aboutsummaryrefslogtreecommitdiff
path: root/src/wal.rs
blob: 119fdf306902e4c024961ac07a3b5629408e0cc3 (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
use async_trait::async_trait;
use futures::future::{self, FutureExt, TryFutureExt};
use std::cell::{RefCell, UnsafeCell};
use std::collections::{hash_map, BinaryHeap, HashMap};
use std::future::Future;
use std::mem::MaybeUninit;
use std::pin::Pin;

#[repr(u8)]
enum WALRingType {
    #[allow(dead_code)]
    Null = 0x0,
    Full,
    First,
    Middle,
    Last,
}

#[repr(packed)]
struct WALRingBlob {
    crc32: u32,
    rsize: u32,
    rtype: WALRingType,
    // payload follows
}

type WALFileId = u64;
pub type WALBytes = Box<[u8]>;
pub type WALPos = u64;

#[derive(Eq, PartialEq, Copy, Clone, Debug, Hash)]
pub struct WALRingId {
    start: WALPos,
    end: WALPos,
}

impl WALRingId {
    pub fn empty_id() -> Self {
        WALRingId { start: 0, end: 0 }
    }
    pub fn get_start(&self) -> WALPos {
        self.start
    }
    pub fn get_end(&self) -> WALPos {
        self.end
    }
}

impl Ord for WALRingId {
    fn cmp(&self, other: &WALRingId) -> std::cmp::Ordering {
        other
            .start
            .cmp(&self.start)
            .then_with(|| other.end.cmp(&self.end))
    }
}

impl PartialOrd for WALRingId {
    fn partial_cmp(&self, other: &WALRingId) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

pub trait Record {
    fn serialize(&self) -> WALBytes;
}

impl Record for WALBytes {
    fn serialize(&self) -> WALBytes {
        self[..].into()
    }
}

impl Record for String {
    fn serialize(&self) -> WALBytes {
        self.as_bytes().into()
    }
}

impl Record for &str {
    fn serialize(&self) -> WALBytes {
        self.as_bytes().into()
    }
}

/// the state for a WAL writer
struct WALState {
    /// the first file id of WAL
    first_fid: WALFileId,
    /// the next position for a record, addressed in the entire WAL space
    next: WALPos,
    /// number of bits for a file
    file_nbit: u64,
    next_complete: WALPos,
    io_complete: BinaryHeap<WALRingId>,
}

#[async_trait(?Send)]
pub trait WALFile {
    /// Initialize the file space in [offset, offset + length) to zero.
    async fn allocate(&self, offset: WALPos, length: usize) -> Result<(), ()>;
    /// Write data with offset. We assume all previous `allocate`/`truncate` invocations are visible
    /// if ordered earlier (should be guaranteed by most OS).  Additionally, the write caused
    /// by each invocation of this function should be _atomic_ (the entire single write should be
    /// all or nothing).
    async fn write(&self, offset: WALPos, data: WALBytes) -> Result<(), ()>;
    /// Read data with offset. Return `Ok(None)` when it reaches EOF.
    async fn read(
        &self,
        offset: WALPos,
        length: usize,
    ) -> Result<Option<WALBytes>, ()>;
    /// Truncate a file to a specified length.
    fn truncate(&self, length: usize) -> Result<(), ()>;
}

#[async_trait(?Send)]
pub trait WALStore {
    type FileNameIter: Iterator<Item = String>;

    /// Open a file given the filename, create the file if not exists when `touch` is `true`.
    async fn open_file(
        &self,
        filename: &str,
        touch: bool,
    ) -> Result<Box<dyn WALFile>, ()>;
    /// Unlink a file given the filename.
    async fn remove_file(&self, filename: String) -> Result<(), ()>;
    /// Enumerate all WAL filenames. It should include all WAL files that are previously opened
    /// (created) but not removed. The list could be unordered.
    fn enumerate_files(&self) -> Result<Self::FileNameIter, ()>;
}

struct WALFileHandle<'a, F: WALStore> {
    fid: WALFileId,
    handle: &'a dyn WALFile,
    pool: *const WALFilePool<F>,
}

impl<'a, F: WALStore> std::ops::Deref for WALFileHandle<'a, F> {
    type Target = dyn WALFile + 'a;
    fn deref(&self) -> &Self::Target {
        self.handle
    }
}

impl<'a, F: WALStore> Drop for WALFileHandle<'a, F> {
    fn drop(&mut self) {
        unsafe {
            (&*self.pool).release_file(self.fid);
        }
    }
}

