Add owner semantics to RBToken

Stop exporting `_ringbuf` on `tractor.ipc`
Use absolute imports on `_ringbuf` module
Add more comments and acm helpers for ringbuf allocation functions
Create generic FD sharing actor module in `tractor.linux._fdshare`
Include original allocator actor name as `owner` in RBToken
Auto share FDs of allocated ringbufs
On `attach_ringbuf_*` functions request fds from owner
Adapt all ringbuf tests to new system

tests/test_ringbuf.py

@@ -7,6 +7,7 @@ import pytest
 import tractor
 from tractor.ipc._ringbuf import (
     open_ringbuf,
+    open_ringbuf_pair,
     attach_to_ringbuf_receiver,
     attach_to_ringbuf_sender,
     attach_to_ringbuf_channel,
@@ -68,7 +69,7 @@ async def child_write_shm(
     msg_amount: int,
     rand_min: int,
     rand_max: int,
-    token: RBToken,
+    buf_size: int
 ) -> None:
     '''
     Sub-actor used in `test_ringbuf`
@@ -85,9 +86,12 @@ async def child_write_shm(
         rand_min=rand_min,
         rand_max=rand_max,
     )
-    await ctx.started()
+    async with (
-    print('writer started')
+        open_ringbuf('test_ringbuf', buf_size=buf_size) as token,
-    async with attach_to_ringbuf_sender(token, cleanup=False) as sender:
+        attach_to_ringbuf_sender(token) as sender
+    ):
+        await ctx.started(token)
+        print('writer started')
         for msg in rng:
             await sender.send(msg)
 
@@ -133,55 +137,53 @@ def test_ringbuf(
 
     '''
     async def main():
-        with open_ringbuf(
+        async with tractor.open_nursery() as an:
-            'test_ringbuf',
+            send_p = await an.start_actor(
-            buf_size=buf_size
+                'ring_sender',
-        ) as token:
+                enable_modules=[
-            proc_kwargs = {'pass_fds': token.fds}
+                    __name__,
+                    'tractor.linux._fdshare'
+                ],
+            )
+            recv_p = await an.start_actor(
+                'ring_receiver',
+                enable_modules=[
+                    __name__,
+                    'tractor.linux._fdshare'
+                ],
+            )
+            async with (
+                send_p.open_context(
+                    child_write_shm,
+                    msg_amount=msg_amount,
+                    rand_min=rand_min,
+                    rand_max=rand_max,
+                    buf_size=buf_size
+                ) as (sctx, token),
 
-            async with tractor.open_nursery() as an:
+                recv_p.open_context(
-                send_p = await an.start_actor(
+                    child_read_shm,
-                    'ring_sender',
+                    token=token,
-                    enable_modules=[__name__],
+                ) as (rctx, _),
-                    proc_kwargs=proc_kwargs
+            ):
-                )
+                sent_hash = await sctx.result()
-                recv_p = await an.start_actor(
+                recvd_hash = await rctx.result()
-                    'ring_receiver',
-                    enable_modules=[__name__],
-                    proc_kwargs=proc_kwargs
-                )
-                async with (
-                    send_p.open_context(
-                        child_write_shm,
-                        token=token,
-                        msg_amount=msg_amount,
-                        rand_min=rand_min,
-                        rand_max=rand_max,
-                    ) as (sctx, _),
 
-                    recv_p.open_context(
+                assert sent_hash == recvd_hash
-                        child_read_shm,
-                        token=token,
-                    ) as (rctx, _),
-                ):
-                    sent_hash = await sctx.result()
-                    recvd_hash = await rctx.result()
 
-                    assert sent_hash == recvd_hash
+            await an.cancel()
-
-                await send_p.cancel_actor()
-                await recv_p.cancel_actor()
 
     trio.run(main)
 
 
 @tractor.context
-async def child_blocked_receiver(
+async def child_blocked_receiver(ctx: tractor.Context):
-    ctx: tractor.Context,
+    async with (
-    token: RBToken
+        open_ringbuf('test_ring_cancel_reader') as token,
-):
+
-    async with attach_to_ringbuf_receiver(token) as receiver:
+        attach_to_ringbuf_receiver(token) as receiver
-        await ctx.started()
+    ):
+        await ctx.started(token)
         await receiver.receive_some()
 
