about summary refs log tree commit diff
path: root/tvix/castore/src/fs/mod.rs
blob: 176199f64aaf0c736381aa41d005b74451fa28fb (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
mod file_attr;
mod inode_tracker;
mod inodes;
mod root_nodes;

#[cfg(feature = "fuse")]
pub mod fuse;

#[cfg(feature = "virtiofs")]
pub mod virtiofs;

pub use self::root_nodes::RootNodes;
use self::{
    file_attr::ROOT_FILE_ATTR,
    inode_tracker::InodeTracker,
    inodes::{DirectoryInodeData, InodeData},
};
use crate::proto as castorepb;
use crate::{
    blobservice::{BlobReader, BlobService},
    directoryservice::DirectoryService,
    proto::{node::Node, NamedNode},
    B3Digest,
};
use bstr::ByteVec;
use bytes::Bytes;
use fuse_backend_rs::abi::fuse_abi::{stat64, OpenOptions};
use fuse_backend_rs::api::filesystem::{
    Context, FileSystem, FsOptions, GetxattrReply, ListxattrReply, ROOT_ID,
};
use futures::StreamExt;
use parking_lot::RwLock;
use std::sync::Mutex;
use std::{
    collections::HashMap,
    io,
    sync::atomic::AtomicU64,
    sync::{atomic::Ordering, Arc},
    time::Duration,
};
use std::{ffi::CStr, io::Cursor};
use tokio::{
    io::{AsyncReadExt, AsyncSeekExt},
    sync::mpsc,
};
use tracing::{debug, error, instrument, warn, Span};

/// This implements a read-only FUSE filesystem for a tvix-store
/// with the passed [BlobService], [DirectoryService] and [RootNodes].
///
/// Linux uses inodes in filesystems. When implementing FUSE, most calls are
/// *for* a given inode.
///
/// This means, we need to have a stable mapping of inode numbers to the
/// corresponding store nodes.
///
/// We internally delegate all inode allocation and state keeping to the
/// inode tracker.
/// We store a mapping from currently "explored" names in the root to their
/// inode.
///
/// There's some places where inodes are allocated / data inserted into
/// the inode tracker, if not allocated before already:
///  - Processing a `lookup` request, either in the mount root, or somewhere
///    deeper.
///  - Processing a `readdir` request
///
///  Things pointing to the same contents get the same inodes, irrespective of
///  their own location.
///  This means:
///  - Symlinks with the same target will get the same inode.
///  - Regular/executable files with the same contents will get the same inode
///  - Directories with the same contents will get the same inode.
///
/// Due to the above being valid across the whole store, and considering the
/// merkle structure is a DAG, not a tree, this also means we can't do "bucketed
/// allocation", aka reserve Directory.size inodes for each directory node we
/// explore.
/// Tests for this live in the tvix-store crate.
pub struct TvixStoreFs<BS, DS, RN> {
    blob_service: BS,
    directory_service: DS,
    root_nodes_provider: RN,

    /// Whether to (try) listing elements in the root.
    list_root: bool,

    /// Whether to expose blob and directory digests as extended attributes.
    show_xattr: bool,

    /// This maps a given basename in the root to the inode we allocated for the node.
    root_nodes: RwLock<HashMap<Bytes, u64>>,

    /// This keeps track of inodes and data alongside them.
    inode_tracker: RwLock<InodeTracker>,

    // FUTUREWORK: have a generic container type for dir/file handles and handle
    // allocation.
    /// Maps from the handle returned from an opendir to
    /// This holds all opendir handles (for the root inode)
    /// They point to the rx part of the channel producing the listing.
    #[allow(clippy::type_complexity)]
    dir_handles: RwLock<
        HashMap<
            u64,
            (
                Span,
                Arc<Mutex<mpsc::Receiver<(usize, Result<Node, crate::Error>)>>>,
            ),
        >,
    >,

    next_dir_handle: AtomicU64,

    /// This holds all open file handles
    #[allow(clippy::type_complexity)]
    file_handles: RwLock<HashMap<u64, (Span, Arc<Mutex<Box<dyn BlobReader>>>)>>,

