more

pkg/idempotency/store.go

@@ -14,19 +14,23 @@ import (
 	"bufio"
 	"encoding/json"
 	"errors"
+	"hash/fnv"
 	"os"
 	"sync"
 )
 
+// lockShards bounds the per-key lock memory: keys hash into a fixed set of
+// mutexes rather than a map that grows with every distinct key ever seen. Two
+// unrelated keys that collide on a shard serialise occasionally — harmless.
+const lockShards = 256
+
 // Store maps an idempotency key to a result id (e.g. an order id), cached in
 // memory and persisted to an append-only file.
 type Store struct {
-	mu sync.Mutex
-	m  map[string]uint64
-	f  *os.File
-
-	klMu     sync.Mutex
-	keyLocks map[string]*sync.Mutex
+	mu    sync.Mutex
+	m     map[string]uint64
+	f     *os.File
+	locks [lockShards]sync.Mutex
 }
 
 type record struct {
@@ -62,7 +66,7 @@ func Open(path string) (*Store, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &Store{m: m, f: f, keyLocks: map[string]*sync.Mutex{}}, nil
+	return &Store{m: m, f: f}, nil
 }
 
 // Get returns the recorded id for key and whether it was present.
@@ -103,13 +107,9 @@ func (s *Store) Lock(key string) func() {
 	if key == "" {
 		return func() {}
 	}
-	s.klMu.Lock()
-	mu, ok := s.keyLocks[key]
-	if !ok {
-		mu = &sync.Mutex{}
-		s.keyLocks[key] = mu
-	}
-	s.klMu.Unlock()
+	h := fnv.New32a()
+	_, _ = h.Write([]byte(key))
+	mu := &s.locks[h.Sum32()%lockShards]
 	mu.Lock()
 	return mu.Unlock
 }