/// The middle layer that manages WAL file handles and invokes public trait functions to actually
/// manipulate files and their contents.
struct WALFilePool<F: WALStore> {
    store: F,
    handle_cache: RefCell<lru::LruCache<WALFileId, Box<dyn WALFile>>>,
    handle_used:
        RefCell<HashMap<WALFileId, UnsafeCell<(Box<dyn WALFile>, usize)>>>,
    last_write:
        UnsafeCell<MaybeUninit<Pin<Box<dyn Future<Output = Result<(), ()>>>>>>,
    last_peel:
        UnsafeCell<MaybeUninit<Pin<Box<dyn Future<Output = Result<(), ()>>>>>>,
    file_nbit: u64,
    file_size: u64,
    block_nbit: u64,
}

impl<F: WALStore> WALFilePool<F> {
    fn new(store: F, file_nbit: u8, block_nbit: u8, cache_size: usize) -> Self {
        let file_nbit = file_nbit as u64;
        let block_nbit = block_nbit as u64;
        WALFilePool {
            store,
            handle_cache: RefCell::new(lru::LruCache::new(cache_size)),
            handle_used: RefCell::new(HashMap::new()),
            last_write: UnsafeCell::new(MaybeUninit::new(Box::pin(
                future::ready(Ok(())),
            ))),
            last_peel: UnsafeCell::new(MaybeUninit::new(Box::pin(
                future::ready(Ok(())),
            ))),
            file_nbit,
            file_size: 1 << (file_nbit as u64),
            block_nbit,
        }
    }

    fn get_fname(fid: WALFileId) -> String {
        format!("{:08x}.log", fid)
    }

    fn get_file<'a>(
        &'a self,
        fid: u64,
        touch: bool,
    ) -> impl Future<Output = Result<WALFileHandle<'a, F>, ()>> {
        async move {
            let pool = self as *const WALFilePool<F>;
            if let Some(h) = self.handle_cache.borrow_mut().pop(&fid) {
                let handle = match self.handle_used.borrow_mut().entry(fid) {
                    hash_map::Entry::Vacant(e) => unsafe {
                        &*(*e.insert(UnsafeCell::new((h, 1))).get()).0
                    },
                    _ => unreachable!(),
                };
                Ok(WALFileHandle { fid, handle, pool })
            } else {
                let v = unsafe {
                    &mut *match self.handle_used.borrow_mut().entry(fid) {
                        hash_map::Entry::Occupied(e) => e.into_mut(),
                        hash_map::Entry::Vacant(e) => {
                            e.insert(UnsafeCell::new((
                                self.store
                                    .open_file(&Self::get_fname(fid), touch)
                                    .await?,
                                0,
                            )))
                        }
                    }
                    .get()
                };
                v.1 += 1;
                Ok(WALFileHandle {
                    fid,
                    handle: &*v.0,
                    pool,
                })
            }
        }
    }

    fn release_file(&self, fid: WALFileId) {
        match self.handle_used.borrow_mut().entry(fid) {
            hash_map::Entry::Occupied(e) => {
                let v = unsafe { &mut *e.get().get() };
                v.1 -= 1;
                if v.1 == 0 {
                    self.handle_cache
                        .borrow_mut()
                        .put(fid, e.remove().into_inner().0);
                }
            }
            _ => unreachable!(),
        }
    }

    fn get_fid(&mut self, fname: &str) -> WALFileId {
        scan_fmt!(fname, "{x}.log", [hex WALFileId]).unwrap()
    }

