re-implement Windows ReadDir on top of GetFileInformationByHandleEx by RalfJung · Pull Request #164529 · rust-lang/rust

RalfJung · 2026-10-18T17:44:35Z

The current implementation based on FindFirstFileExW fundamentally relies on knowing the path to the directory, so implementing Dir::read_dir (iterating a directory based on a Dir handle) is not possible. remove_dir_all already uses a completely different implementation based on GetFileInformationByHandleEx that can work with a directory handle. This PR adjusts fs::read_dir to also use an implementation based on GetFileInformationByHandleEx.

The directory entry buffer is currently 1 KiB large, as that's what we are using for remove_dir_all. That is fairly small; it means we have to do a new syscall around every 9 entries. For comparison, the glibc implementation uses a buffer of 32 KiB by default.

try-job: test-x86_64-msvc*

RalfJung · 2026-10-18T17:48:08Z

 impl From<c::WIN32_FIND_DATAW> for FileAttr {
    fn from(wfd: c::WIN32_FIND_DATAW) -> Self {


FWIW this conversion from WIN32_FIND_DATAW is still used by fs::metadata. File::metadata uses yet another conversion, from BY_HANDLE_FILE_INFORMATION. And DirEntry::metadata now uses FILE_ID_BOTH_DIR_INFO.

No idea if it makes sense to consolidate that, but I'd say that would be a job for a different PR.

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RalfJung · 2026-10-18T17:48:34Z

I don't have a Windows machine so this is only build-tested so far...
@bors try

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

RalfJung · 2026-10-18T20:50:56Z

@bors try

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

rust-bors · 2026-10-18T23:47:12Z

☀️ Try build successful (CI)
Build commit: c9ad66c (c9ad66c5d99336ce105e9b2591e1c8f87d9ecf22)
Base parent: 15c1044 (15c10447a46529b68e38c7f138aeb95240e4d7d6)

RalfJung · 2026-10-19T06:06:27Z

@bors try

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

RalfJung · 2026-10-19T06:08:35Z

+        // `read_dir` on a non-directory should fail, but Windows `Dir::open` suceeds.
+        // So we have to specifically check for this.
+        return Err(io::const_error!(io::ErrorKind::NotADirectory, "path is not a directory"));
+    }


Would it maybe be better to call buf.fill() already here? Would that "naturall" throw an error for non-directories, or would we still need a dedicated check then?
Currently we delay that until the first next() call. That matches what glibc does.

View changes since the review

I think what you currently have is fine for now. Ideally we would be using Dir to limit it to opening a directory but, as I said, I'm uncomfortable tying this to a new unstable feature.

I think Dir::open (open dir with just a path) will happily open files on Windows, so using Dir would not help.

Dir::open_dir (open dir relative to Dir handle) ensures that it has a directory, but that requires us to already have a Dir to begin with.

(Yeah I find that naming quite confusing... but it's what got approved in the ACP so I guess other people like it.)

rustbot · 2026-10-19T06:09:10Z

r? @Mark-Simulacrum

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RalfJung · 2026-10-19T06:09:15Z

r? @ChrisDenton

rustbot · 2026-10-19T06:09:21Z

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rust-bors · 2026-10-19T09:01:49Z

☀️ Try build successful (CI)
Build commit: 60c0f2e (60c0f2e84cba1dcbd8c1f6b32d9cfae5fc6af11b)
Base parent: b6eeb93 (b6eeb93c992027a03e1fd120b6652546396b142c)

ChrisDenton · 2026-10-20T03:19:15Z

While I think this is the right long-term direction I admit I am concerned about re-implementing an important and long-stable function on top of an experimental and only partially implemented new feature. This would make std::fs::read_dir sensitive to any changes or problems with Dir.

RalfJung · 2026-10-20T06:16:56Z

Note that the only thing we do with Dir is Dir::open, which is a fairly "traditional" method (it uses CreateFileW, not NtCreateFile). I can inline that CreateFileW call if you prefer.