 
@@ -192,26 +194,23 @@ def test_reader_cancel():
 
     '''
     async def main():
-        with open_ringbuf('test_ring_cancel_reader') as token:
+        async with tractor.open_nursery() as an:
+            recv_p = await an.start_actor(
+                'ring_blocked_receiver',
+                enable_modules=[
+                    __name__,
+                    'tractor.linux._fdshare'
+                ],
+            )
             async with (
-                tractor.open_nursery() as an,
+                recv_p.open_context(
-                attach_to_ringbuf_sender(token) as _sender,
+                    child_blocked_receiver,
+                ) as (sctx, token),
+
+                attach_to_ringbuf_sender(token),
             ):
-                recv_p = await an.start_actor(
+                await trio.sleep(.1)
-                    'ring_blocked_receiver',
+                await an.cancel()
-                    enable_modules=[__name__],
-                    proc_kwargs={
-                        'pass_fds': token.fds
-                    }
-                )
-                async with (
-                    recv_p.open_context(
-                        child_blocked_receiver,
-                        token=token
-                    ) as (sctx, _sent),
-                ):
-                    await trio.sleep(1)
-                    await an.cancel()
 
 
     with pytest.raises(tractor._exceptions.ContextCancelled):
@@ -219,12 +218,16 @@ def test_reader_cancel():
 
 
 @tractor.context
-async def child_blocked_sender(
+async def child_blocked_sender(ctx: tractor.Context):
-    ctx: tractor.Context,
+    async with (
-    token: RBToken
+        open_ringbuf(
-):
+            'test_ring_cancel_sender',
-    async with attach_to_ringbuf_sender(token) as sender:
+            buf_size=1
-        await ctx.started()
+        ) as token,
+
+        attach_to_ringbuf_sender(token) as sender
+    ):
+        await ctx.started(token)
         await sender.send_all(b'this will wrap')
 
 
@@ -235,26 +238,23 @@ def test_sender_cancel():
 
     '''
     async def main():
-        with open_ringbuf(
+        async with tractor.open_nursery() as an:
-            'test_ring_cancel_sender',
+            recv_p = await an.start_actor(
-            buf_size=1
+                'ring_blocked_sender',
-        ) as token:
+                enable_modules=[
-            async with tractor.open_nursery() as an:
+                    __name__,
-                recv_p = await an.start_actor(
+                    'tractor.linux._fdshare'
-                    'ring_blocked_sender',
+                ],
-                    enable_modules=[__name__],
+            )
-                    proc_kwargs={
+            async with (
-                        'pass_fds': token.fds
+                recv_p.open_context(
-                    }
+                    child_blocked_sender,
-                )
+                ) as (sctx, token),
-                async with (
+
-                    recv_p.open_context(
+                attach_to_ringbuf_receiver(token)
-                        child_blocked_sender,
+            ):
-                        token=token
+                await trio.sleep(.1)
-                    ) as (sctx, _sent),
+                await an.cancel()
-                ):
-                    await trio.sleep(1)
-                    await an.cancel()
 
 
     with pytest.raises(tractor._exceptions.ContextCancelled):
@@ -274,24 +274,28 @@ def test_receiver_max_bytes():
     msgs = []
 
     async def main():
-        with open_ringbuf(
+        async with (
-            'test_ringbuf_max_bytes',
+            tractor.open_nursery(),
-            buf_size=10
+            open_ringbuf(
-        ) as token:
+                'test_ringbuf_max_bytes',
-            async with (
+                buf_size=10
-                trio.open_nursery() as n,
+            ) as token,
-                attach_to_ringbuf_sender(token, cleanup=False) as sender,
-                attach_to_ringbuf_receiver(token, cleanup=False) as receiver
-            ):
-                async def _send_and_close():
-                    await sender.send_all(msg)
-                    await sender.aclose()
 