    fn write<'a>(
        &'a mut self,
        writes: Vec<(WALPos, WALBytes)>,
    ) -> Vec<Pin<Box<dyn Future<Output = Result<(), ()>> + 'a>>> {
        if writes.is_empty() {
            return Vec::new()
        }
        let file_size = self.file_size;
        let file_nbit = self.file_nbit;
        let meta: Vec<(u64, u64)> = writes
            .iter()
            .map(|(off, w)| ((*off) >> file_nbit, w.len() as u64))
            .collect();
        let mut files: Vec<Pin<Box<dyn Future<Output = _> + 'a>>> = Vec::new();
        for &(fid, _) in meta.iter() {
            files.push(Box::pin(self.get_file(fid, true))
                as Pin<Box<dyn Future<Output = _> + 'a>>)
        }
        let mut fid = writes[0].0 >> file_nbit;
        let mut alloc_start = writes[0].0 & (self.file_size - 1);
        let mut alloc_end = alloc_start + writes[0].1.len() as u64;
        let last_write = unsafe {
            std::mem::replace(
                &mut *self.last_write.get(),
                std::mem::MaybeUninit::uninit(),
            )
            .assume_init()
        };
        // pre-allocate the file space
        let alloc = async move {
            last_write.await?;
            let mut last_h: Option<
                Pin<
                    Box<
                        dyn Future<Output = Result<WALFileHandle<'a, F>, ()>>
                            + 'a,
                    >,
                >,
            > = None;
            for ((next_fid, wl), h) in meta.into_iter().zip(files.into_iter()) {
                if let Some(lh) = last_h.take() {
                    if next_fid != fid {
                        lh.await?
                            .allocate(
                                alloc_start,
                                (alloc_end - alloc_start) as usize,
                            )
                            .await?;
                        last_h = Some(h);
                        alloc_start = 0;
                        alloc_end = alloc_start + wl;
                        fid = next_fid;
                    } else {
                        last_h = Some(lh);
                        alloc_end += wl;
                    }
                } else {
                    last_h = Some(h);
                }
            }
            if let Some(lh) = last_h {
                lh.await?
                    .allocate(alloc_start, (alloc_end - alloc_start) as usize)
                    .await?
            }
            Ok(())
        };
        let mut res = Vec::new();
        let mut prev = Box::pin(alloc) as Pin<Box<dyn Future<Output = _> + 'a>>;
        for (off, w) in writes.into_iter() {
            let f = self.get_file(off >> file_nbit, true);
            let w = (async move {
                prev.await?;
                f.await?.write(off & (file_size - 1), w).await
            })
            .shared();
            prev = Box::pin(w.clone());
            res.push(Box::pin(w) as Pin<Box<dyn Future<Output = _> + 'a>>)
        }
        unsafe {
            (*self.last_write.get()) = MaybeUninit::new(std::mem::transmute::<
                Pin<Box<dyn Future<Output = _> + 'a>>,
                Pin<Box<dyn Future<Output = _> + 'static>>,
            >(prev))
        }
        res
    }

    fn remove_files<'a>(
        &'a mut self,
        fid_s: u64,
        fid_e: u64,
    ) -> impl Future<Output = Result<(), ()>> + 'a {
        let last_peel = unsafe {
            std::mem::replace(
                &mut *self.last_peel.get(),
                std::mem::MaybeUninit::uninit(),
            )
            .assume_init()
        };

        let mut removes = Vec::new();
        for fid in fid_s..fid_e {
            removes.push(self.store.remove_file(Self::get_fname(fid))
                as Pin<Box<dyn Future<Output = _> + 'a>>)
        }
        let p = async move {
            last_peel.await?;
            for r in removes.into_iter() {
                r.await?
            }
            Ok(())
        }
        .shared();
        unsafe {
            (*self.last_peel.get()) =
                MaybeUninit::new(std::mem::transmute(Box::pin(p.clone())
                    as Pin<Box<dyn Future<Output = _> + 'a>>))
        }
        p
    }

    fn in_use_len(&self) -> usize {
        self.handle_used.borrow().len()
    }

    fn reset(&mut self) {
        self.handle_cache.borrow_mut().clear();
        self.handle_used.borrow_mut().clear()
    }
}

pub struct WALWriter<F: WALStore> {
    state: WALState,
    file_pool: WALFilePool<F>,
    block_buffer: WALBytes,
    block_size: u32,
    msize: usize,
}

impl<F: WALStore> WALWriter<F> {
    fn new(state: WALState, file_pool: WALFilePool<F>) -> Self {
        let mut b = Vec::new();
        let block_size = 1 << file_pool.block_nbit as u32;
        let msize = std::mem::size_of::<WALRingBlob>();
        b.resize(block_size as usize, 0);
        WALWriter {
            state,
            file_pool,
            block_buffer: b.into_boxed_slice(),
            block_size,
            msize,
        }
    }

