| 1 | // Package filelock provides bounded, cross-process advisory file locks. |
| 2 | package filelock |
| 3 | |
| 4 | import ( |
| 5 | "context" |
| 6 | "errors" |
| 7 | "fmt" |
| 8 | "path/filepath" |
| 9 | "sort" |
| 10 | "strings" |
| 11 | "sync" |
| 12 | "time" |
| 13 | ) |
| 14 | |
| 15 | const retryInterval = 20 * time.Millisecond |
| 16 | |
| 17 | // ErrHeld reports that another file descriptor currently owns the lock. |
| 18 | // Callers normally see their context error after Acquire's bounded retry loop. |
| 19 | var ErrHeld = errors.New("file lock held") |
| 20 | |
| 21 | // localLock is a process-local reader-writer lock for one canonical path. |
| 22 | // refs counts acquirers currently between registry entry and release/timeout |
| 23 | // so the registry can reclaim entries when no one is waiting or holding — |
| 24 | // important for short-lived paths such as session-temp owner locks. |
| 25 | type localLock struct { |
| 26 | mu sync.Mutex |
| 27 | cond *sync.Cond |
| 28 | exclusive bool |
| 29 | readers int |
| 30 | waitingWriters int |
| 31 | refs int |
| 32 | } |
| 33 | |
| 34 | var localRegistry = struct { |
| 35 | sync.Mutex |
| 36 | locks map[string]*localLock |
| 37 | }{locks: map[string]*localLock{}} |
| 38 | |
| 39 | // Acquire obtains an exclusive lock on path until the returned release |
| 40 | // function is called. It serializes both goroutines in this process and other |
| 41 | // Reasonix processes, and never waits past ctx's deadline. |
| 42 | func Acquire(ctx context.Context, path string) (func(), error) { |
| 43 | return acquire(ctx, path, 0, ModeExclusive) |
| 44 | } |
| 45 | |
| 46 | // AcquireMode obtains a lock in exclusive or shared mode. |
| 47 | func AcquireMode(ctx context.Context, path string, mode Mode) (func(), error) { |
| 48 | return acquire(ctx, path, 0, mode) |
| 49 | } |
| 50 | |
| 51 | // AcquireModeWithKey uses localKey for the process-local queue while opening |
| 52 | // path for the cross-process lock. Callers with filesystem identity knowledge |
| 53 | // use this to make aliases share a queue without using a comparison key for IO. |
| 54 | func AcquireModeWithKey(ctx context.Context, path, localKey string, mode Mode) (func(), error) { |
| 55 | if localKey == "" { |
| 56 | return nil, errors.New("file lock local key is empty") |
| 57 | } |
| 58 | return acquireWithKey(ctx, path, localKey, 0, mode) |
| 59 | } |
| 60 | |
| 61 | // AcquireWithExternalTimeout obtains an exclusive lock while keeping the |
| 62 | // in-process queue and cross-process file-lock budgets separate. ctx bounds |
| 63 | // only the wait for another goroutine in this process; externalTimeout starts |
| 64 | // after that queue is acquired and bounds retries against other processes. |
| 65 | func AcquireWithExternalTimeout(ctx context.Context, path string, externalTimeout time.Duration) (func(), error) { |
| 66 | if externalTimeout <= 0 { |
| 67 | return nil, errors.New("external file lock timeout must be positive") |
| 68 | } |
| 69 | return acquire(ctx, path, externalTimeout, ModeExclusive) |
| 70 | } |
| 71 | |
| 72 | // AcquireWithExternalTimeoutAndKey is AcquireWithExternalTimeout with an |
| 73 | // explicit process-local identity key. |
| 74 | func AcquireWithExternalTimeoutAndKey(ctx context.Context, path, localKey string, externalTimeout time.Duration) (func(), error) { |
| 75 | if externalTimeout <= 0 { |
| 76 | return nil, errors.New("external file lock timeout must be positive") |
| 77 | } |
| 78 | if localKey == "" { |
| 79 | return nil, errors.New("file lock local key is empty") |
| 80 | } |
| 81 | return acquireWithKey(ctx, path, localKey, externalTimeout, ModeExclusive) |
| 82 | } |
| 83 | |
| 84 | func acquire(ctx context.Context, path string, externalTimeout time.Duration, mode Mode) (func(), error) { |
| 85 | return acquireWithKey(ctx, path, "", externalTimeout, mode) |
| 86 | } |
| 87 | |