-                n.start_soon(_send_and_close)
+            trio.open_nursery() as n,
-                while len(msgs) < len(msg):
+
-                    msg_part = await receiver.receive_some(max_bytes=1)
+            attach_to_ringbuf_sender(token, cleanup=False) as sender,
-                    assert len(msg_part) == 1
+
-                    msgs.append(msg_part)
+            attach_to_ringbuf_receiver(token, cleanup=False) as receiver
+        ):
+            async def _send_and_close():
+                await sender.send_all(msg)
+                await sender.aclose()
+
+            n.start_soon(_send_and_close)
+            while len(msgs) < len(msg):
+                msg_part = await receiver.receive_some(max_bytes=1)
+                assert len(msg_part) == 1
+                msgs.append(msg_part)
 
     trio.run(main)
     assert msg == b''.join(msgs)
@@ -329,42 +333,46 @@ def test_channel():
     msg_amount_min = 100
     msg_amount_max = 1000
 
+    mods = [
+        __name__,
+        'tractor.linux._fdshare'
+    ]
+
     async def main():
-        with tractor.ipc.open_ringbuf_pair(
+        async with (
-            'test_ringbuf_transport'
+            tractor.open_nursery(enable_modules=mods) as an,
-        ) as (send_token, recv_token):
+
+            open_ringbuf_pair(
+                'test_ringbuf_transport'
+            ) as (send_token, recv_token),
+
+            attach_to_ringbuf_channel(send_token, recv_token) as chan,
+        ):
+            sender = await an.start_actor(
+                'test_ringbuf_transport_sender',
+                enable_modules=mods,
+            )
             async with (
-                attach_to_ringbuf_channel(send_token, recv_token) as chan,
+                sender.open_context(
-                tractor.open_nursery() as an
+                    child_channel_sender,
+                    msg_amount_min=msg_amount_min,
+                    msg_amount_max=msg_amount_max,
+                    token_in=recv_token,
+                    token_out=send_token
+                ) as (ctx, _),
             ):
-                sender = await an.start_actor(
+                recvd_hash = hashlib.sha256()
-                    'test_ringbuf_transport_sender',
+                async for msg in chan:
-                    enable_modules=[__name__],
+                    if msg == b'bye':
-                    proc_kwargs={
+                        await chan.send(b'bye')
-                        'pass_fds': send_token.fds + recv_token.fds
+                        break
-                    }
-                )
-                async with (
-                    sender.open_context(
-                        child_channel_sender,
-                        msg_amount_min=msg_amount_min,
-                        msg_amount_max=msg_amount_max,
-                        token_in=recv_token,
-                        token_out=send_token
-                    ) as (ctx, _),
-                ):
-                    recvd_hash = hashlib.sha256()
-                    async for msg in chan:
-                        if msg == b'bye':
-                            await chan.send(b'bye')
-                            break
 
-                        recvd_hash.update(msg)
+                    recvd_hash.update(msg)
 
-                    sent_hash = await ctx.result()
+                sent_hash = await ctx.result()
 
-                    assert recvd_hash.hexdigest() == sent_hash
+                assert recvd_hash.hexdigest() == sent_hash
 
-                await an.cancel()
+            await an.cancel()
 
     trio.run(main)

tractor/ipc/__init__.py

@@ -17,26 +17,7 @@
 A modular IPC layer supporting the power of cross-process SC!
 
 '''
-import platform
-
 from ._chan import (
     _connect_chan as _connect_chan,
     Channel as Channel
 )
-
-if platform.system() == 'Linux':
-    from ._ringbuf import (
-        RBToken as RBToken,
-
-        open_ringbuf as open_ringbuf,
-        open_ringbuf_pair as open_ringbuf_pair,
-
-        RingBufferSendChannel as RingBufferSendChannel,
-        attach_to_ringbuf_sender as attach_to_ringbuf_sender,
-
-        RingBufferReceiveChannel as RingBufferReceiveChannel,
-        attach_to_ringbuf_receiver as attach_to_ringbuf_receiver,
-
-        RingBufferChannel as RingBufferChannel,
-        attach_to_ringbuf_channel as attach_to_ringbuf_channel,
-    )

tractor/ipc/_ringbuf/__init__.py

@@ -35,16 +35,22 @@ from msgspec import (
     to_builtins
 )
 
-from ...log import get_logger
+from tractor.log import get_logger
-from ..._exceptions import (
+from tractor._exceptions import (
     InternalError
 )
-from .._mp_bs import disable_mantracker
+from tractor.ipc._mp_bs import disable_mantracker
-from ...linux.eventfd import (
+from tractor.linux._fdshare import (
+    share_fds,
+    unshare_fds,
+    request_fds_from
+)
+from tractor.linux.eventfd import (
     open_eventfd,
     EFDReadCancelled,
     EventFD
 )
+from tractor._state import current_actor
 
 
 log = get_logger(__name__)
@@ -57,17 +63,19 @@ _DEFAULT_RB_SIZE = 10 * 1024
 
 class RBToken(Struct, frozen=True):
     '''
-    RingBuffer token contains necesary info to open the three
+    RingBuffer token contains necesary info to open resources of a ringbuf,
-    eventfds and the shared memory
+    even in the case that ringbuf was not allocated by current actor.
 
