From 76a795de1aa803d41bdfaeca6bf3855c12c5d940 Mon Sep 17 00:00:00 2001
From: William Manley <will@williammanley.net>
Date: Sat, 24 Apr 2021 23:17:56 +0100
Subject: [PATCH] impl Send for `DirIter`

This will allow using `DirIter` with rayon's `par_bridge`.

It may not be thread-safe to call readdir concurrently from multiple
threads on a single `DIR*`, but all `Send` requires is that we can call
it from different threads non-concurrently - so this is fine.

`man readdir` says:

> It is expected that a future version of POSIX.1 will require that
> `readdir()` be thread-safe when concurrently employed on different
> directory streams.

so in the future we may also be able to implement `Sync`.  The `std`
implements both `Send` and `Sync` on [`ReadDir`], but they implement
iteration using the deprecated [`readdir_r`].

To be sure I've also looked at the implementation of readdir on [glibc],
[musl] and [freebsd] and they all use per `struct DIR` allocations for
the dirent buffer.

[`ReadDir`]: https://github.com/rust-lang/rust/blob/7e11f3a8f3c1b2683125e7def0acb68a6d684f92/library/std/src/sys/unix/fs.rs#L478-L511
[`readdir_r`]: https://man7.org/linux/man-pages/man3/readdir_r.3.html
[glibc]: https://code.woboq.org/userspace/glibc/sysdeps/posix/readdir.c.html
[musl]: https://git.musl-libc.org/cgit/musl/tree/src/dirent/readdir.c
[freebsd]: https://github.com/freebsd/freebsd-src/blob/373ffc62c158e52cde86a5b934ab4a51307f9f2e/lib/libc/gen/opendir.c#L337
---
 src/list.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/list.rs b/src/list.rs
index 5b4d3cd..e329f20 100644
--- a/src/list.rs
+++ b/src/list.rs
@@ -21,6 +21,18 @@ pub struct DirIter {
     dir: *mut libc::DIR,
 }
 
+// It may not be thread-safe to call readdir concurrently from multiple threads on a single
+// `DIR*`, but all `Send` requires is that we can call it from different threads
+// non-concurrently - so this is fine.
+//
+// `man readdir` says:
+//
+// > It is expected that a future version of POSIX.1 will require that readdir() be
+// > thread-safe when concurrently employed on different directory streams.
+//
+// so in the future we may also be able to implement `Sync`.
+unsafe impl Send for DirIter {}
+
 /// Position in a DirIter as obtained by 'DirIter::current_position()'
 ///
 /// The position is only valid for the DirIter it was retrieved from.