    next_file_handle: AtomicU64,

    tokio_handle: tokio::runtime::Handle,
}

impl<BS, DS, RN> TvixStoreFs<BS, DS, RN>
where
    BS: AsRef<dyn BlobService> + Clone + Send,
    DS: AsRef<dyn DirectoryService> + Clone + Send + 'static,
    RN: RootNodes + Clone + 'static,
{
    pub fn new(
        blob_service: BS,
        directory_service: DS,
        root_nodes_provider: RN,
        list_root: bool,
        show_xattr: bool,
    ) -> Self {
        Self {
            blob_service,
            directory_service,
            root_nodes_provider,

            list_root,
            show_xattr,

            root_nodes: RwLock::new(HashMap::default()),
            inode_tracker: RwLock::new(Default::default()),

            dir_handles: RwLock::new(Default::default()),
            next_dir_handle: AtomicU64::new(1),

            file_handles: RwLock::new(Default::default()),
            next_file_handle: AtomicU64::new(1),
            tokio_handle: tokio::runtime::Handle::current(),
        }
    }

    /// Retrieves the inode for a given root node basename, if present.
    /// This obtains a read lock on self.root_nodes.
    fn get_inode_for_root_name(&self, name: &[u8]) -> Option<u64> {
        self.root_nodes.read().get(name).cloned()
    }

    /// For a given inode, look up the given directory behind it (from
    /// self.inode_tracker), and return its children.
    /// The inode_tracker MUST know about this inode already, and it MUST point
    /// to a [InodeData::Directory].
    /// It is ok if it's a [DirectoryInodeData::Sparse] - in that case, a lookup
    /// in self.directory_service is performed, and self.inode_tracker is updated with the
    /// [DirectoryInodeData::Populated].
    #[instrument(skip(self), err)]
    fn get_directory_children(&self, ino: u64) -> io::Result<(B3Digest, Vec<(u64, Node)>)> {
        let data = self.inode_tracker.read().get(ino).unwrap();
        match *data {
            // if it's populated already, return children.
            InodeData::Directory(DirectoryInodeData::Populated(
                ref parent_digest,
                ref children,
            )) => Ok((parent_digest.clone(), children.clone())),
            // if it's sparse, fetch data using directory_service, populate child nodes
            // and update it in [self.inode_tracker].
            InodeData::Directory(DirectoryInodeData::Sparse(ref parent_digest, _)) => {
                let directory = self
                    .tokio_handle
                    .block_on({
                        let directory_service = self.directory_service.clone();
                        let parent_digest = parent_digest.to_owned();
                        async move { directory_service.as_ref().get(&parent_digest).await }
                    })?
                    .ok_or_else(|| {
                        warn!(directory.digest=%parent_digest, "directory not found");
                        // If the Directory can't be found, this is a hole, bail out.
                        io::Error::from_raw_os_error(libc::EIO)
                    })?;

                // Turn the retrieved directory into a InodeData::Directory(DirectoryInodeData::Populated(..)),
                // allocating inodes for the children on the way.
                // FUTUREWORK: there's a bunch of cloning going on here, which we can probably avoid.
                let children = {
                    let mut inode_tracker = self.inode_tracker.write();

                    let children: Vec<(u64, castorepb::node::Node)> = directory
                        .nodes()
                        .map(|child_node| {
                            let (inode_data, _) = InodeData::from_node(child_node.clone());

                            let child_ino = inode_tracker.put(inode_data);
                            (child_ino, child_node)
                        })
                        .collect();

                    // replace.
                    inode_tracker.replace(
                        ino,
                        Arc::new(InodeData::Directory(DirectoryInodeData::Populated(
                            parent_digest.clone(),
                            children.clone(),
                        ))),
                    );

                    children
                };

                Ok((parent_digest.clone(), children))
            }
            // if the parent inode was not a directory, this doesn't make sense
            InodeData::Regular(..) | InodeData::Symlink(_) => {
                Err(io::Error::from_raw_os_error(libc::ENOTDIR))
            }
        }
    }

