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chore: purge file cache entries on error
1 parent f44969b commit 8a6deb1

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+130
-96
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1 file changed

+130
-96
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‎coderd/files/cache.go

Lines changed: 130 additions & 96 deletions
Original file line numberDiff line numberDiff line change
@@ -25,60 +25,61 @@ type FileAcquirer interface {
2525

2626
// New returns a file cache that will fetch files from a database
2727
func New(registerer prometheus.Registerer, authz rbac.Authorizer) *Cache {
28-
return (&Cache{
29-
lock: sync.Mutex{},
30-
data: make(map[uuid.UUID]*cacheEntry),
31-
authz: authz,
32-
}).registerMetrics(registerer)
28+
return &Cache{
29+
lock: sync.Mutex{},
30+
data: make(map[uuid.UUID]*cacheEntry),
31+
authz: authz,
32+
cacheMetrics: newCacheMetrics(registerer),
33+
}
3334
}
3435

35-
func (c *Cache) registerMetrics(registerer prometheus.Registerer) *Cache {
36+
func newCacheMetrics(registerer prometheus.Registerer) cacheMetrics {
3637
subsystem := "file_cache"
3738
f := promauto.With(registerer)
3839

39-
c.currentCacheSize = f.NewGauge(prometheus.GaugeOpts{
40-
Namespace: "coderd",
41-
Subsystem: subsystem,
42-
Name: "open_files_size_bytes_current",
43-
Help: "The current amount of memory of all files currently open in the file cache.",
44-
})
45-
46-
c.totalCacheSize = f.NewCounter(prometheus.CounterOpts{
47-
Namespace: "coderd",
48-
Subsystem: subsystem,
49-
Name: "open_files_size_bytes_total",
50-
Help: "The total amount of memory ever opened in the file cache. This number never decrements.",
51-
})
52-
53-
c.currentOpenFiles = f.NewGauge(prometheus.GaugeOpts{
54-
Namespace: "coderd",
55-
Subsystem: subsystem,
56-
Name: "open_files_current",
57-
Help: "The count of unique files currently open in the file cache.",
58-
})
59-
60-
c.totalOpenedFiles = f.NewCounter(prometheus.CounterOpts{
61-
Namespace: "coderd",
62-
Subsystem: subsystem,
63-
Name: "open_files_total",
64-
Help: "The total count of unique files ever opened in the file cache.",
65-
})
66-
67-
c.currentOpenFileReferences = f.NewGauge(prometheus.GaugeOpts{
68-
Namespace: "coderd",
69-
Subsystem: subsystem,
70-
Name: "open_file_refs_current",
71-
Help: "The count of file references currently open in the file cache. Multiple references can be held for the same file.",
72-
})
73-
74-
c.totalOpenFileReferences = f.NewCounterVec(prometheus.CounterOpts{
75-
Namespace: "coderd",
76-
Subsystem: subsystem,
77-
Name: "open_file_refs_total",
78-
Help: "The total number of file references ever opened in the file cache. The 'hit' label indicates if the file was loaded from the cache.",
79-
}, []string{"hit"})
80-
81-
return c
40+
return cacheMetrics{
41+
currentCacheSize: f.NewGauge(prometheus.GaugeOpts{
42+
Namespace: "coderd",
43+
Subsystem: subsystem,
44+
Name: "open_files_size_bytes_current",
45+
Help: "The current amount of memory of all files currently open in the file cache.",
46+
}),
47+
48+
totalCacheSize: f.NewCounter(prometheus.CounterOpts{
49+
Namespace: "coderd",
50+
Subsystem: subsystem,
51+
Name: "open_files_size_bytes_total",
52+
Help: "The total amount of memory ever opened in the file cache. This number never decrements.",
53+
}),
54+
55+
currentOpenFiles: f.NewGauge(prometheus.GaugeOpts{
56+
Namespace: "coderd",
57+
Subsystem: subsystem,
58+
Name: "open_files_current",
59+
Help: "The count of unique files currently open in the file cache.",
60+
}),
61+
62+
totalOpenedFiles: f.NewCounter(prometheus.CounterOpts{
63+
Namespace: "coderd",
64+
Subsystem: subsystem,
65+
Name: "open_files_total",
66+
Help: "The total count of unique files ever opened in the file cache.",
67+
}),
68+
69+
currentOpenFileReferences: f.NewGauge(prometheus.GaugeOpts{
70+
Namespace: "coderd",
71+
Subsystem: subsystem,
72+
Name: "open_file_refs_current",
73+
Help: "The count of file references currently open in the file cache. Multiple references can be held for the same file.",
74+
}),
75+
76+
totalOpenFileReferences: f.NewCounterVec(prometheus.CounterOpts{
77+
Namespace: "coderd",
78+
Subsystem: subsystem,
79+
Name: "open_file_refs_total",
80+
Help: "The total number of file references ever opened in the file cache. The 'hit' label indicates if the file was loaded from the cache.",
81+
}, []string{"hit"}),
82+
}
8283
}
8384

