sneak
ea7621de29
Introduces the keyed exclusion primitive that StoreSource and evictSourceBlob will hold across their full operation, so a store and an eviction racing on identical content bytes cannot interleave. Covered here in isolation: same-key exclusion, independence across distinct keys, and that the entry map does not grow unbounded.
65 lines
1.8 KiB
Go
65 lines
1.8 KiB
Go
package imgcache
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import "sync"
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// contentLock provides per-key mutual exclusion for content-hash keyed
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// operations. StoreSource and evictSourceBlob each hold a content
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// hash's lock for the full duration of their file-plus-accounting-row
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// work, so a store and an eviction racing on identical content bytes
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// (the real SHA-256 content-addressed dedup case, not a contrived one)
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// can never interleave: the unlink of an evicted blob's file can never
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// race the creation of a fresh database row for a concurrently
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// re-stored copy of the same content. Entries are removed once no
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// goroutine holds or is waiting for them, so a long-running process
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// does not accumulate memory proportional to the number of distinct
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// content hashes it has ever seen.
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type contentLock struct {
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mu sync.Mutex
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entries map[string]*contentLockEntry
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}
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// contentLockEntry is one key's exclusion lock plus a count of
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// goroutines currently holding or waiting to acquire it, used to know
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// when it is safe to remove the entry from the map.
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type contentLockEntry struct {
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mu sync.Mutex
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count int
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}
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// newContentLock creates an empty contentLock.
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func newContentLock() *contentLock {
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return &contentLock{entries: make(map[string]*contentLockEntry)}
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}
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// Lock acquires exclusive access for key, blocking until it is
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// available, and returns a function that releases it. The caller must
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// invoke the returned function exactly once to release the lock.
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func (c *contentLock) Lock(key string) func() {
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c.mu.Lock()
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entry, ok := c.entries[key]
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if !ok {
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entry = &contentLockEntry{}
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c.entries[key] = entry
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}
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entry.count++
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c.mu.Unlock()
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entry.mu.Lock()
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return func() {
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entry.mu.Unlock()
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c.mu.Lock()
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entry.count--
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if entry.count == 0 {
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delete(c.entries, key)
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}
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c.mu.Unlock()
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}
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}
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