    /// This will turn a lookup request for a name in the root to a ino and
    /// [InodeData].
    /// It will peek in [self.root_nodes], and then either look it up from
    /// [self.inode_tracker],
    /// or otherwise fetch from [self.root_nodes], and then insert into
    /// [self.inode_tracker].
    /// In the case the name can't be found, a libc::ENOENT is returned.
    fn name_in_root_to_ino_and_data(
        &self,
        name: &std::ffi::CStr,
    ) -> io::Result<(u64, Arc<InodeData>)> {
        // Look up the inode for that root node.
        // If there's one, [self.inode_tracker] MUST also contain the data,
        // which we can then return.
        if let Some(inode) = self.get_inode_for_root_name(name.to_bytes()) {
            return Ok((
                inode,
                self.inode_tracker
                    .read()
                    .get(inode)
                    .expect("must exist")
                    .to_owned(),
            ));
        }

        // We don't have it yet, look it up in [self.root_nodes].
        match self.tokio_handle.block_on({
            let root_nodes_provider = self.root_nodes_provider.clone();
            async move { root_nodes_provider.get_by_basename(name.to_bytes()).await }
        }) {
            // if there was an error looking up the root node, propagate up an IO error.
            Err(_e) => Err(io::Error::from_raw_os_error(libc::EIO)),
            // the root node doesn't exist, so the file doesn't exist.
            Ok(None) => Err(io::Error::from_raw_os_error(libc::ENOENT)),
            // The root node does exist
            Ok(Some(root_node)) => {
                // The name must match what's passed in the lookup, otherwise this is also a ENOENT.
                if root_node.get_name() != name.to_bytes() {
                    debug!(root_node.name=?root_node.get_name(), found_node.name=%name.to_string_lossy(), "node name mismatch");
                    return Err(io::Error::from_raw_os_error(libc::ENOENT));
                }

                // Let's check if someone else beat us to updating the inode tracker and
                // root_nodes map. This avoids locking inode_tracker for writing.
                if let Some(ino) = self.root_nodes.read().get(name.to_bytes()) {
                    return Ok((
                        *ino,
                        self.inode_tracker.read().get(*ino).expect("must exist"),
                    ));
                }

                // Only in case it doesn't, lock [self.root_nodes] and
                // [self.inode_tracker] for writing.
                let mut root_nodes = self.root_nodes.write();
                let mut inode_tracker = self.inode_tracker.write();

                // insert the (sparse) inode data and register in
                // self.root_nodes.
                let (inode_data, name) = InodeData::from_node(root_node);
                let ino = inode_tracker.put(inode_data.clone());
                root_nodes.insert(name, ino);

                Ok((ino, Arc::new(inode_data)))
            }
        }
    }
}

/// Buffer size of the channel providing nodes in the mount root
const ROOT_NODES_BUFFER_SIZE: usize = 16;

const XATTR_NAME_DIRECTORY_DIGEST: &[u8] = b"user.tvix.castore.directory.digest";
const XATTR_NAME_BLOB_DIGEST: &[u8] = b"user.tvix.castore.blob.digest";

impl<BS, DS, RN> FileSystem for TvixStoreFs<BS, DS, RN>
where
    BS: AsRef<dyn BlobService> + Clone + Send + 'static,
    DS: AsRef<dyn DirectoryService> + Send + Clone + 'static,
    RN: RootNodes + Clone + 'static,
{
    type Handle = u64;
    type Inode = u64;

    fn init(&self, _capable: FsOptions) -> io::Result<FsOptions> {
        Ok(FsOptions::empty())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn getattr(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _handle: Option<Self::Handle>,
    ) -> io::Result<(stat64, Duration)> {
        if inode == ROOT_ID {
            return Ok((ROOT_FILE_ATTR.into(), Duration::MAX));
        }

        match self.inode_tracker.read().get(inode) {
            None => Err(io::Error::from_raw_os_error(libc::ENOENT)),
            Some(inode_data) => {
                debug!(inode_data = ?inode_data, "found node");
                Ok((inode_data.as_fuse_file_attr(inode).into(), Duration::MAX))
            }
        }
    }

    #[tracing::instrument(skip_all, fields(rq.parent_inode = parent, rq.name = ?name))]
    fn lookup(
        &self,
        _ctx: &Context,
        parent: Self::Inode,
        name: &std::ffi::CStr,
    ) -> io::Result<fuse_backend_rs::api::filesystem::Entry> {
        debug!("lookup");