8485
// Cache persists the files for template versions, and is used by dynamic
@@ -106,18 +107,21 @@ type cacheMetrics struct {
106107
totalCacheSize prometheus.Counter
107108
}
108109

110+
type cacheEntry struct {
111+
// refCount must only be accessed while the cacheEntry lock is held.
112+
lock sync.Mutex
113+
refCount int
114+
value *lazy.ValueWithError[CacheEntryValue]
115+
116+
close func()
117+
}
118+
109119
type CacheEntryValue struct {
110120
fs.FS
111121
Object rbac.Object
112122
Size int64
113123
}
114124

115-
type cacheEntry struct {
116-
// refCount must only be accessed while the Cache lock is held.
117-
refCount int
118-
value *lazy.ValueWithError[CacheEntryValue]
119-
}
120-
121125
var _ fs.FS = (*CloseFS)(nil)
122126

123127
// CloseFS is a wrapper around fs.FS that implements io.Closer. The Close()
@@ -142,93 +146,116 @@ func (c *Cache) Acquire(ctx context.Context, db database.Store, fileID uuid.UUID
142146
// mutex has been released, or we would continue to hold the lock until the
143147
// entire file has been fetched, which may be slow, and would prevent other
144148
// files from being fetched in parallel.
145-
it, err := c.prepare(ctx, db, fileID).Load()
149+
e := c.prepare(ctx, db, fileID)
150+
ev, err := e.value.Load()
146151
if err != nil {
147-
c.release(fileID)
152+
c.purge(fileID)
153+
return nil, err
154+
}
155+
156+
// We always run the fetch under a system context and actor, so we need to check the caller's
157+
// context manually before returning.
158+
159+
// Check if the caller's context was canceled
160+
if err := ctx.Err(); err != nil {
148161
return nil, err
149162
}
150163

164+
// Check that the caller is authorized to access the file
151165
subject, ok := dbauthz.ActorFromContext(ctx)
152166
if !ok {
153167
return nil, dbauthz.ErrNoActor
154168
}
155-
// Always check the caller can actually read the file.
156-
if err := c.authz.Authorize(ctx, subject, policy.ActionRead, it.Object); err != nil {
157-
c.release(fileID)
169+
if err := c.authz.Authorize(ctx, subject, policy.ActionRead, ev.Object); err != nil {
170+
e.close()
158171
return nil, err
159172
}
160173

161-
var once sync.Once
174+
var closeOnce sync.Once
162175
return &CloseFS{
163-
FS: it.FS,
176+
FS: ev.FS,
164177
close: func() {
165178
// sync.Once makes the Close() idempotent, so we can call it
166179
// multiple times without worrying about double-releasing.
167-
once.Do(func() { c.release(fileID) })
180+
closeOnce.Do(func() {
181+
e.close()
182+
})
168183
},
169184
}, nil
170185
}
171186

172-
func (c *Cache) prepare(ctx context.Context, db database.Store, fileID uuid.UUID) *lazy.ValueWithError[CacheEntryValue] {
187+
func (c *Cache) prepare(ctx context.Context, db database.Store, fileID uuid.UUID) *cacheEntry {
173188
c.lock.Lock()
174189
defer c.lock.Unlock()
175190

176191
hitLabel := "true"
177192
entry, ok := c.data[fileID]
178193
if !ok {
179-
value := lazy.NewWithError(func() (CacheEntryValue, error) {
180-
val, err := fetch(ctx, db, fileID)
194+
hitLabel = "false"
181195

