From aad9870a284410b9b7fa4d5c5d84f4a4d697c71b Mon Sep 17 00:00:00 2001
From: Ryan Liptak <squeek502@hotmail.com>
Date: Sun, 19 Jul 2026 14:26:35 -0700
Subject: [PATCH] Io.Threaded: Decouple follow_symlinks and asynchronous IO on
 Windows

The logic around follow_symlinks and asynchronous IO was first introduced in 66bbe4ec4c11839217d6a9d65771d60d45cd6bc1, and then cemented in 390194431e7fa439d67686e5a0e9efceed2d898a with the added comment:

> If we're not following symlinks, we need to ensure we don't pass in any synchronization flags such as FILE_SYNCHRONOUS_IO_NONALERT.

As far as I can tell, this comment is erroneous. There is no documented incompatibility between OPEN_REPARSE_POINT and SYNCHRONOUS_IO_NONALERT, and empirically everything works fine when using SYNCHRONOUS_IO_NONALERT. In fact, in 68ed787751ed36af3db0c52031741a5c31413034 the ReadLink implementation (a main usage of OPEN_REPARSE_POINT) was specifically switched to *not* use asynchronous IO (although that was effectively reverted during the move to std.Io).

This also fixes a bug with `.follow_symlinks = false` since, before this commit, the returned File would always have `.nonblocking = false` even though that was not correct when `.follow_symlinks` was false.
---
 lib/std/Io/Threaded.zig |  8 ++++----
 lib/std/fs/test.zig     | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 5b230064199c4d90df6c9e5ce7ecf155dcd46313..04384b3d68e4e8c26f610731479f4ec435c7cc44 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -5053,7 +5053,7 @@ pub fn dirOpenFileWtf16(
             .VALID_FLAGS,
             .OPEN,
             .{
-                .IO = if (flags.follow_symlinks) .SYNCHRONOUS_NONALERT else .ASYNCHRONOUS,
+                .IO = .SYNCHRONOUS_NONALERT,
                 .NON_DIRECTORY_FILE = !allow_directory,
                 .OPEN_REPARSE_POINT = !flags.follow_symlinks,
             },
@@ -8136,7 +8136,7 @@ fn dirReadLinkWindows(dir: Dir, sub_path: []const u8, buffer: []u8) Dir.ReadLink
         .{
             .DIRECTORY_FILE = false,
             .NON_DIRECTORY_FILE = false,
-            .IO = .ASYNCHRONOUS,
+            .IO = .SYNCHRONOUS_NONALERT,
             .OPEN_REPARSE_POINT = true,
         },
         null,
@@ -8202,7 +8202,7 @@ fn dirReadLinkWindows(dir: Dir, sub_path: []const u8, buffer: []u8) Dir.ReadLink
 
     var reparse_buf: [windows.MAXIMUM_REPARSE_DATA_BUFFER_SIZE]u8 align(@alignOf(windows.REPARSE_DATA_BUFFER)) = undefined;
     switch ((try deviceIoControl(&.{
-        .file = .{ .handle = result_handle, .flags = .{ .nonblocking = true } },
+        .file = .{ .handle = result_handle, .flags = .{ .nonblocking = false } },
         .code = .GET_REPARSE_POINT,
         .out = &reparse_buf,
     })).u.Status) {
@@ -18995,7 +18995,7 @@ fn OpenFile(sub_path_w: []const u16, options: OpenFileOptions) OpenError!windows
             .{
                 .DIRECTORY_FILE = options.filter == .dir_only,
                 .NON_DIRECTORY_FILE = options.filter == .non_directory_only,
-                .IO = if (options.follow_symlinks) .SYNCHRONOUS_NONALERT else .ASYNCHRONOUS,
+                .IO = .SYNCHRONOUS_NONALERT,
                 .OPEN_REPARSE_POINT = !options.follow_symlinks,
             },
             null,
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index bf16a7c216b4f9087195c7a98160e6b765687960..fc613ccdaa313d46dab643f849149293f422a9a5 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -758,6 +758,43 @@ test "readFileAlloc" {
     );
 }
 
+test "file operations with follow_symlinks=false" {
+    const io = testing.io;
+
+    var tmp_dir = tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    const contents = "this is a test.\nthis is a test.\nthis is a test.\nthis is a test.\n";
+    try tmp_dir.dir.writeFile(io, .{
+        .sub_path = "test_file",
+        .data = contents,
+    });
+
+    // Without lock
+    {
+        var file = try tmp_dir.dir.openFile(io, "test_file", .{ .follow_symlinks = false });
+        defer file.close(io);
+
+        var file_reader = file.reader(io, &.{});
+        const actual_contents = try file_reader.interface.allocRemaining(testing.allocator, .unlimited);
+        defer testing.allocator.free(actual_contents);
+
+        try std.testing.expectEqualSlices(u8, contents, actual_contents);
+    }
+
+    // With lock
+    {
+        var file = try tmp_dir.dir.openFile(io, "test_file", .{ .follow_symlinks = false, .lock = .exclusive });
+        defer file.close(io);
+
+        var file_reader = file.reader(io, &.{});
+        const actual_contents = try file_reader.interface.allocRemaining(testing.allocator, .unlimited);
+        defer testing.allocator.free(actual_contents);
+
+        try std.testing.expectEqualSlices(u8, contents, actual_contents);
+    }
+}
+
 test "Dir.statFile" {
     try testWithAllSupportedPathTypes(struct {
         fn impl(ctx: *TestContext) !void {
-- 
2.54.0