ceremonyclient/pebble/read_state.go

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2024-01-03 07:31:42 +00:00
// Copyright 2019 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package pebble
import "sync/atomic"
// readState encapsulates the state needed for reading (the current version and
// list of memtables). Loading the readState is done without grabbing
// DB.mu. Instead, a separate DB.readState.RWMutex is used for
// synchronization. This mutex solely covers the current readState object which
// means it is rarely or ever contended.
//
// Note that various fancy lock-free mechanisms can be imagined for loading the
// readState, but benchmarking showed the ones considered to purely be
// pessimizations. The RWMutex version is a single atomic increment for the
// RLock and an atomic decrement for the RUnlock. It is difficult to do better
// than that without something like thread-local storage which isn't available
// in Go.
type readState struct {
db *DB
refcnt atomic.Int32
current *version
memtables flushableList
}
// ref adds a reference to the readState.
func (s *readState) ref() {
s.refcnt.Add(1)
}
// unref removes a reference to the readState. If this was the last reference,
// the reference the readState holds on the version is released. Requires DB.mu
// is NOT held as version.unref() will acquire it. See unrefLocked() if DB.mu
// is held by the caller.
func (s *readState) unref() {
if s.refcnt.Add(-1) != 0 {
return
}
s.current.Unref()
for _, mem := range s.memtables {
mem.readerUnref(true)
}
// The last reference to the readState was released. Check to see if there
// are new obsolete tables to delete.
s.db.maybeScheduleObsoleteTableDeletion()
}
// unrefLocked removes a reference to the readState. If this was the last
// reference, the reference the readState holds on the version is
// released.
//
// DB.mu must be held. See unref() if DB.mu is NOT held by the caller.
func (s *readState) unrefLocked() {
if s.refcnt.Add(-1) != 0 {
return
}
s.current.UnrefLocked()
for _, mem := range s.memtables {
mem.readerUnrefLocked(true)
}
// In this code path, the caller is responsible for scheduling obsolete table
// deletion as necessary.
}
// loadReadState returns the current readState. The returned readState must be
// unreferenced when the caller is finished with it.
func (d *DB) loadReadState() *readState {
d.readState.RLock()
state := d.readState.val
state.ref()
d.readState.RUnlock()
return state
}
// updateReadStateLocked creates a new readState from the current version and
// list of memtables. Requires DB.mu is held. If checker is not nil, it is
// called after installing the new readState.
func (d *DB) updateReadStateLocked(checker func(*DB) error) {
s := &readState{
db: d,
current: d.mu.versions.currentVersion(),
memtables: d.mu.mem.queue,
}
s.refcnt.Store(1)
s.current.Ref()
for _, mem := range s.memtables {
mem.readerRef()
}
d.readState.Lock()
old := d.readState.val
d.readState.val = s
d.readState.Unlock()
if checker != nil {
if err := checker(d); err != nil {
d.opts.Logger.Fatalf("checker failed with error: %s", err)
}
}
if old != nil {
old.unrefLocked()
}
}