    /// Submit a sequence of records to WAL. It returns a vector of futures, each of which
    /// corresponds to one record. When a future resolves to `WALRingId`, it is guaranteed the
    /// record is already logged. Then, after finalizing the changes encoded by that record to
    /// the persistent storage, the caller can recycle the WAL files by invoking the given
    /// `peel` with the given `WALRingId`s. Note: each serialized record should contain at least 1
    /// byte (empty record payload will result in assertion failure).
    pub fn grow<'a, R: Record + 'a>(
        &'a mut self,
        records: Vec<R>,
    ) -> Vec<impl Future<Output = Result<(R, WALRingId), ()>> + 'a> {
        let mut res = Vec::new();
        let mut writes = Vec::new();
        let msize = self.msize as u32;
        // the global offest of the begining of the block
        // the start of the unwritten data
        let mut bbuff_start = self.state.next as u32 & (self.block_size - 1);
        // the end of the unwritten data
        let mut bbuff_cur = bbuff_start;

        for rec in records.iter() {
            let bytes = rec.serialize();
            let mut rec = &bytes[..];
            let mut rsize = rec.len() as u32;
            let mut ring_start = None;
            assert!(rsize > 0);
            while rsize > 0 {
                let remain = self.block_size - bbuff_cur;
                if remain > msize {
                    let d = remain - msize;
                    let rs0 =
                        self.state.next + (bbuff_cur - bbuff_start) as u64;
                    let blob = unsafe {
                        std::mem::transmute::<*mut u8, &mut WALRingBlob>(
                            (&mut self.block_buffer[bbuff_cur as usize..])
                                .as_mut_ptr(),
                        )
                    };
                    bbuff_cur += msize;
                    if d >= rsize {
                        // the remaining rec fits in the block
                        let payload = rec;
                        blob.crc32 = crc::crc32::checksum_ieee(payload);
                        blob.rsize = rsize;
                        let (rs, rt) = if let Some(rs) = ring_start.take() {
                            (rs, WALRingType::Last)
                        } else {
                            (rs0, WALRingType::Full)
                        };
                        blob.rtype = rt;
                        &mut self.block_buffer[bbuff_cur as usize..
                            bbuff_cur as usize + payload.len()]
                            .copy_from_slice(payload);
                        bbuff_cur += rsize;
                        rsize = 0;
                        let end =
                            self.state.next + (bbuff_cur - bbuff_start) as u64;
                        res.push((WALRingId { start: rs, end }, Vec::new()));
                    } else {
                        // the remaining block can only accommodate partial rec
                        let payload = &rec[..d as usize];
                        blob.crc32 = crc::crc32::checksum_ieee(payload);
                        blob.rsize = d;
                        blob.rtype = if ring_start.is_some() {
                            WALRingType::Middle
                        } else {
                            ring_start = Some(rs0);
                            WALRingType::First
                        };
                        &mut self.block_buffer[bbuff_cur as usize..
                            bbuff_cur as usize + payload.len()]
                            .copy_from_slice(payload);
                        bbuff_cur += d;
                        rsize -= d;
                        rec = &rec[d as usize..];
                    }
                } else {
                    // add padding space by moving the point to the end of the block
                    bbuff_cur = self.block_size;
                }
                if bbuff_cur == self.block_size {
                    writes.push((
                        self.state.next,
                        self.block_buffer[bbuff_start as usize..]
                            .to_vec()
                            .into_boxed_slice(),
                    ));
                    self.state.next += (self.block_size - bbuff_start) as u64;
                    bbuff_start = 0;
                    bbuff_cur = 0;
                }
            }
        }
        if bbuff_cur > bbuff_start {
            writes.push((
                self.state.next,
                self.block_buffer[bbuff_start as usize..bbuff_cur as usize]
                    .to_vec()
                    .into_boxed_slice(),
            ));
            self.state.next += (bbuff_cur - bbuff_start) as u64;
        }

