libutil: Fix canonPath, makeTempPath and createTempDir on windows

This at least makes canonPath not consider the drive letter as a path
component. There still some issues with it on windows, but at least
this gets us through some of the libutil-tests.

Also since we don't want to change which env variables nix considers
we don't use std::filesystem::temp_directory_path and implement the
windows version directly.

src/libutil-tests/file-system.cc

@@ -416,4 +416,11 @@ TEST(FdSource, restartWorks)
     EXPECT_EQ(source.drain(), "");
 }
 
+TEST(createTempDir, works)
+{
+    auto tmpDir = createTempDir();
+    nix::AutoDelete delTmpDir(tmpDir, /*recursive=*/true);
+    ASSERT_TRUE(std::filesystem::is_directory(tmpDir));
+}
+
 } // namespace nix

src/libutil/file-system.cc

@@ -115,9 +115,6 @@ Path canonPath(PathView path, bool resolveSymlinks)
     if (!isAbsolute(path))
         throw Error("not an absolute path: '%1%'", path);
 
-    // For Windows
-    auto rootName = std::filesystem::path{path}.root_name();
-
     /* This just exists because we cannot set the target of `remaining`
        (the callback parameter) directly to a newly-constructed string,
        since it is `std::string_view`. */
@@ -147,8 +144,6 @@ Path canonPath(PathView path, bool resolveSymlinks)
             }
         });
 
-    if (!rootName.empty())
-        ret = rootName.string() + std::move(ret);
     return ret;
 }
 
@@ -672,11 +667,6 @@ void AutoUnmount::cancel()
 
 //////////////////////////////////////////////////////////////////////
 
-std::filesystem::path defaultTempDir()
-{
-    return getEnvNonEmpty("TMPDIR").value_or("/tmp");
-}
-
 std::filesystem::path createTempDir(const std::filesystem::path & tmpRoot, const std::string & prefix, mode_t mode)
 {
     while (1) {

src/libutil/include/nix/util/file-path-impl.hh

@@ -40,6 +40,11 @@ struct UnixPathTrait
     {
         return path.rfind('/', from);
     }
+
+    static size_t rootNameLen(StringView)
+    {
+        return 0;
+    }
 };
 
 /**
@@ -83,6 +88,18 @@ struct WindowsPathTrait
         size_t p2 = path.rfind(preferredSep, from);
         return p1 == String::npos ? p2 : p2 == String::npos ? p1 : std::max(p1, p2);
     }
+
+    static size_t rootNameLen(StringView path)
+    {
+        if (path.size() >= 2 && path[1] == ':') {
+            char driveLetter = path[0];
+            if ((driveLetter >= 'A' && driveLetter <= 'Z') || (driveLetter >= 'a' && driveLetter <= 'z'))
+                return 2;
+        }
+        /* TODO: This needs to also handle UNC paths.
+         * https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats#unc-paths */
+        return 0;
+    }
 };
 
 template<typename CharT>
@@ -116,6 +133,11 @@ typename PathDict::String canonPathInner(typename PathDict::StringView remaining
     typename PathDict::String result;
     result.reserve(256);
 
+    if (auto rootNameLength = PathDict::rootNameLen(remaining)) {
+        result += remaining.substr(0, rootNameLength); /* Copy drive letter verbatim. */
+        remaining.remove_prefix(rootNameLength);
+    }
+
     while (true) {
 
         /* Skip slashes. */

src/libutil/include/nix/util/file-system.hh

@@ -362,6 +362,8 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
 
 /**
  * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ * Uses GetTempPathW on windows which respects TMP, TEMP, USERPROFILE env variables.
+ * Does not resolve symlinks and the returned path might not be directory or exist at all.
  */
 std::filesystem::path defaultTempDir();
 

src/libutil/unix/file-system.cc

@@ -9,6 +9,7 @@
 #include <unistd.h>
 
 #include "nix/util/file-system.hh"
+#include "nix/util/environment-variables.hh"
 
 #include "util-unix-config-private.hh"
 
@@ -19,6 +20,11 @@ Descriptor openDirectory(const std::filesystem::path & path)
     return open(path.c_str(), O_RDONLY | O_DIRECTORY | O_CLOEXEC);
 }
 
+std::filesystem::path defaultTempDir()
+{
+    return getEnvNonEmpty("TMPDIR").value_or("/tmp");
+}
+
 void setWriteTime(
     const std::filesystem::path & path, time_t accessedTime, time_t modificationTime, std::optional<bool> optIsSymlink)
 {

src/libutil/windows/file-system.cc

@@ -1,9 +1,11 @@
 #include "nix/util/file-system.hh"
+#include "nix/util/windows-error.hh"
 #include "nix/util/logging.hh"
 
-#ifdef _WIN32
 namespace nix {
 
+using namespace nix::windows;
+
 void setWriteTime(
     const std::filesystem::path & path, time_t accessedTime, time_t modificationTime, std::optional<bool> optIsSymlink)
 {
@@ -28,5 +30,13 @@ Descriptor openDirectory(const std::filesystem::path & path)
         NULL);
 }
 
+std::filesystem::path defaultTempDir()
+{
+    wchar_t buf[MAX_PATH + 1];
+    DWORD len = GetTempPathW(MAX_PATH + 1, buf);
+    if (len == 0 || len > MAX_PATH)
+        throw WinError("getting default temporary directory");
+    return std::filesystem::path(buf);
+}
+
 } // namespace nix
-#endif