test: add contentLock, a per-key mutex for content-hash exclusion

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.

internal/imgcache/contentlock.go

@@ -0,0 +1,64 @@
+package imgcache
+
+import "sync"
+
+// contentLock provides per-key mutual exclusion for content-hash keyed
+// operations. StoreSource and evictSourceBlob each hold a content
+// hash's lock for the full duration of their file-plus-accounting-row
+// work, so a store and an eviction racing on identical content bytes
+// (the real SHA-256 content-addressed dedup case, not a contrived one)
+// can never interleave: the unlink of an evicted blob's file can never
+// race the creation of a fresh database row for a concurrently
+// re-stored copy of the same content. Entries are removed once no
+// goroutine holds or is waiting for them, so a long-running process
+// does not accumulate memory proportional to the number of distinct
+// content hashes it has ever seen.
+type contentLock struct {
+	mu      sync.Mutex
+	entries map[string]*contentLockEntry
+}
+
+// contentLockEntry is one key's exclusion lock plus a count of
+// goroutines currently holding or waiting to acquire it, used to know
+// when it is safe to remove the entry from the map.
+type contentLockEntry struct {
+	mu    sync.Mutex
+	count int
+}
+
+// newContentLock creates an empty contentLock.
+func newContentLock() *contentLock {
+	return &contentLock{entries: make(map[string]*contentLockEntry)}
+}
+
+// Lock acquires exclusive access for key, blocking until it is
+// available, and returns a function that releases it. The caller must
+// invoke the returned function exactly once to release the lock.
+func (c *contentLock) Lock(key string) func() {
+	c.mu.Lock()
+
+	entry, ok := c.entries[key]
+	if !ok {
+		entry = &contentLockEntry{}
+		c.entries[key] = entry
+	}
+
+	entry.count++
+
+	c.mu.Unlock()
+
+	entry.mu.Lock()
+
+	return func() {
+		entry.mu.Unlock()
+
+		c.mu.Lock()
+
+		entry.count--
+		if entry.count == 0 {
+			delete(c.entries, key)
+		}
+
+		c.mu.Unlock()
+	}
+}

internal/imgcache/contentlock_test.go

@@ -0,0 +1,133 @@
+package imgcache
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// TestContentLockExcludesSameKey verifies that two goroutines locking
+// the same key never run their critical sections concurrently.
+func TestContentLockExcludesSameKey(t *testing.T) {
+	lock := newContentLock()
+
+	var (
+		active  int32
+		maxSeen int32
+		wg      sync.WaitGroup
+	)
+
+	const goroutines = 20
+
+	wg.Add(goroutines)
+
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+
+			unlock := lock.Lock("same-key")
+			defer unlock()
+
+			n := atomic.AddInt32(&active, 1)
+
+			for {
+				seen := atomic.LoadInt32(&maxSeen)
+				if n <= seen || atomic.CompareAndSwapInt32(&maxSeen, seen, n) {
+					break
+				}
+			}
+
+			time.Sleep(time.Millisecond)
+
+			atomic.AddInt32(&active, -1)
+		}()
+	}
+
+	wg.Wait()
+
+	if maxSeen != 1 {
+		t.Errorf("max concurrent holders of the same key = %d, want 1", maxSeen)
+	}
+}
+
+// TestContentLockAllowsDifferentKeys verifies that locking distinct
+// keys does not serialize unrelated work: all goroutines must be able
+// to enter their critical sections at once, proven by every one of
+// them reaching the rendezvous point before any is allowed to
+// proceed.
+func TestContentLockAllowsDifferentKeys(t *testing.T) {
+	lock := newContentLock()
+
+	const goroutines = 20
+
+	var (
+		wg      sync.WaitGroup
+		inside  int32
+		reached = make(chan struct{}, goroutines)
+	)
+
+	wg.Add(goroutines)
+
+	release := make(chan struct{})
+
+	for i := 0; i < goroutines; i++ {
+		key := string(rune('a' + i))
+
+		go func() {
+			defer wg.Done()
+
+			unlock := lock.Lock(key)
+			defer unlock()
+
+			atomic.AddInt32(&inside, 1)
+			reached <- struct{}{}
+			<-release
+		}()
+	}
+
+	// Every goroutine must reach the rendezvous point (i.e. acquire its
+	// own key's lock) without needing any other to release first. If
+	// keys were incorrectly serialized onto one underlying lock, only
+	// one would get here and this would time out.
+	for i := 0; i < goroutines; i++ {
+		select {
+		case <-reached:
+		case <-time.After(2 * time.Second):
+			t.Fatalf("only %d/%d goroutines locking distinct keys made progress; "+
+				"keys may be incorrectly serialized", i, goroutines)
+		}
+	}
+
+	if n := atomic.LoadInt32(&inside); n != goroutines {
+		t.Errorf("goroutines inside their critical section = %d, want %d", n, goroutines)
+	}
+
+	close(release)
+	wg.Wait()
+}
+
+// TestContentLockRemovesEntryAfterUnlock verifies that the internal
+// entries map does not grow without bound: once no goroutine holds or
+// awaits a key, its entry is removed.
+func TestContentLockRemovesEntryAfterUnlock(t *testing.T) {
+	lock := newContentLock()
+
+	unlock := lock.Lock("k")
+
+	lock.mu.Lock()
+	if _, ok := lock.entries["k"]; !ok {
+		lock.mu.Unlock()
+		t.Fatal("entry missing while lock is held")
+	}
+	lock.mu.Unlock()
+
+	unlock()
+
+	lock.mu.Lock()
+	defer lock.mu.Unlock()
+
+	if _, ok := lock.entries["k"]; ok {
+		t.Error("entry for key still present after the last holder unlocked")
+	}
+}