fs: introduce `opendir()` and `fs.Dir`

This adds long-requested methods for asynchronously interacting and
iterating through directory entries by using `uv_fs_opendir`,
`uv_fs_readdir`, and `uv_fs_closedir`.

`fs.opendir()` and friends return an `fs.Dir`, which contains methods
for doing reads and cleanup. `fs.Dir` also has the async iterator
symbol exposed.

The `read()` method and friends only return `fs.Dirent`s for this API.
Having a entry type or doing a `stat` call is deemed to be necessary in
the majority of cases, so just returning dirents seems like the logical
choice for a new api.

Reading when there are no more entries returns `null` instead of a
dirent. However the async iterator hides that (and does automatic
cleanup).

The code lives in separate files from the rest of fs, this is done
partially to prevent over-pollution of those (already very large)
files, but also in the case of js allows loading into `fsPromises`.

Due to async_hooks, this introduces a new handle type of `DIRHANDLE`.

This PR does not attempt to make complete optimization of
this feature. Notable future improvements include:
- Moving promise work into C++ land like FileHandle.
- Possibly adding `readv()` to do multi-entry directory reads.
- Aliasing `fs.readdir` to `fs.scandir` and doing a deprecation.

Refs: https://github.com/nodejs/node-v0.x-archive/issues/388
Refs: https://github.com/nodejs/node/issues/583
Refs: https://github.com/libuv/libuv/pull/2057

PR-URL: https://github.com/nodejs/node/pull/29349
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/api/errors.md b/doc/api/errors.md
index e13e25827e..8c86012630 100644
--- a/doc/api/errors.md
+++ b/doc/api/errors.md
@@ -826,6 +826,11 @@ A signing `key` was not provided to the [`sign.sign()`][] method.
 [`crypto.timingSafeEqual()`][] was called with `Buffer`, `TypedArray`, or
 `DataView` arguments of different lengths.
 
+<a id="ERR_DIR_CLOSED"></a>
+### ERR_DIR_CLOSED
+
+The [`fs.Dir`][] was previously closed.
+
 <a id="ERR_DNS_SET_SERVERS_FAILED"></a>
 ### ERR_DNS_SET_SERVERS_FAILED
 
@@ -2388,6 +2393,7 @@ such as `process.stdout.on('data')`.
 [`dgram.disconnect()`]: dgram.html#dgram_socket_disconnect
 [`dgram.remoteAddress()`]: dgram.html#dgram_socket_remoteaddress
 [`errno`(3) man page]: http://man7.org/linux/man-pages/man3/errno.3.html
+[`fs.Dir`]: fs.html#fs_class_fs_dir
 [`fs.readFileSync`]: fs.html#fs_fs_readfilesync_path_options
 [`fs.readdir`]: fs.html#fs_fs_readdir_path_options_callback
 [`fs.symlink()`]: fs.html#fs_fs_symlink_target_path_type_callback