        // mark the block info for each record
        let mut i = 0;
        'outer: for (j, (off, w)) in writes.iter().enumerate() {
            let blk_s = *off;
            let blk_e = blk_s + w.len() as u64;
            while res[i].0.end <= blk_s {
                i += 1;
                if i >= res.len() {
                    break 'outer;
                }
            }
            while res[i].0.start < blk_e {
                res[i].1.push(j);
                if res[i].0.end >= blk_e {
                    break;
                }
                i += 1;
                if i >= res.len() {
                    break 'outer;
                }
            }
        }

        let writes: Vec<future::Shared<_>> = self
            .file_pool
            .write(writes)
            .into_iter()
            .map(move |f| async move { f.await }.shared())
            .collect();
        let res = res
            .into_iter()
            .zip(records.into_iter())
            .map(|((ringid, blks), rec)| {
                future::try_join_all(
                    blks.into_iter().map(|idx| writes[idx].clone()),
                )
                .or_else(|_| future::ready(Err(())))
                .and_then(move |_| future::ready(Ok((rec, ringid))))
            })
            .collect();
        res
    }

    /// Inform the `WALWriter` that some data writes are complete so that it could automatically
    /// remove obsolete WAL files. The given list of `WALRingId` does not need to be ordered and
    /// could be of arbitrary length.
    pub fn peel<'a, T: AsRef<[WALRingId]>>(
        &'a mut self,
        records: T,
    ) -> impl Future<Output = Result<(), ()>> + 'a {
        let msize = self.msize as u64;
        let block_size = self.block_size as u64;
        let state = &mut self.state;
        for rec in records.as_ref() {
            state.io_complete.push(*rec);
        }
        let orig_fid = state.first_fid;
        while let Some(s) =
            state.io_complete.peek().and_then(|&e| Some(e.start))
        {
            if s != state.next_complete {
                break;
            }
            let mut m = state.io_complete.pop().unwrap();
            let block_remain = block_size - (m.end & (block_size - 1));
            if block_remain <= msize as u64 {
                m.end += block_remain
            }
            state.next_complete = m.end
        }
        let next_fid = state.next_complete >> state.file_nbit;
        state.first_fid = next_fid;
        self.file_pool.remove_files(orig_fid, next_fid)
    }

    pub fn file_pool_in_use(&self) -> usize {
        self.file_pool.in_use_len()
    }
}

#[derive(Copy, Clone)]
pub enum RecoverPolicy {
    /// all checksums must be correct, otherwise recovery fails
    Strict,
    /// stop recovering when hitting the first corrupted record
    BestEffort,
}

pub struct WALLoader {
    file_nbit: u8,
    block_nbit: u8,
    cache_size: usize,
    recover_policy: RecoverPolicy,
}

impl Default for WALLoader {
    fn default() -> Self {
        WALLoader {
            file_nbit: 22,  // 4MB
            block_nbit: 15, // 32KB,
            cache_size: 16,
            recover_policy: RecoverPolicy::Strict,
        }
    }
}

impl WALLoader {
    pub fn new() -> Self {
        Default::default()
    }

    pub fn file_nbit(&mut self, v: u8) -> &mut Self {
        self.file_nbit = v;
        self
    }

    pub fn block_nbit(&mut self, v: u8) -> &mut Self {
        self.block_nbit = v;
        self
    }

    pub fn cache_size(&mut self, v: usize) -> &mut Self {
        self.cache_size = v;
        self
    }

    pub fn recover_policy(&mut self, p: RecoverPolicy) -> &mut Self {
        self.recover_policy = p;
        self
    }

    fn verify_checksum(&self, data: &[u8], checksum: u32) -> Result<bool, ()> {
        if checksum == crc::crc32::checksum_ieee(data) {
            Ok(true)
        } else {
            match self.recover_policy {
                RecoverPolicy::Strict => Err(()),
                RecoverPolicy::BestEffort => Ok(false),
            }
        }
    }

