Support Windows paths in canonPath and absPath

`canonPath` and `absPath` work on native paths, and so should switch
between supporting Unix paths and Windows paths accordingly.

The templating is because `CanonPath`, which shares the implementation,
should always be Unix style. It is the pure "nix-native" path type for
virtual file operations --- it is part of Nix's "business logic", and
should not vary with the host OS accordingly.

src/libutil/file-path-impl.hh

@@ -43,6 +43,67 @@ struct UnixPathTrait
 };
 
 
+/**
+ * Windows-style path primitives.
+ *
+ * The character type is a parameter because while windows paths rightly
+ * work over UTF-16 (*) using `wchar_t`, at the current time we are
+ * often manipulating them converted to UTF-8 (*) using `char`.
+ *
+ * (Actually neither are guaranteed to be valid unicode; both are
+ * arbitrary non-0 8- or 16-bit bytes. But for charcters with specifical
+ * meaning like '/', '\\', ':', etc., we refer to an encoding scheme,
+ * and also for sake of UIs that display paths a text.)
+ */
+template<class CharT0>
+struct WindowsPathTrait
+{
+    using CharT = CharT0;
+
+    using String = std::basic_string<CharT>;
+
+    using StringView = std::basic_string_view<CharT>;
+
+    constexpr static CharT preferredSep = '\\';
+
+    static inline bool isPathSep(CharT c)
+    {
+        return c == '/' || c == preferredSep;
+    }
+
+    static size_t findPathSep(StringView path, size_t from = 0)
+    {
+        size_t p1 = path.find('/', from);
+        size_t p2 = path.find(preferredSep, from);
+        return p1 == String::npos ? p2 :
+               p2 == String::npos ? p1 :
+               std::min(p1, p2);
+    }
+
+    static size_t rfindPathSep(StringView path, size_t from = String::npos)
+    {
+        size_t p1 = path.rfind('/', from);
+        size_t p2 = path.rfind(preferredSep, from);
+        return p1 == String::npos ? p2 :
+               p2 == String::npos ? p1 :
+               std::max(p1, p2);
+    }
+};
+
+
+/**
+ * @todo Revisit choice of `char` or `wchar_t` for `WindowsPathTrait`
+ * argument.
+ */
+using NativePathTrait =
+#ifdef _WIN32
+    WindowsPathTrait<char>
+#else
+    UnixPathTrait
+#endif
+    ;
+
+
 /**
  * Core pure path canonicalization algorithm.
  *

src/libutil/file-system.cc

@@ -22,10 +22,14 @@ namespace fs = std::filesystem;
 
 namespace nix {
 
-/** Treat the string as possibly an absolute path, by inspecting the start of it. Return whether it was probably intended to be absolute. */
+/**
+ * Treat the string as possibly an absolute path, by inspecting the
+ * start of it. Return whether it was probably intended to be
+ * absolute.
+ */
 static bool isAbsolute(PathView path)
 {
-    return !path.empty() && path[0] == '/';
+    return fs::path { path }.is_absolute();
 }
 
 
@@ -69,6 +73,9 @@ Path canonPath(PathView path, bool resolveSymlinks)
     if (!isAbsolute(path))
         throw Error("not an absolute path: '%1%'", path);
 
+    // For Windows
+    auto rootName = fs::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`. */
@@ -78,7 +85,7 @@ Path canonPath(PathView path, bool resolveSymlinks)
        arbitrary (but high) limit to prevent infinite loops. */
     unsigned int followCount = 0, maxFollow = 1024;
 
-    return canonPathInner<UnixPathTrait>(
+    auto ret = canonPathInner<NativePathTrait>(
         path,
         [&followCount, &temp, maxFollow, resolveSymlinks]
         (std::string & result, std::string_view & remaining) {
@@ -99,6 +106,10 @@ Path canonPath(PathView path, bool resolveSymlinks)
                 }
             }
         });
+
+    if (!rootName.empty())
+        ret = rootName.string() + std::move(ret);
+    return ret;
 }
 
 

tests/unit/libutil/tests.cc

@@ -9,6 +9,14 @@
 
 #include <numeric>
 
+#ifdef _WIN32
+# define FS_SEP "\\"
+# define FS_ROOT "C:" FS_SEP // Need a mounted one, C drive is likely
+#else
+# define FS_SEP "/"
+# define FS_ROOT FS_SEP
+#endif
+
 namespace nix {
 
 /* ----------- tests for util.hh ------------------------------------------------*/
@@ -18,9 +26,9 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(absPath, doesntChangeRoot) {
-        auto p = absPath("/");
+        auto p = absPath(FS_ROOT);
 
-        ASSERT_EQ(p, "/");
+        ASSERT_EQ(p, FS_ROOT);
     }
 
 
@@ -53,11 +61,11 @@ namespace nix {
 
 
     TEST(absPath, pathIsCanonicalised) {
-        auto path = "/some/path/with/trailing/dot/.";
+        auto path = FS_ROOT "some/path/with/trailing/dot/.";
         auto p1 = absPath(path);
         auto p2 = absPath(p1);
 
-        ASSERT_EQ(p1, "/some/path/with/trailing/dot");
+        ASSERT_EQ(p1, FS_ROOT "some" FS_SEP "path" FS_SEP "with" FS_SEP "trailing" FS_SEP "dot");
         ASSERT_EQ(p1, p2);
     }
 
@@ -66,24 +74,24 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(canonPath, removesTrailingSlashes) {
-        auto path = "/this/is/a/path//";
+        auto path = FS_ROOT "this/is/a/path//";
         auto p = canonPath(path);
 
-        ASSERT_EQ(p, "/this/is/a/path");
+        ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
     }
 
     TEST(canonPath, removesDots) {
-        auto path = "/this/./is/a/path/./";
+        auto path = FS_ROOT "this/./is/a/path/./";
         auto p = canonPath(path);
 
-        ASSERT_EQ(p, "/this/is/a/path");
+        ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
     }
 
     TEST(canonPath, removesDots2) {
-        auto path = "/this/a/../is/a////path/foo/..";
+        auto path = FS_ROOT "this/a/../is/a////path/foo/..";
         auto p = canonPath(path);
 
-        ASSERT_EQ(p, "/this/is/a/path");
+        ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
     }
 
     TEST(canonPath, requiresAbsolutePath) {
@@ -197,7 +205,7 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(pathExists, rootExists) {
-        ASSERT_TRUE(pathExists("/"));
+        ASSERT_TRUE(pathExists(FS_ROOT));
     }
 
     TEST(pathExists, cwdExists) {