Fix swap to keep older one

index.js

@@ -109,7 +109,10 @@ class TreeNode {
       c = b4a.compare(key.value, await this.getKey(mid))
 
       if (c === 0) {
-        if (cas && !(await cas((await this.getKeyNode(mid)).final(encoding), node))) return true
+        const older = (await this.getKeyNode(mid)).final(encoding)
+        const swap = cas ? (await cas(older, node)) : null
+        if (cas && (!swap || swap === older)) return true
+
         this.changed = true
         this.keys[mid] = key
         return true
@@ -764,7 +767,9 @@ class Batch {
         c = b4a.compare(target.value, await node.getKey(mid))
 
         if (c === 0) {
-          if (cas && !(await cas((await node.getKeyNode(mid)).final(encoding), newNode))) return this._unlockMaybe()
+          const older = (await node.getKeyNode(mid)).final(encoding)
+          const swap = cas ? (await cas(older, newNode)) : null
+          if (cas && (!swap || swap === older)) return this._unlockMaybe()
 
           node.setKey(mid, target)
           return this._append(root, seq, key, enc(encoding.value, newNode.value))

test/cas.js

@@ -58,6 +58,24 @@ test('cas - should not swap', async function (t) {
   t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 1 })
 })
 
+test('cas - swap but keep older one', async function (t) {
+  const db = create({ valueEncoding: 'json' })
+
+  await db.put('/a', 0)
+
+  t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 0 })
+
+  await db.put('/a', 99, {
+    cas: function (prev, next) {
+      t.alike(prev, { seq: 1, key: '/a', value: 0 })
+      t.alike(next, { seq: 2, key: '/a', value: 99 })
+      return prev
+    }
+  })
+
+  t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 0 })
+})
+
 test('cas is called when prev does not exists', async function (t) {
   t.plan(6)