        // This goes from a parent inode to a node.
        // - If the parent is [ROOT_ID], we need to check
        //   [self.root_nodes] (fetching from a [RootNode] provider if needed)
        // - Otherwise, lookup the parent in [self.inode_tracker] (which must be
        //   a [InodeData::Directory]), and find the child with that name.
        if parent == ROOT_ID {
            let (ino, inode_data) = self.name_in_root_to_ino_and_data(name)?;

            debug!(inode_data=?&inode_data, ino=ino, "Some");
            return Ok(inode_data.as_fuse_entry(ino));
        }
        // This is the "lookup for "a" inside inode 42.
        // We already know that inode 42 must be a directory.
        let (parent_digest, children) = self.get_directory_children(parent)?;

        Span::current().record("directory.digest", parent_digest.to_string());
        // Search for that name in the list of children and return the FileAttrs.

        // in the children, find the one with the desired name.
        if let Some((child_ino, _)) = children.iter().find(|e| e.1.get_name() == name.to_bytes()) {
            // lookup the child [InodeData] in [self.inode_tracker].
            // We know the inodes for children have already been allocated.
            let child_inode_data = self.inode_tracker.read().get(*child_ino).unwrap();

            // Reply with the file attributes for the child.
            // For child directories, we still have all data we need to reply.
            Ok(child_inode_data.as_fuse_entry(*child_ino))
        } else {
            // Child not found, return ENOENT.
            Err(io::Error::from_raw_os_error(libc::ENOENT))
        }
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn opendir(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
    ) -> io::Result<(Option<Self::Handle>, OpenOptions)> {
        // In case opendir on the root is called, we provide the handle, as re-entering that listing is expensive.
        // For all other directory inodes we just let readdir take care of it.
        if inode == ROOT_ID {
            if !self.list_root {
                return Err(io::Error::from_raw_os_error(libc::EPERM)); // same error code as ipfs/kubo
            }

            let root_nodes_provider = self.root_nodes_provider.clone();
            let (tx, rx) = mpsc::channel(ROOT_NODES_BUFFER_SIZE);

            // This task will run in the background immediately and will exit
            // after the stream ends or if we no longer want any more entries.
            self.tokio_handle.spawn(async move {
                let mut stream = root_nodes_provider.list().enumerate();
                while let Some(node) = stream.next().await {
                    if tx.send(node).await.is_err() {
                        // If we get a send error, it means the sync code
                        // doesn't want any more entries.
                        break;
                    }
                }
            });

            // Put the rx part into [self.dir_handles].
            // TODO: this will overflow after 2**64 operations,
            // which is fine for now.
            // See https://cl.tvl.fyi/c/depot/+/8834/comment/a6684ce0_d72469d1
            // for the discussion on alternatives.
            let dh = self.next_dir_handle.fetch_add(1, Ordering::SeqCst);

            self.dir_handles
                .write()
                .insert(dh, (Span::current(), Arc::new(Mutex::new(rx))));

            return Ok((
                Some(dh),
                fuse_backend_rs::api::filesystem::OpenOptions::empty(), // TODO: non-seekable
            ));
        }

        Ok((None, OpenOptions::empty()))
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle, rq.offset = offset), parent = self.dir_handles.read().get(&handle).and_then(|x| x.0.id()))]
    fn readdir(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        handle: Self::Handle,
        _size: u32,
        offset: u64,
        add_entry: &mut dyn FnMut(fuse_backend_rs::api::filesystem::DirEntry) -> io::Result<usize>,
    ) -> io::Result<()> {
        debug!("readdir");

        if inode == ROOT_ID {
            if !self.list_root {
                return Err(io::Error::from_raw_os_error(libc::EPERM)); // same error code as ipfs/kubo
            }

            // get the handle from [self.dir_handles]
            let (_span, rx) = match self.dir_handles.read().get(&handle) {
                Some(rx) => rx.clone(),
                None => {
                    warn!("dir handle {} unknown", handle);
                    return Err(io::Error::from_raw_os_error(libc::EIO));
                }
            };

            let mut rx = rx
                .lock()
                .map_err(|_| crate::Error::StorageError("mutex poisoned".into()))?;

            while let Some((i, n)) = rx.blocking_recv() {
                let root_node = n.map_err(|e| {
                    warn!("failed to retrieve root node: {}", e);
                    io::Error::from_raw_os_error(libc::EIO)
                })?;

                let (inode_data, name) = InodeData::from_node(root_node);