This is on the critical path for rust-lang/miri#4982 so I'd like to find some way to make progress here. Happy to take suggestions for the best way to do that.

RalfJung · 2026-10-20T08:23:22Z

Also, CreateFileW + GetFileInformationByHandleEx has been used in remove_dir_all on Windows for a long time now. So I'd say that combination is pretty battle-tested at this point.

Rather than using Dir::open I can also use File::open to get the handle. That's what remove_dir_all does. It just seemed kind of hacky.

RalfJung · 2026-10-20T09:36:23Z

I have reused the code from remove_dir_all now for opening the directory, so this no longer has any interaction with fs::Dir.

@bors try

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

rust-bors · 2026-10-20T12:17:39Z

☀️ Try build successful (CI)
Build commit: 54d7313 (54d7313ab1a96805f76681b87b2d85369a5652f3)
Base parent: 39b126b (39b126b8e227197c688fed9f9962658bdd89a761)

ChrisDenton · 2026-10-20T14:51:51Z

    fs::create_dir_all(&tmpdir.join("a")).unwrap();
    File::create(&tmpdir.join("b")).unwrap();
+    num_files += 2;
+    if got_symlink_permission(&tmpdir) {


This either shouldn't use the same tmpdir as is used for the read_dir tests or else got_symlink_permission should cleanup any files it creates.

View changes since the review

Oh I see, good point. I made got_symlink_permission clean up after itself.

RalfJung · 2026-10-21T10:24:49Z

@bors try

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

RalfJung · 2026-10-21T10:26:19Z

        options.read(true);
-        // FIXME: use race-free handle-based approach instead.
-        Dir::open(&self.root, &options)
+        dir_handle.open_dir(Path::new(""), &options)


Now that we have an underlying handle, we can also implement this method properly. :)

(This is only used by the unstable ReadDir::dir.)

View changes since the review

rust-bors · 2026-10-21T12:47:56Z

☀️ Try build successful (CI)
Build commit: 7369f4e (7369f4e2cf33f04741ad8ca17a291ead9e7b1bba)
Base parent: 1731fae (1731faea0da5b4ebf881d8e53f98066c17a74711)

rustbot · 2026-10-23T08:15:26Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added O-windows Operating system: Windows S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 18, 2026

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Auto merge of #164529 - RalfJung:windows-read-dir, r=<try>

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re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

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RalfJung force-pushed the windows-read-dir branch from 25e62dc to 1b5dd74 Compare October 18, 2026 20:50

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Auto merge of #164529 - RalfJung:windows-read-dir, r=<try>

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re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

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Auto merge of #164529 - RalfJung:windows-read-dir, r=<try>

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RalfJung commented Oct 19, 2026

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RalfJung marked this pull request as ready for review October 19, 2026 06:09

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 19, 2026

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RalfJung force-pushed the windows-read-dir branch from fe876a5 to a1a11c2 Compare October 19, 2026 07:01

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Auto merge of #164529 - RalfJung:windows-read-dir, r=<try>

54d7313

re-implement Windows ReadDir on top of GetFileInformationByHandleEx try-job: test-x86_64-msvc*

RalfJung force-pushed the windows-read-dir branch from 4da8841 to 436436c Compare October 20, 2026 09:39

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Auto merge of #164529 - RalfJung:windows-read-dir, r=<try>

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RalfJung added 2 commits October 23, 2026 10:15

re-implement Windows ReadDir on top of GetFileInformationByHandleEx

9309c1c

make ReadDir::dir race-free on Windows

dd36e1b

RalfJung force-pushed the windows-read-dir branch from 9de1464 to dd36e1b Compare October 23, 2026 08:15

		impl From<c::WIN32_FIND_DATAW> for FileAttr {
		fn from(wfd: c::WIN32_FIND_DATAW) -> Self {

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