     '''
+    owner: str  # if owner != `current_actor().name` we must use fdshare
+
     shm_name: str
 
     write_eventfd: int  # used to signal writer ptr advance
     wrap_eventfd: int  # used to signal reader ready after wrap around
     eof_eventfd: int  # used to signal writer closed
 
-    buf_size: int
+    buf_size: int  # size in bytes of underlying shared memory buffer
 
     def as_msg(self):
         return to_builtins(self)
@@ -81,10 +89,6 @@ class RBToken(Struct, frozen=True):
 
     @property
     def fds(self) -> tuple[int, int, int]:
-        '''
-        Useful for `pass_fds` params
-
-        '''
         return (
             self.write_eventfd,
             self.wrap_eventfd,
@@ -92,38 +96,137 @@ class RBToken(Struct, frozen=True):
         )
 
 
-@cm
+def alloc_ringbuf(
-def open_ringbuf(
     shm_name: str,
     buf_size: int = _DEFAULT_RB_SIZE,
-) -> ContextManager[RBToken]:
+) -> tuple[SharedMemory, RBToken]:
     '''
-    Handle resources for a ringbuf (shm, eventfd), yield `RBToken` to
+    Allocate OS resources for a ringbuf.
-    be used with `attach_to_ringbuf_sender` and `attach_to_ringbuf_receiver`
-
     '''
     shm = SharedMemory(
         name=shm_name,
         size=buf_size,
         create=True
     )
-    try:
+    token = RBToken(
-        token = RBToken(
+        owner=current_actor().name,
-            shm_name=shm_name,
+        shm_name=shm_name,
-            write_eventfd=open_eventfd(),
+        write_eventfd=open_eventfd(),
-            wrap_eventfd=open_eventfd(),
+        wrap_eventfd=open_eventfd(),
-            eof_eventfd=open_eventfd(),
+        eof_eventfd=open_eventfd(),
-            buf_size=buf_size
+        buf_size=buf_size
-        )
+    )
-        yield token
+    # register fds for sharing
-
+    share_fds(
-    finally:
+        shm_name,
-        shm.unlink()
+        token.fds,
+    )
+    return shm, token
 