                // obtain the inode, or allocate a new one.
                let ino = self.get_inode_for_root_name(&name).unwrap_or_else(|| {
                    // insert the (sparse) inode data and register in
                    // self.root_nodes.
                    let ino = self.inode_tracker.write().put(inode_data.clone());
                    self.root_nodes.write().insert(name.clone(), ino);
                    ino
                });

                let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
                    ino,
                    offset: offset + (i as u64) + 1,
                    type_: inode_data.as_fuse_type(),
                    name: &name,
                })?;
                // If the buffer is full, add_entry will return `Ok(0)`.
                if written == 0 {
                    break;
                }
            }
            return Ok(());
        }

        // Non root-node case: lookup the children, or return an error if it's not a directory.
        let (parent_digest, children) = self.get_directory_children(inode)?;
        Span::current().record("directory.digest", parent_digest.to_string());

        for (i, (ino, child_node)) in children.into_iter().skip(offset as usize).enumerate() {
            let (inode_data, name) = InodeData::from_node(child_node);

            // the second parameter will become the "offset" parameter on the next call.
            let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
                ino,
                offset: offset + (i as u64) + 1,
                type_: inode_data.as_fuse_type(),
                name: &name,
            })?;
            // If the buffer is full, add_entry will return `Ok(0)`.
            if written == 0 {
                break;
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle), parent = self.dir_handles.read().get(&handle).and_then(|x| x.0.id()))]
    fn readdirplus(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        handle: Self::Handle,
        _size: u32,
        offset: u64,
        add_entry: &mut dyn FnMut(
            fuse_backend_rs::api::filesystem::DirEntry,
            fuse_backend_rs::api::filesystem::Entry,
        ) -> io::Result<usize>,
    ) -> io::Result<()> {
        debug!("readdirplus");

        if inode == ROOT_ID {
            if !self.list_root {
                return Err(io::Error::from_raw_os_error(libc::EPERM)); // same error code as ipfs/kubo
            }

            // get the handle from [self.dir_handles]
            let (_span, rx) = match self.dir_handles.read().get(&handle) {
                Some(rx) => rx.clone(),
                None => {
                    warn!("dir handle {} unknown", handle);
                    return Err(io::Error::from_raw_os_error(libc::EIO));
                }
            };

            let mut rx = rx
                .lock()
                .map_err(|_| crate::Error::StorageError("mutex poisoned".into()))?;

            while let Some((i, n)) = rx.blocking_recv() {
                let root_node = n.map_err(|e| {
                    warn!("failed to retrieve root node: {}", e);
                    io::Error::from_raw_os_error(libc::EPERM)
                })?;

                let (inode_data, name) = InodeData::from_node(root_node);

                // obtain the inode, or allocate a new one.
                let ino = self.get_inode_for_root_name(&name).unwrap_or_else(|| {
                    // insert the (sparse) inode data and register in
                    // self.root_nodes.
                    let ino = self.inode_tracker.write().put(inode_data.clone());
                    self.root_nodes.write().insert(name.clone(), ino);
                    ino
                });

                let written = add_entry(
                    fuse_backend_rs::api::filesystem::DirEntry {
                        ino,
                        offset: offset + (i as u64) + 1,
                        type_: inode_data.as_fuse_type(),
                        name: &name,
                    },
                    inode_data.as_fuse_entry(ino),
                )?;
                // If the buffer is full, add_entry will return `Ok(0)`.
                if written == 0 {
                    break;
                }
            }
            return Ok(());
        }

        // Non root-node case: lookup the children, or return an error if it's not a directory.
        let (parent_digest, children) = self.get_directory_children(inode)?;
        Span::current().record("directory.digest", parent_digest.to_string());

        for (i, (ino, child_node)) in children.into_iter().skip(offset as usize).enumerate() {
            let (inode_data, name) = InodeData::from_node(child_node);

