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clockstorage.go
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clockstorage.go
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package youyouayedee
import (
"encoding/json"
"fmt"
"io"
"io/fs"
"os"
"strings"
"sync"
"time"
)
// ClockStorage provides an interface for loading and storing clock counters on
// behalf of UUID generators.
//
// UUID generators based on timestamps (versions 1, 6, and 7) need some form of
// persistent storage in order to prevent collisions. This interface provides
// that persistent storage.
//
type ClockStorage interface {
// Load retrieves the last known timestamp and the last known counter
// value for the given Node.
//
// If the implementation returns ErrClockNotFound for any reason, the
// UUID generator is required to use the current time as the last known
// timestamp and to generate a new random counter value from scratch.
//
// Implementations are not required to retain a (timestamp, counter)
// tuple for each node; if there is no tuple stored for the given Node,
// then ErrClockNotFound is the best choice of return value.
//
Load(Node) (time.Time, uint32, error)
// Store associates the given last known timestamp and last known
// counter value with the given Node, synchronizing the data to
// persistent storage.
//
// Implementations are free to discard the (timestamp, counter) tuples
// associated with any previous node identifiers upon receiving a Store
// method call with a new node identifier. However, it may be
// beneficial to retain one such tuple for each past node identifier,
// at least when such node identifiers are flagged as both IsGlobal and
// IsUnicast.
//
Store(Node, time.Time, uint32) error
}
// ClockStorageUnavailable is a dummy implementation of ClockStorage that does
// not store anything.
type ClockStorageUnavailable struct{}
func (ClockStorageUnavailable) Load(Node) (time.Time, uint32, error) {
return time.Time{}, 0, ErrClockNotFound{}
}
func (ClockStorageUnavailable) Store(Node, time.Time, uint32) error {
return nil
}
var _ ClockStorage = ClockStorageUnavailable{}
// ClockStorageFile is an implementation of ClockStorage that reads from and
// writes to a file while holding a lock.
type ClockStorageFile struct {
mu sync.Mutex
name string
file *os.File
data map[string]*clockRow
closed bool
}
type clockRow struct {
Time time.Time `json:"time"`
Counter uint32 `json:"counter"`
}
// OpenClockStorageFile constructs an instance of ClockStorageFile.
func OpenClockStorageFile(fileName string) (*ClockStorageFile, error) {
if !lockFileSupported {
return nil, fmt.Errorf("clock sequence files must be locked for exclusive access, but package youyouayedee doesn't know how to lock files on your OS")
}
f, err := os.OpenFile(fileName, os.O_RDWR|os.O_CREATE, 0666)
if err != nil {
return nil, fmt.Errorf("failed to open clock sequence file: %q: %w", fileName, err)
}
needClose := true
defer func() {
if needClose {
_ = f.Close()
}
}()
err = lockFile(f)
if err != nil {
return nil, fmt.Errorf("failed to acquire exclusive lock on clock sequence file: %q: %w", fileName, err)
}
cs := &ClockStorageFile{
name: fileName,
file: f,
}
raw, err := io.ReadAll(f)
if err != nil {
return nil, fmt.Errorf("failed to read clock sequence data from file: %q: %w", fileName, err)
}
if len(raw) == 0 {
cs.data = make(map[string]*clockRow)
} else {
err = json.Unmarshal(raw, &cs.data)
if err != nil {
return nil, fmt.Errorf("failed to unmarshal clock sequence data from file as JSON: %q: %w", fileName, err)
}
}
needClose = false
return cs, nil
}
func (cs *ClockStorageFile) Load(node Node) (time.Time, uint32, error) {
if cs == nil {
return time.Time{}, 0, ErrClockNotFound{}
}
cs.mu.Lock()
defer cs.mu.Unlock()
if cs.file == nil || cs.closed {
return time.Time{}, 0, fs.ErrClosed
}
key := node.String()
if row, found := cs.data[key]; found {
return row.Time, row.Counter, nil
}
return time.Time{}, 0, ErrClockNotFound{}
}
func (cs *ClockStorageFile) Store(node Node, t time.Time, c uint32) error {
if cs == nil {
return nil
}
cs.mu.Lock()
defer cs.mu.Unlock()
if cs.file == nil || cs.closed {
return fs.ErrClosed
}
key := node.String()
row := cs.data[key]
if row == nil {
row = new(clockRow)
cs.data[key] = row
}
row.Time = t
row.Counter = c
// Delete all IsLocal && IsMulticast keys except the current key.
//
// This reduces clutter in the database if the node identifier is being
// generated at random each time the application runs.
//
for rowKey := range cs.data {
if rowKey != key && len(rowKey) >= 2 && strings.ContainsRune("37bf", rune(rowKey[1])) {
delete(cs.data, rowKey)
}
}
raw, err := json.Marshal(cs.data)
if err != nil {
return fmt.Errorf("failed to marshal clock sequence data to JSON: %q: %w", cs.name, err)
}
_, err = cs.file.Seek(0, os.SEEK_SET)
if err != nil {
return fmt.Errorf("failed to seek to start of the clock sequence file: %q: %w", cs.name, err)
}
err = cs.file.Truncate(0)
if err != nil {
return fmt.Errorf("failed to truncate the clock sequence file to zero bytes: %q: %w", cs.name, err)
}
_, err = cs.file.Write(raw)
if err != nil {
return fmt.Errorf("failed to write clock sequence data to file: %q: %w", cs.name, err)
}
err = cs.file.Sync()
if err != nil {
return fmt.Errorf("failed to sync clock sequence data to disk: %q: %w", cs.name, err)
}
return nil
}
func (cs *ClockStorageFile) Close() error {
if cs == nil {
return nil
}
cs.mu.Lock()
defer cs.mu.Unlock()
if cs.file == nil || cs.closed {
return fs.ErrClosed
}
cs.closed = true
return cs.file.Close()
}
var (
_ ClockStorage = (*ClockStorageFile)(nil)
_ io.Closer = (*ClockStorageFile)(nil)
)