.cpp
@@ -5,6 +5,7 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <algorithm>
+#include <cctype>
 #include <iostream>
 #include <sstream>
 #include "common/container/unordered_map.h"
@@ -480,28 +481,80 @@ std::string SanitizePath(std::string_view path_, DirectorySeparator directory_se
                            [type2](char c1, char c2) { return c1 == type2 && c2 == type2; }),
                path.end());
 
-    const bool absolute = !path.empty() && path[0] == type2;
-    std::vector<std::string_view> parts;
+    std::string root;
+    std::string_view components{path};
+    bool drive_relative = false;
+
+#ifdef _WIN32
+    const bool network = path.size() > 1 && path[0] == type2 && path[1] == type2;
+    const bool drive =
+        path.size() > 1 && std::isalpha(static_cast<unsigned char>(path[0])) && path[1] == ':';
+
+    if (network) {
+        root.assign(2, type2);
+        components.remove_prefix(2);
+    } else if (drive) {
+        root.assign(path.data(), 2);
+        components.remove_prefix(2);
+        if (!components.empty() && components.front() == type2) {
+            root += type2;
+            components.remove_prefix(1);
+        } else {
+            drive_relative = true;
+        }
+    }
+#endif
+
+    if (root.empty() && !components.empty() && components.front() == type2) {
+        root += type2;
+        components.remove_prefix(1);
+    }
+
+    const auto path_parts = SplitPathComponents(components);
+    std::size_t root_component_count = 0;
+#ifdef _WIN32
+    if (network) {
+        root_component_count = 2;
+
+        const auto is_unc = [](std::string_view part) {
+            return part.size() == 3 && (part[0] == 'U' || part[0] == 'u') &&
+                   (part[1] == 'N' || part[1] == 'n') && (part[2] == 'C' || part[2] == 'c');
+        };
+        if (path_parts.size() >= 2 && path_parts[0] == "?" && is_unc(path_parts[1])) {
+            root_component_count = 4;
+        }
+    }
+#endif
 
-    for (const auto part : SplitPathComponents(path))
-    {
-        if (part.empty() || part == ".")
+    std::vector<std::string_view> parts;
+    for (std::size_t i = 0; i < path_parts.size(); ++i) {
+        const auto part = path_parts[i];
+        if (i < root_component_count) {
+            parts.push_back(part);
+        } else if (part.empty() || part == ".") {
             continue;
-        if (part == ".." && !parts.empty() && parts.back() != "..")
-            parts.pop_back();
-        else if (part != "..") parts.push_back(part);
+        } else if (part == "..") {
+            if (parts.size() > root_component_count) {
+                parts.pop_back();
+            }
+        } else {
+            parts.push_back(part);
+        }
     }
 
-    std::string resolved = absolute ? std::string(1, type2) : std::string{};
-    for (std::size_t i = 0; i < parts.size(); ++i)
-    {
-        if (i != 0)
+    const std::size_t root_length = root.size();
+    std::string resolved = std::move(root);
+    for (std::size_t i = 0; i < parts.size(); ++i) {
+        if (i != 0 || (!resolved.empty() && resolved.back() != type2 && !drive_relative))
             resolved += type2;
         resolved.append(parts[i].data(), parts[i].size());
     }
 
     path = std::move(resolved);
 
+    if (!path.empty() && path.size() == root_length) {
+        return path;
+    }
     return std::string(RemoveTrailingSlash(path));
 }
 
diff --git a/src/common/fs/path_util.h b/src/common/fs/path_util.h
index 983e4917e8..e8fa5614ae 100644
--- a/src/common/fs/path_util.h
+++ b/src/common/fs/path_util.h
@@ -347,8 +347,9 @@ enum class DirectorySeparator {
 // i.e. "C:\Users\Yuzu\Documents\save.bin" becomes {"C:", "Users", "Yuzu", "Documents", "save.bin" }
 [[nodiscard]] std::vector<std::string> SplitPathComponentsCopy(std::string_view filename);
 
-// Removes trailing slash, makes all '\\' into '/', and removes duplicate '/'. Makes '/' into '\\'
-// depending if directory_separator is BackwardSlash or PlatformDefault and running on windows
+// Normalizes directory separators, removes duplicate and non-root trailing separators, and resolves
+// '.' and '..' components without traversing above the path root. Windows drive and UNC roots are
+// preserved.
 [[nodiscard]] std::string SanitizePath(
     std::string_view path,
     DirectorySeparator directory_separator = DirectorySeparator::ForwardSlash);