| 88 | func acquireWithKey(ctx context.Context, path, localKey string, externalTimeout time.Duration, mode Mode) (func(), error) { |
| 89 | if ctx == nil { |
| 90 | ctx = context.Background() |
| 91 | } |
| 92 | lockPath, err := canonicalLockPath(path) |
| 93 | if err != nil { |
| 94 | return nil, err |
| 95 | } |
| 96 | key := localKey |
| 97 | if key == "" { |
| 98 | key = lockPath |
| 99 | } |
| 100 | releaseLocal, err := acquireLocal(ctx, key, mode) |
| 101 | if err != nil { |
| 102 | return nil, err |
| 103 | } |
| 104 | fileCtx := ctx |
| 105 | cancel := func() {} |
| 106 | if externalTimeout > 0 { |
| 107 | fileCtx, cancel = context.WithTimeout(context.Background(), externalTimeout) |
| 108 | } |
| 109 | defer cancel() |
| 110 | |
| 111 | for { |
| 112 | releaseFile, err := tryLockFileMode(lockPath, mode) |
| 113 | if err == nil { |
| 114 | var once sync.Once |
| 115 | return func() { |
| 116 | once.Do(func() { |
| 117 | releaseFile() |
| 118 | releaseLocal() |
| 119 | }) |
| 120 | }, nil |
| 121 | } |
| 122 | if !errors.Is(err, ErrHeld) { |
| 123 | releaseLocal() |
| 124 | return nil, fmt.Errorf("acquire file lock: %w", err) |
| 125 | } |
| 126 | timer := time.NewTimer(retryInterval) |
| 127 | select { |
| 128 | case <-timer.C: |
| 129 | case <-fileCtx.Done(): |
| 130 | if !timer.Stop() { |
| 131 | select { |
| 132 | case <-timer.C: |
| 133 | default: |
| 134 | } |
| 135 | } |
| 136 | releaseLocal() |
| 137 | return nil, fmt.Errorf("acquire file lock: %w", fileCtx.Err()) |
| 138 | } |
| 139 | } |
| 140 | } |
| 141 | |
| 142 | // TryAcquire attempts a non-blocking exclusive lock. It returns ErrHeld when |
| 143 | // another holder (in this process or another) currently owns the lock. |
| 144 | func TryAcquire(path string) (func(), error) { |
| 145 | return TryAcquireMode(path, ModeExclusive) |
| 146 | } |
| 147 | |
| 148 | // TryAcquireMode attempts a non-blocking lock in exclusive or shared mode. |
| 149 | func TryAcquireMode(path string, mode Mode) (func(), error) { |
| 150 | return tryAcquireModeWithKey(path, "", mode) |
| 151 | } |
| 152 | |
| 153 | // TryAcquireModeWithKey is the non-blocking form of AcquireModeWithKey. |
| 154 | func TryAcquireModeWithKey(path, localKey string, mode Mode) (func(), error) { |
| 155 | if localKey == "" { |
| 156 | return nil, errors.New("file lock local key is empty") |
| 157 | } |
| 158 | return tryAcquireModeWithKey(path, localKey, mode) |
| 159 | } |
| 160 | |
| 161 | func tryAcquireModeWithKey(path, localKey string, mode Mode) (func(), error) { |
| 162 | lockPath, err := canonicalLockPath(path) |
| 163 | if err != nil { |
| 164 | return nil, err |
| 165 | } |
| 166 | key := localKey |
| 167 | if key == "" { |
| 168 | key = lockPath |
| 169 | } |
| 170 | releaseLocal, ok := tryAcquireLocal(key, mode) |
| 171 | if !ok { |
| 172 | return nil, ErrHeld |
| 173 | } |
| 174 | |
| 175 | releaseFile, err := tryLockFileMode(lockPath, mode) |
| 176 | if err != nil { |
| 177 | releaseLocal() |
| 178 | if errors.Is(err, ErrHeld) { |
| 179 | return nil, ErrHeld |
| 180 | } |
| 181 | return nil, fmt.Errorf("try acquire file lock: %w", err) |
| 182 | } |
| 183 | var once sync.Once |
| 184 | return func() { |
| 185 | once.Do(func() { |
| 186 | releaseFile() |
| 187 | releaseLocal() |
| 188 | }) |
| 189 | }, nil |
| 190 | } |
| 191 | |
| 192 | func lookupLocal(key string) *localLock { |
| 193 | local := localRegistry.locks[key] |
| 194 | if local == nil { |
| 195 | local = &localLock{} |
| 196 | local.cond = sync.NewCond(&local.mu) |
| 197 | localRegistry.locks[key] = local |
| 198 | } |
| 199 | local.refs++ |
| 200 | return local |
| 201 | } |
| 202 | |
| 203 | func acquireLocal(ctx context.Context, key string, mode Mode) (func(), error) { |
| 204 | localRegistry.Lock() |
| 205 | local := lookupLocal(key) |
| 206 | localRegistry.Unlock() |
| 207 | |
| 208 | stop := context.AfterFunc(ctx, func() { |
| 209 | local.mu.Lock() |
| 210 | local.cond.Broadcast() |
| 211 | local.mu.Unlock() |