182-
// Always add to the cache size the bytes of the file loaded.
183-
if err == nil {
196+
var releaseOnce sync.Once
197+
entry = &cacheEntry{
198+
refCount: 0,
199+
value: lazy.NewWithError(func() (CacheEntryValue, error) {
200+
val, err := fetch(db, fileID)
201+
if err != nil {
202+
// Force future calls to Acquire to trigger a new fetch as soon as
203+
// a fetch has failed, even if references are still held.
204+
delete(c.data, fileID)
205+
return val, err
206+
}
207+
208+
// Add the size of the file to the cache size metrics.
184209
c.currentCacheSize.Add(float64(val.Size))
185210
c.totalCacheSize.Add(float64(val.Size))
186-
}
187211

188-
return val, err
189-
})
212+
return val, err
213+
}),
190214

191-
entry = &cacheEntry{
192-
value: value,
193-
refCount: 0,
215+
close: func() {
216+
entry.lock.Lock()
217+
defer entry.lock.Unlock()
218+
219+
entry.refCount--
220+
c.currentOpenFileReferences.Dec()
221+
222+
if entry.refCount == 0 {
223+
releaseOnce.Do(func() {
224+
c.purge(fileID)
225+
})
226+
}
227+
},
194228
}
195229
c.data[fileID] = entry
230+
196231
c.currentOpenFiles.Inc()
197232
c.totalOpenedFiles.Inc()
198-
hitLabel = "false"
199233
}
200234

235+
entry.lock.Lock()
236+
defer entry.lock.Unlock()
201237
c.currentOpenFileReferences.Inc()
202238
c.totalOpenFileReferences.WithLabelValues(hitLabel).Inc()
203239
entry.refCount++
204-
return entry.value
240+
return entry
205241
}
206242

207-
// release decrements the reference count for the given fileID, and frees the
208-
// backing data if there are no further references being held.
209-
//
210-
// release should only be called after a successful call to Acquire using the Release()
211-
// method on the returned *CloseFS.
212-
func (c *Cache) release(fileID uuid.UUID) {
243+
// purge immediately removes an entry from the cache. It should be called
244+
func (c *Cache) purge(fileID uuid.UUID) {
213245
c.lock.Lock()
214246
defer c.lock.Unlock()
215247

216248
entry, ok := c.data[fileID]
217249
if !ok {
218-
// If we land here, it's almost certainly because a bug already happened,
219-
// and we're freeing something that's already been freed, or we're calling
220-
// this function with an incorrect ID. Should this function return an error?
221-
return
222-
}
223-
224-
c.currentOpenFileReferences.Dec()
225-
entry.refCount--
226-
if entry.refCount > 0 {
250+
// If we land here, it's probably because of a fetch attempt that
251+
// resulted in an error, and got purged already. It may also be an
252+
// erroneous extra close, but we can't really distinguish between those
253+
// two cases currently.
227254
return
228255
}
229256

257+
// Purge the file from the cache.
230258
c.currentOpenFiles.Dec()
231-
232259
ev, err := entry.value.Load()
233260
if err == nil {
234261
c.currentCacheSize.Add(-1 * float64(ev.Size))
@@ -246,11 +273,18 @@ func (c *Cache) Count() int {
246273
return len(c.data)
247274
}
248275

249-
func fetch(ctx context.Context, store database.Store, fileID uuid.UUID) (CacheEntryValue, error) {
250-
// Make sure the read does not fail due to authorization issues.
251-
// Authz is checked on the Acquire call, so this is safe.
276+
func fetch(store database.Store, fileID uuid.UUID) (CacheEntryValue, error) {
277+
// Because many callers can be waiting on the same file fetch concurrently, we
278+
// want to prevent any failures that would cause them all to receive errors
279+
// because the caller who initiated the fetch would fail.
280+
// - We always run the fetch with an uncancelable context, and then check
281+
// context cancellation for each acquirer afterwards.
282+
// - We always run the fetch as a system user, and then check authorization
283+
// for each acquirer afterwards.
284+
// This prevents a canceled context or an unauthorized user from "holding up
285+
// the queue".
252286
//nolint:gocritic
253-
file, err := store.GetFileByID(dbauthz.AsFileReader(ctx), fileID)
287+
file, err := store.GetFileByID(dbauthz.AsFileReader(context.Background()), fileID)
254288
if err != nil {
255289
return CacheEntryValue{}, xerrors.Errorf("failed to read file from database: %w", err)
256290
}

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