            // the second parameter will become the "offset" parameter on the next call.
            let written = add_entry(
                fuse_backend_rs::api::filesystem::DirEntry {
                    ino,
                    offset: offset + (i as u64) + 1,
                    type_: inode_data.as_fuse_type(),
                    name: &name,
                },
                inode_data.as_fuse_entry(ino),
            )?;
            // If the buffer is full, add_entry will return `Ok(0)`.
            if written == 0 {
                break;
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle), parent = self.dir_handles.read().get(&handle).and_then(|x| x.0.id()))]
    fn releasedir(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
        handle: Self::Handle,
    ) -> io::Result<()> {
        if inode == ROOT_ID {
            // drop the rx part of the channel.
            match self.dir_handles.write().remove(&handle) {
                // drop it, which will close it.
                Some(rx) => drop(rx),
                None => {
                    warn!("dir handle not found");
                }
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn open(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
        _fuse_flags: u32,
    ) -> io::Result<(
        Option<Self::Handle>,
        fuse_backend_rs::api::filesystem::OpenOptions,
    )> {
        if inode == ROOT_ID {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // lookup the inode
        match *self.inode_tracker.read().get(inode).unwrap() {
            // read is invalid on non-files.
            InodeData::Directory(..) | InodeData::Symlink(_) => {
                warn!("is directory");
                Err(io::Error::from_raw_os_error(libc::EISDIR))
            }
            InodeData::Regular(ref blob_digest, _blob_size, _) => {
                Span::current().record("blob.digest", blob_digest.to_string());

                match self.tokio_handle.block_on({
                    let blob_service = self.blob_service.clone();
                    let blob_digest = blob_digest.clone();
                    async move { blob_service.as_ref().open_read(&blob_digest).await }
                }) {
                    Ok(None) => {
                        warn!("blob not found");
                        Err(io::Error::from_raw_os_error(libc::EIO))
                    }
                    Err(e) => {
                        warn!(e=?e, "error opening blob");
                        Err(io::Error::from_raw_os_error(libc::EIO))
                    }
                    Ok(Some(blob_reader)) => {
                        // get a new file handle
                        // TODO: this will overflow after 2**64 operations,
                        // which is fine for now.
                        // See https://cl.tvl.fyi/c/depot/+/8834/comment/a6684ce0_d72469d1
                        // for the discussion on alternatives.
                        let fh = self.next_file_handle.fetch_add(1, Ordering::SeqCst);

                        self.file_handles
                            .write()
                            .insert(fh, (Span::current(), Arc::new(Mutex::new(blob_reader))));

                        Ok((
                            Some(fh),
                            fuse_backend_rs::api::filesystem::OpenOptions::empty(),
                        ))
                    }
                }
            }
        }
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle), parent = self.file_handles.read().get(&handle).and_then(|x| x.0.id()))]
    fn release(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
        handle: Self::Handle,
        _flush: bool,
        _flock_release: bool,
        _lock_owner: Option<u64>,
    ) -> io::Result<()> {
        match self.file_handles.write().remove(&handle) {
            // drop the blob reader, which will close it.
            Some(blob_reader) => drop(blob_reader),
            None => {
                // These might already be dropped if a read error occured.
                warn!("file handle not found");
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle, rq.offset = offset, rq.size = size), parent = self.file_handles.read().get(&handle).and_then(|x| x.0.id()))]
    fn read(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        handle: Self::Handle,
        w: &mut dyn fuse_backend_rs::api::filesystem::ZeroCopyWriter,
        size: u32,
        offset: u64,
        _lock_owner: Option<u64>,
        _flags: u32,
    ) -> io::Result<usize> {
        debug!("read");

        // We need to take out the blob reader from self.file_handles, so we can
        // interact with it in the separate task.
        // On success, we pass it back out of the task, so we can put it back in self.file_handles.
        let (_span, blob_reader) = self
            .file_handles
            .read()
            .get(&handle)
            .ok_or_else(|| {
                warn!("file handle {} unknown", handle);
                io::Error::from_raw_os_error(libc::EIO)
            })
            .cloned()?;

        let mut blob_reader = blob_reader
            .lock()
            .map_err(|_| crate::Error::StorageError("mutex poisoned".into()))?;

        let buf = self.tokio_handle.block_on(async move {
            // seek to the offset specified, which is relative to the start of the file.
            let pos = blob_reader
                .seek(io::SeekFrom::Start(offset))
                .await
                .map_err(|e| {
                    warn!("failed to seek to offset {}: {}", offset, e);
                    io::Error::from_raw_os_error(libc::EIO)
                })?;

            debug_assert_eq!(offset, pos);