 
 @cm
-def open_ringbuf_pair(
+def open_ringbuf_sync(
-    name: str,
+    shm_name: str,
+    buf_size: int = _DEFAULT_RB_SIZE,
+) -> ContextManager[RBToken]:
+    '''
+    Handle resources for a ringbuf (shm, eventfd), yield `RBToken` to
+    be used with `attach_to_ringbuf_sender` and `attach_to_ringbuf_receiver`,
+    post yield maybe unshare fds and unlink shared memory
+
+    '''
+    shm: SharedMemory | None = None
+    token: RBToken | None = None
+    try:
+        shm, token = alloc_ringbuf(shm_name, buf_size=buf_size)
+        yield token
+
+    finally:
+        if token:
+            unshare_fds(shm_name)
+
+        if shm:
+            shm.unlink()
+
+@acm
+async def open_ringbuf(
+    shm_name: str,
+    buf_size: int = _DEFAULT_RB_SIZE,
+) -> AsyncContextManager[RBToken]:
+    '''
+    Helper to use `open_ringbuf_sync` inside an async with block.
+
+    '''
+    with open_ringbuf_sync(
+        shm_name,
+        buf_size=buf_size
+    ) as token:
+        yield token
+
+
+@cm
+def open_ringbufs_sync(
+    shm_names: list[str],
+    buf_sizes: int | list[str] = _DEFAULT_RB_SIZE,
+) -> ContextManager[tuple[RBToken]]:
+    '''
+    Handle resources for multiple ringbufs at once.
+
+    '''
+    # maybe convert single int into list
+    if isinstance(buf_sizes, int):
+        buf_size = [buf_sizes] * len(shm_names)
+
+    # ensure len(shm_names) == len(buf_sizes)
+    if (
+        isinstance(buf_sizes, list)
+        and
+        len(buf_sizes) != len(shm_names)
+    ):
+        raise ValueError(
+            'Expected buf_size list to be same length as shm_names'
+        )
+
+    # allocate resources
+    rings: list[tuple[SharedMemory, RBToken]] = [
+        alloc_ringbuf(shm_name, buf_size=buf_size)
+        for shm_name, buf_size in zip(shm_names, buf_size)
+    ]
+
+    try:
+        yield tuple([token for _, token in rings])
+
+    finally:
+        # attempt fd unshare and shm unlink for each
+        for shm, token in rings:
+            try:
+                unshare_fds(token.shm_name)
+
+            except RuntimeError:
+                log.exception(f'while unsharing fds of {token}')
+
+            shm.unlink()
+
+
+@acm
+async def open_ringbufs(
+    shm_names: list[str],
+    buf_sizes: int | list[str] = _DEFAULT_RB_SIZE,
+) -> AsyncContextManager[tuple[RBToken]]:
+    '''
+    Helper to use `open_ringbufs_sync` inside an async with block.
+
+    '''
+    with open_ringbufs_sync(
+        shm_names,
+        buf_sizes=buf_sizes
+    ) as tokens:
+        yield tokens
+
+
+@cm
+def open_ringbuf_pair_sync(
+    shm_name: str,
     buf_size: int = _DEFAULT_RB_SIZE
 ) -> ContextManager[tuple(RBToken, RBToken)]:
     '''
@@ -131,18 +234,30 @@ def open_ringbuf_pair(
     bidirectional messaging.
 
     '''
-    with (
+    with open_ringbufs_sync(
-        open_ringbuf(
+        [
-            name + '.send',
+            f'{shm_name}.send',
-            buf_size=buf_size
+            f'{shm_name}.recv'
-        ) as send_token,
+        ],
+        buf_sizes=buf_size
+    ) as tokens:
+        yield tokens
 
-        open_ringbuf(
+
-            name + '.recv',
+@acm
-            buf_size=buf_size
+async def open_ringbuf_pair(
-        ) as recv_token
+    shm_name: str,
-    ):
+    buf_size: int = _DEFAULT_RB_SIZE
-        yield send_token, recv_token
+) -> AsyncContextManager[tuple[RBToken, RBToken]]:
+    '''
+    Helper to use `open_ringbuf_pair_sync` inside an async with block.
+
+    '''
+    with open_ringbuf_pair_sync(
+        shm_name,
+        buf_size=buf_size
+    ) as tokens:
+        yield tokens
 
 
 Buffer = bytes | bytearray | memoryview
@@ -640,6 +755,29 @@ class RingBufferReceiveChannel(trio.abc.ReceiveChannel[bytes]):
         return self
 
 
+async def _maybe_obtain_shared_resources(token: RBToken):
+    token = RBToken.from_msg(token)
+
+    # maybe token wasn't allocated by current actor
+    if token.owner != current_actor().name:
+        # use fdshare module to retrieve a copy of the FDs
+        fds = await request_fds_from(
+            token.owner,
+            token.shm_name
+        )
+        write, wrap, eof = fds
+        # rebuild token using FDs copies
+        token = RBToken(
+            owner=token.owner,
+            shm_name=token.shm_name,
+            write_eventfd=write,
+            wrap_eventfd=wrap,
+            eof_eventfd=eof,
+            buf_size=token.buf_size
+        )
+
+    return token
+
 @acm
 async def attach_to_ringbuf_receiver(
 
@@ -651,8 +789,13 @@ async def attach_to_ringbuf_receiver(
     Attach a RingBufferReceiveChannel from a previously opened
     RBToken.
 
+    Requires tractor runtime to be up in order to support opening a ringbuf
+    originally allocated by a different actor.
+
     Launches `receiver._eof_monitor_task` in a `trio.Nursery`.
     '''
+    token = await _maybe_obtain_shared_resources(token)
+
     async with (
         trio.open_nursery(strict_exception_groups=False) as n,
         RingBufferReceiveChannel(
@@ -676,7 +819,12 @@ async def attach_to_ringbuf_sender(
     Attach a RingBufferSendChannel from a previously opened
     RBToken.
 