    /// Recover by reading the WAL files.
    pub async fn load<
        S: WALStore,
        F: FnMut(WALBytes, WALRingId) -> Result<(), ()>,
    >(
        &self,
        store: S,
        mut recover_func: F,
    ) -> Result<WALWriter<S>, ()> {
        let msize = std::mem::size_of::<WALRingBlob>();
        assert!(self.file_nbit > self.block_nbit);
        assert!(msize < 1 << self.block_nbit);
        let filename_fmt = regex::Regex::new(r"[0-9a-f]+\.log").unwrap();
        let mut file_pool = WALFilePool::new(
            store,
            self.file_nbit,
            self.block_nbit,
            self.cache_size,
        );
        let block_size = 1 << file_pool.block_nbit;
        let mut logfiles: Vec<String> = file_pool
            .store
            .enumerate_files()?
            .filter(|f| filename_fmt.is_match(f))
            .collect();
        // TODO: check for missing logfiles
        logfiles.sort();
        let mut chunks = None;
        let mut skip = false;
        for fname in logfiles.into_iter() {
            let fid = file_pool.get_fid(&fname);
            let f = file_pool.get_file(fid, false).await?;
            let mut off = 0;
            if skip {
                f.truncate(0)?;
                file_pool.store.remove_file(fname).await?;
                continue;
            }
            while let Some(header_raw) = f.read(off, msize as usize).await? {
                let ringid_start = (fid << file_pool.file_nbit) + off;
                off += msize as u64;
                let header = unsafe {
                    std::mem::transmute::<*const u8, &WALRingBlob>(
                        header_raw.as_ptr(),
                    )
                };
                let rsize = header.rsize;
                match header.rtype {
                    WALRingType::Full => {
                        assert!(chunks.is_none());
                        let payload =
                            f.read(off, rsize as usize).await?.ok_or(())?;
                        // TODO: improve the behavior when CRC32 fails
                        if !self.verify_checksum(&payload, header.crc32)? {
                            skip = true;
                            break;
                        }
                        off += rsize as u64;
                        recover_func(
                            payload,
                            WALRingId {
                                start: ringid_start,
                                end: (fid << file_pool.file_nbit) + off,
                            },
                        )?;
                    }
                    WALRingType::First => {
                        assert!(chunks.is_none());
                        let chunk =
                            f.read(off, rsize as usize).await?.ok_or(())?;
                        if !self.verify_checksum(&chunk, header.crc32)? {
                            skip = true;
                            break;
                        }
                        chunks = Some((vec![chunk], ringid_start));
                        off += rsize as u64;
                    }
                    WALRingType::Middle => {
                        if let Some((chunks, _)) = &mut chunks {
                            let chunk =
                                f.read(off, rsize as usize).await?.ok_or(())?;
                            if !self.verify_checksum(&chunk, header.crc32)? {
                                skip = true;
                                break;
                            }
                            chunks.push(chunk);
                        } // otherwise ignore the leftover
                        off += rsize as u64;
                    }
                    WALRingType::Last => {
                        if let Some((mut chunks, ringid_start)) = chunks.take()
                        {
                            let chunk =
                                f.read(off, rsize as usize).await?.ok_or(())?;
                            off += rsize as u64;
                            if !self.verify_checksum(&chunk, header.crc32)? {
                                skip = true;
                                break;
                            }
                            chunks.push(chunk);
                            let mut payload = Vec::new();
                            payload.resize(
                                chunks.iter().fold(0, |acc, v| acc + v.len()),
                                0,
                            );
                            let mut ps = &mut payload[..];
                            for c in chunks {
                                ps[..c.len()].copy_from_slice(&*c);
                                ps = &mut ps[c.len()..];
                            }
                            recover_func(
                                payload.into_boxed_slice(),
                                WALRingId {
                                    start: ringid_start,
                                    end: (fid << file_pool.file_nbit) + off,
                                },
                            )?;
                        }
                        // otherwise ignore the leftover
                        else {
                            off += rsize as u64;
                        }
                    }
                    WALRingType::Null => break,
                }
                let block_remain = block_size - (off & (block_size - 1));
                if block_remain <= msize as u64 {
                    off += block_remain;
                }
            }
            f.truncate(0)?;
            file_pool.store.remove_file(fname).await?;
        }
        file_pool.reset();
        Ok(WALWriter::new(
            WALState {
                first_fid: 0,
                next: 0,
                file_nbit: file_pool.file_nbit,
                next_complete: 0,
                io_complete: BinaryHeap::new(),
            },
            file_pool,
        ))
    }
}