            // As written in the fuse docs, read should send exactly the number
            // of bytes requested except on EOF or error.

            let mut buf: Vec<u8> = Vec::with_capacity(size as usize);

            // copy things from the internal buffer into buf to fill it till up until size
            tokio::io::copy(&mut blob_reader.as_mut().take(size as u64), &mut buf).await?;

            Ok::<_, std::io::Error>(buf)
        })?;

        // We cannot use w.write() here, we're required to call write multiple
        // times until we wrote the entirety of the buffer (which is `size`, except on EOF).
        let buf_len = buf.len();
        let bytes_written = io::copy(&mut Cursor::new(buf), w)?;
        if bytes_written != buf_len as u64 {
            error!(bytes_written=%bytes_written, "unable to write all of buf to kernel");
            return Err(io::Error::from_raw_os_error(libc::EIO));
        }

        Ok(bytes_written as usize)
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn readlink(&self, _ctx: &Context, inode: Self::Inode) -> io::Result<Vec<u8>> {
        if inode == ROOT_ID {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // lookup the inode
        match *self.inode_tracker.read().get(inode).unwrap() {
            InodeData::Directory(..) | InodeData::Regular(..) => {
                Err(io::Error::from_raw_os_error(libc::EINVAL))
            }
            InodeData::Symlink(ref target) => Ok(target.to_vec()),
        }
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, name=?name))]
    fn getxattr(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        name: &CStr,
        size: u32,
    ) -> io::Result<GetxattrReply> {
        if !self.show_xattr {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // Peek at the inode requested, and construct the response.
        let digest_str = match *self
            .inode_tracker
            .read()
            .get(inode)
            .ok_or_else(|| io::Error::from_raw_os_error(libc::ENODATA))?
        {
            InodeData::Directory(DirectoryInodeData::Sparse(ref digest, _))
            | InodeData::Directory(DirectoryInodeData::Populated(ref digest, _))
                if name.to_bytes() == XATTR_NAME_DIRECTORY_DIGEST =>
            {
                digest.to_string()
            }
            InodeData::Regular(ref digest, _, _) if name.to_bytes() == XATTR_NAME_BLOB_DIGEST => {
                digest.to_string()
            }
            _ => {
                return Err(io::Error::from_raw_os_error(libc::ENODATA));
            }
        };

        if size == 0 {
            Ok(GetxattrReply::Count(digest_str.len() as u32))
        } else if size < digest_str.len() as u32 {
            Err(io::Error::from_raw_os_error(libc::ERANGE))
        } else {
            Ok(GetxattrReply::Value(digest_str.into_bytes()))
        }
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn listxattr(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        size: u32,
    ) -> io::Result<ListxattrReply> {
        if !self.show_xattr {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // determine the (\0-terminated list) to of xattr keys present, depending on the type of the inode.
        let xattrs_names = {
            let mut out = Vec::new();
            if let Some(inode_data) = self.inode_tracker.read().get(inode) {
                match *inode_data {
                    InodeData::Directory(_) => {
                        out.extend_from_slice(XATTR_NAME_DIRECTORY_DIGEST);
                        out.push_byte(b'\x00');
                    }
                    InodeData::Regular(..) => {
                        out.extend_from_slice(XATTR_NAME_BLOB_DIGEST);
                        out.push_byte(b'\x00');
                    }
                    _ => {}
                }
            }
            out
        };

        if size == 0 {
            Ok(ListxattrReply::Count(xattrs_names.len() as u32))
        } else if size < xattrs_names.len() as u32 {
            Err(io::Error::from_raw_os_error(libc::ERANGE))
        } else {
            Ok(ListxattrReply::Names(xattrs_names.to_vec()))
        }
    }
}