+    Requires tractor runtime to be up in order to support opening a ringbuf
+    originally allocated by a different actor.
+
     '''
+    token = await _maybe_obtain_shared_resources(token)
+
     async with RingBufferSendChannel(
         token,
         batch_size=batch_size,

tractor/linux/__init__.py

@@ -13,7 +13,3 @@
 
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
-from ._fdshare import (
-    send_fds as send_fds,
-    recv_fds as recv_fds
-)

tractor/linux/_fdshare.py

@@ -21,13 +21,19 @@ https://github.com/python/cpython/blob/275056a7fdcbe36aaac494b4183ae59943a338eb/
 '''
 import os
 import array
+import tempfile
+from pathlib import Path
 from typing import AsyncContextManager
 from contextlib import asynccontextmanager as acm
 
 import trio
+import tractor
 from trio import socket
 
 
+log = tractor.log.get_logger(__name__)
+
+
 class FDSharingError(Exception):
     ...
 
@@ -157,3 +163,126 @@ async def recv_fds(sock_path: str, amount: int) -> tuple:
         )
 
     return tuple(a)
+
+
+'''
+Share FD actor module
+
+Add "tractor.linux._fdshare" to enabled modules on actors to allow sharing of
+FDs with other actors.
+
+Use `share_fds` function to register a set of fds with a name, then other
+actors can use `request_fds_from` function to retrieve the fds.
+
+Use `unshare_fds` to disable sharing of  a set of FDs.
+
+'''
+
+FDType = tuple[int]
+
+_fds: dict[str, FDType] = {}
+
+
+def maybe_get_fds(name: str) -> FDType | None:
+    '''
+    Get registered FDs with a given name or return None
+
+    '''
+    return _fds.get(name, None)
+
+
+def get_fds(name: str) -> FDType:
+    '''
+    Get registered FDs with a given name or raise
+    '''
+    fds = maybe_get_fds(name)
+
+    if not fds:
+        raise RuntimeError(f'No FDs with name {name} found!')
+
+    return fds
+
+
+def share_fds(
+    name: str,
+    fds: tuple[int],
+) -> None:
+    '''
+    Register a set of fds to be shared under a given name.
+
+    '''
+    maybe_fds = maybe_get_fds(name)
+    if maybe_fds:
+        raise RuntimeError(f'share FDs: {maybe_fds} already tied to name {name}')
+
+    _fds[name] = fds
+
+
+def unshare_fds(name: str) -> None:
+    '''
+    Unregister a set of fds to disable sharing them.
+
+    '''
+    get_fds(name)  # raise if not exists
+
+    del _fds[name]
+
+
+@tractor.context
+async def _pass_fds(
+    ctx: tractor.Context,
+    name: str,
+    sock_path: str
+) -> None:
+    '''
+    Endpoint to request a set of FDs from current actor, will use `ctx.started`
+    to send original FDs, then `send_fds` will block until remote side finishes
+    the `recv_fds` call.
+
+    '''
+    # get fds or raise error
+    fds = get_fds(name)
+
+    # start fd passing context using socket on `sock_path`
+    async with send_fds(fds, sock_path):
+        # send original fds through ctx.started
+        await ctx.started(fds)
+
+
+async def request_fds_from(
+    actor_name: str,
+    fds_name: str
+) -> FDType:
+    '''
+    Use this function to retreive shared FDs from `actor_name`.
+
+    '''
+    this_actor = tractor.current_actor()
+
+    # create a temporary path for the UDS sock
+    sock_path = str(
+        Path(tempfile.gettempdir())
+        /
+        f'{fds_name}-from-{actor_name}-to-{this_actor.name}.sock'
+    )
+
+    async with (
+        tractor.find_actor(actor_name) as portal,
+
+        portal.open_context(
+            _pass_fds,
+            name=fds_name,
+            sock_path=sock_path
+        ) as (ctx, fds_info),
+    ):
+        # get original FDs
+        og_fds = fds_info
+
+        # retrieve copies of FDs
+        fds = await recv_fds(sock_path, len(og_fds))
+
+        log.info(
+            f'{this_actor.name} received fds: {og_fds} -> {fds}'
+        )
+
+        return fds