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, 1 insertions, 0 deletions

diff --git a/src/env.h b/src/env.h
index 534b888e2c7..df0df0b99ca 100644
--- a/src/env.h
+++ b/src/env.h
@@ -382,6 +382,7 @@ constexpr size_t kFsStatsBufferLength =
   V(async_wrap_ctor_template, v8::FunctionTemplate)                            \
   V(async_wrap_object_ctor_template, v8::FunctionTemplate)                     \
   V(compiled_fn_entry_template, v8::ObjectTemplate)                            \
+  V(dir_instance_template, v8::ObjectTemplate)                                 \
   V(fd_constructor_template, v8::ObjectTemplate)                               \
   V(fdclose_constructor_template, v8::ObjectTemplate)                          \
   V(filehandlereadwrap_template, v8::ObjectTemplate)                           \