| 212 | }) |
| 213 | defer stop() |
| 214 | |
| 215 | local.mu.Lock() |
| 216 | writer := mode == ModeExclusive |
| 217 | if writer { |
| 218 | local.waitingWriters++ |
| 219 | } |
| 220 | for { |
| 221 | if ctx.Err() != nil { |
| 222 | if writer { |
| 223 | local.waitingWriters-- |
| 224 | local.cond.Broadcast() |
| 225 | } |
| 226 | local.mu.Unlock() |
| 227 | dropLocalRef(key, local) |
| 228 | return nil, fmt.Errorf("acquire file lock: %w", ctx.Err()) |
| 229 | } |
| 230 | if mode == ModeShared { |
| 231 | if !local.exclusive && local.waitingWriters == 0 { |
| 232 | local.readers++ |
| 233 | local.mu.Unlock() |
| 234 | return releaseLocalFunc(key, local, mode), nil |
| 235 | } |
| 236 | } else if !local.exclusive && local.readers == 0 { |
| 237 | local.waitingWriters-- |
| 238 | local.exclusive = true |
| 239 | local.mu.Unlock() |
| 240 | return releaseLocalFunc(key, local, mode), nil |
| 241 | } |
| 242 | local.cond.Wait() |
| 243 | } |
| 244 | } |
| 245 | |
| 246 | func tryAcquireLocal(key string, mode Mode) (func(), bool) { |
| 247 | localRegistry.Lock() |
| 248 | local := lookupLocal(key) |
| 249 | localRegistry.Unlock() |
| 250 | |
| 251 | local.mu.Lock() |
| 252 | if mode == ModeShared { |
| 253 | if local.exclusive || local.waitingWriters > 0 { |
| 254 | local.mu.Unlock() |
| 255 | dropLocalRef(key, local) |
| 256 | return nil, false |
| 257 | } |
| 258 | local.readers++ |
| 259 | local.mu.Unlock() |
| 260 | return releaseLocalFunc(key, local, mode), true |
| 261 | } |
| 262 | if local.exclusive || local.readers > 0 { |
| 263 | local.mu.Unlock() |
| 264 | dropLocalRef(key, local) |
| 265 | return nil, false |
| 266 | } |
| 267 | local.exclusive = true |
| 268 | local.mu.Unlock() |
| 269 | return releaseLocalFunc(key, local, mode), true |
| 270 | } |
| 271 | |
| 272 | func dropLocalRef(key string, local *localLock) { |
| 273 | localRegistry.Lock() |
| 274 | local.refs-- |
| 275 | if local.refs <= 0 { |
| 276 | local.refs = 0 |
| 277 | delete(localRegistry.locks, key) |
| 278 | } |
| 279 | localRegistry.Unlock() |
| 280 | } |
| 281 | |
| 282 | func releaseLocalFunc(key string, local *localLock, mode Mode) func() { |
| 283 | var once sync.Once |
| 284 | return func() { |
| 285 | once.Do(func() { |
| 286 | local.mu.Lock() |
| 287 | if mode == ModeShared { |
| 288 | if local.readers > 0 { |
| 289 | local.readers-- |
| 290 | } |
| 291 | } else { |
| 292 | local.exclusive = false |
| 293 | } |
| 294 | local.cond.Broadcast() |
| 295 | local.mu.Unlock() |
| 296 | dropLocalRef(key, local) |
| 297 | }) |
| 298 | } |
| 299 | } |
| 300 | |
| 301 | // RegistrySizeForTest returns the number of live local-lock entries (tests). |
| 302 | func RegistrySizeForTest() int { |
| 303 | localRegistry.Lock() |
| 304 | defer localRegistry.Unlock() |
| 305 | return len(localRegistry.locks) |
| 306 | } |
| 307 | |
| 308 | // HeldPathsForTest returns the canonical paths currently holding a lock. A |
| 309 | // package TestMain uses it to turn a leaked lease into a failure everywhere: |
| 310 | // Windows cannot remove a directory containing an open lock file, so a leak |
| 311 | // that only breaks t.TempDir cleanup there is otherwise invisible on POSIX. |
| 312 | func HeldPathsForTest() []string { |
| 313 | localRegistry.Lock() |
| 314 | defer localRegistry.Unlock() |
| 315 | held := make([]string, 0, len(localRegistry.locks)) |
| 316 | for path := range localRegistry.locks { |
| 317 | held = append(held, path) |
| 318 | } |
| 319 | sort.Strings(held) |
| 320 | return held |
| 321 | } |
| 322 | |
| 323 | func canonicalLockPath(path string) (string, error) { |
| 324 | path = strings.TrimSpace(path) |
| 325 | if path == "" { |
| 326 | return "", errors.New("file lock path is empty") |
| 327 | } |
| 328 | abs, err := filepath.Abs(path) |
| 329 | if err != nil { |
| 330 | return "", fmt.Errorf("resolve file lock path: %w", err) |
| 331 | } |
| 332 | return filepath.Clean(abs), nil |
| 333 | } |
| 334 |