Finish symlink implementation on Windows

lib/std/os.zig

@@ -1541,6 +1541,10 @@ pub const SymLinkError = error{
 /// Creates a symbolic link named `sym_link_path` which contains the string `target_path`.
 /// A symbolic link (also known as a soft link) may point to an existing file or to a nonexistent
 /// one; the latter case is known as a dangling link.
+/// On Windows, it is only legal to create a symbolic link to an existing resource. Furthermore,
+/// this function will by default try creating a symbolic link to a file. If you would like to
+/// create a symbolic link to a directory instead, see `symlinkW` for more information how to
+/// do that.
 /// If `sym_link_path` exists, it will not be overwritten.
 /// See also `symlinkC` and `symlinkW`.
 pub fn symlink(target_path: []const u8, sym_link_path: []const u8) SymLinkError!void {
@@ -1550,7 +1554,7 @@ pub fn symlink(target_path: []const u8, sym_link_path: []const u8) SymLinkError!
     if (builtin.os.tag == .windows) {
         const target_path_w = try windows.sliceToPrefixedFileW(target_path);
         const sym_link_path_w = try windows.sliceToPrefixedFileW(sym_link_path);
-        return windows.CreateSymbolicLinkW(sym_link_path_w.span().ptr, target_path_w.span().ptr, 0);
+        return symlinkW(sym_link_path_w.span().ptr, target_path_w.span().ptr);
     }
     const target_path_c = try toPosixPath(target_path);
     const sym_link_path_c = try toPosixPath(sym_link_path);
@@ -1559,6 +1563,15 @@ pub fn symlink(target_path: []const u8, sym_link_path: []const u8) SymLinkError!
 
 pub const symlinkC = @compileError("deprecated: renamed to symlinkZ");
 
+/// Windows-only. Same as `symlink` except the parameters are null-terminated, WTF16 encoded.
+/// Note that this function will by default try creating a symbolic link to a file. If you would
+/// like to create a symbolic link to a directory, use `std.os.windows.CreateSymbolicLinkW` directly
+/// specifying as flags `std.os.windows.CreateSymbolicLinkFlags.Directory`.
+pub fn symlinkW(target_path: [*:0]const u16, sym_link_path: [*:0]const u16) SymLinkError!void {
+    const flags = windows.CreateSymbolicLinkFlags.File;
+    return windows.CreateSymbolicLinkW(sym_link_path, target_path, flags);
+}
+
 /// This is the same as `symlink` except the parameters are null-terminated pointers.
 /// See also `symlink`.
 pub fn symlinkZ(target_path: [*:0]const u8, sym_link_path: [*:0]const u8) SymLinkError!void {

lib/std/os/windows.zig

@@ -601,12 +601,17 @@ pub fn GetCurrentDirectory(buffer: []u8) GetCurrentDirectoryError![]u8 {
     return buffer[0..end_index];
 }
 
-pub const CreateSymbolicLinkError = error{Unexpected};
+pub const CreateSymbolicLinkError = error{AccessDenied, FileNotFound, Unexpected};
+
+pub const CreateSymbolicLinkFlags = enum(DWORD) {
+    File = SYMBOLIC_LINK_FLAG_FILE,
+    Directory = SYMBOLIC_LINK_FLAG_DIRECTORY,
+};
 
 pub fn CreateSymbolicLink(
     sym_link_path: []const u8,
     target_path: []const u8,
-    flags: DWORD,
+    flags: CreateSymbolicLinkFlags,
 ) CreateSymbolicLinkError!void {
     const sym_link_path_w = try sliceToPrefixedFileW(sym_link_path);
     const target_path_w = try sliceToPrefixedFileW(target_path);
@@ -616,10 +621,32 @@ pub fn CreateSymbolicLink(
 pub fn CreateSymbolicLinkW(
     sym_link_path: [*:0]const u16,
     target_path: [*:0]const u16,
-    flags: DWORD,
+    flags: CreateSymbolicLinkFlags,
 ) CreateSymbolicLinkError!void {
-    if (kernel32.CreateSymbolicLinkW(sym_link_path, target_path, flags) == 0) {
+    // Previously, until Win 10 Creators Update, creating symbolic links required
+    // SeCreateSymbolicLink privilege. Currently, this is no longer required if the
+    // OS is in Developer Mode; however, SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+    // must be added to the input flags.
+    if (kernel32.CreateSymbolicLinkW(sym_link_path, target_path, @enumToInt(flags) | SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE) == 0) {
         switch (kernel32.GetLastError()) {
+            .INVALID_PARAMETER => {
+                // If we're on Windows pre Creators Update, SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+                // flag is an invalid parameter, in which case repeat without the flag.
+                if (kernel32.CreateSymbolicLinkW(sym_link_path, target_path, @enumToInt(flags)) == 0) {
+                    switch (kernel32.GetLastError()) {
+                        .PRIVILEGE_NOT_HELD => return error.AccessDenied,
+                        .FILE_NOT_FOUND => return error.FileNotFound,
+                        .PATH_NOT_FOUND => return error.FileNotFound,
+                        .ACCESS_DENIED => return error.AccessDenied,
+                        else => |err| return unexpectedError(err),
+                    }
+                }
+                return;
+            },
+            .PRIVILEGE_NOT_HELD => return error.AccessDenied,
+            .FILE_NOT_FOUND => return error.FileNotFound,
+            .PATH_NOT_FOUND => return error.FileNotFound,
+            .ACCESS_DENIED => return error.AccessDenied,
             else => |err| return unexpectedError(err),
         }
     }

lib/std/os/windows/bits.zig

@@ -1569,3 +1569,7 @@ pub const MAXIMUM_REPARSE_DATA_BUFFER_SIZE: usize = 16 * 1024;
 pub const FSCTL_GET_REPARSE_POINT: DWORD = 0x900a8;
 pub const IO_REPARSE_TAG_SYMLINK: ULONG = 0xa000000c;
 pub const IO_REPARSE_TAG_MOUNT_POINT: ULONG = 0xa0000003;
+
+pub const SYMBOLIC_LINK_FLAG_FILE: DWORD = 0x0;
+pub const SYMBOLIC_LINK_FLAG_DIRECTORY: DWORD = 0x1;
+pub const SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE: DWORD = 0x2;