Copy node-cache entries to reduce mem usage (#521)

* Copy node-cache entries to reduce mem usage

* undo unneeded formatting change

* Close tree at end of test

* Only copy hash

* Rm unneeded variables

* Add clarifying comment

* One-line comment

lib/merkle-tree.js

@@ -1224,7 +1224,16 @@ function getStoredNode (storage, index, cache, error) {
       }
 
       const node = { index, size, hash }
-      if (cache !== null) cache.set(index, node)
+
+      if (cache !== null) {
+        // Copy hash to a new buffer to avoid blocking gc of its original slab
+        const hashCopy = b4a.alloc(hash.byteLength)
+        hashCopy.set(hash)
+        node.hash = hashCopy
+
+        cache.set(index, node)
+      }
+
       resolve(node)
     })
   })

test/merkle-tree.js

@@ -1,6 +1,9 @@
+const path = require('path')
 const test = require('brittle')
 const RAM = require('random-access-memory')
 const b4a = require('b4a')
+const RandomAccessFile = require('random-access-file')
+const createTempDir = require('test-tmp')
 const Tree = require('../lib/merkle-tree')
 
 test('nodes', async function (t) {
@@ -599,6 +602,28 @@ test('checkout nodes in a batch', async t => {
   t.alike(nodes, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 28])
 })
 
+test('buffer of cached nodes is copied to small slab', async function (t) {
+  // RAM does not use slab-allocated memory,
+  // so we need to us random-access-file to reproduce this issue
+  const dir = await createTempDir(t)
+  const storage = new RandomAccessFile(path.join(dir, 'tree'))
+
+  const tree = await Tree.open(storage)
+
+  const b = tree.batch()
+  b.append(b4a.from('tree-entry'))
+  b.commit()
+
+  // Note: if the batch is committed but not yet flushed,
+  // it still uses the original slab-allocated memory
+  await tree.flush()
+
+  const node = await tree.get(0)
+  t.is(node.hash.buffer.byteLength, 32, 'created a new memory slab of the correct (small) size')
+
+  await tree.close()
+})
+
 async function audit (tree) {
   const flat = require('flat-tree')
   const expectedRoots = flat.fullRoots(tree.length * 2)