github.com/petermattis/pebble@v0.0.0-20190905164901-ab51a2166067/internal/ackseq/ackseq.go (about)

     1  // Copyright 2019 The LevelDB-Go and Pebble Authors. All rights reserved. Use
     2  // of this source code is governed by a BSD-style license that can be found in
     3  // the LICENSE file.
     4  
     5  package ackseq
     6  
     7  import (
     8  	"fmt"
     9  	"sync"
    10  	"sync/atomic"
    11  )
    12  
    13  const (
    14  	// The window size constants. These values specify a window that can hold ~1m
    15  	// pending unacknowledged sequence numbers using 128 KB of memory.
    16  	windowSize  = 1 << 20
    17  	windowMask  = windowSize - 1
    18  	windowBytes = (windowSize + 7) / 8
    19  )
    20  
    21  // S keeps track of the largest sequence number such that all sequence numbers
    22  // in the range [0,v) have been acknowledged.
    23  type S struct {
    24  	next uint64
    25  	mu   struct {
    26  		sync.Mutex
    27  		base   uint64
    28  		window [windowBytes]uint8
    29  	}
    30  }
    31  
    32  // New creates a new acknowledged sequence tracker with the specified base
    33  // sequence number. All of the sequence numbers in the range [0,base) are
    34  // considered acknowledged. Next() will return base upon first call.
    35  func New(base uint64) *S {
    36  	s := &S{
    37  		next: base,
    38  	}
    39  	s.mu.base = base
    40  	return s
    41  }
    42  
    43  // Next returns the next sequence number to use.
    44  func (s *S) Next() uint64 {
    45  	return atomic.AddUint64(&s.next, 1) - 1
    46  }
    47  
    48  // Ack acknowledges the specified seqNum, adjusting base as necessary,
    49  // returning the number of newly acknowledged sequence numbers.
    50  func (s *S) Ack(seqNum uint64) (int, error) {
    51  	s.mu.Lock()
    52  	if s.getLocked(seqNum) {
    53  		defer s.mu.Unlock()
    54  		return 0, fmt.Errorf(
    55  			"pending acks exceeds window size: %d has been acked, but %d has not",
    56  			seqNum, s.mu.base)
    57  	}
    58  
    59  	var count int
    60  	s.setLocked(seqNum)
    61  	for s.getLocked(s.mu.base) {
    62  		s.clearLocked(s.mu.base)
    63  		s.mu.base++
    64  		count++
    65  	}
    66  	s.mu.Unlock()
    67  	return count, nil
    68  }
    69  
    70  func (s *S) getLocked(seqNum uint64) bool {
    71  	bit := seqNum & windowMask
    72  	return (s.mu.window[bit/8] & (1 << (bit % 8))) != 0
    73  }
    74  
    75  func (s *S) setLocked(seqNum uint64) {
    76  	bit := seqNum & windowMask
    77  	s.mu.window[bit/8] |= (1 << (bit % 8))
    78  }
    79  
    80  func (s *S) clearLocked(seqNum uint64) {
    81  	bit := seqNum & windowMask
    82  	s.mu.window[bit/8] &^= (1 << (bit % 8))
    83  }