From 9a2d9d92ad87611ad5f920de9a154874ae89aa5e Mon Sep 17 00:00:00 2001
From: Vincent Weevers <mail@vincentweevers.nl>
Date: Sun, 4 Feb 2024 19:49:44 +0100
Subject: [PATCH] Fix cache bug in `iterator.nextv()`

---
 iterator.js           | 10 +++++++++-
 test/iterator-test.js | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/iterator.js b/iterator.js
index b9f46a57..cc8d29de 100644
--- a/iterator.js
+++ b/iterator.js
@@ -76,10 +76,18 @@ class Iterator extends AbstractIterator {
     }
   }
 
-  // TODO (v2): read from cache first?
   async _nextv (size, options) {
     this[kFirst] = false
 
+    // If next() was called then empty the cache first
+    if (this[kPosition] < this[kCache].length) {
+      const length = Math.min(size, this[kCache].length - this[kPosition])
+      const chunk = this[kCache].slice(this[kPosition], this[kPosition] + length)
+
+      this[kPosition] += length
+      return chunk
+    }
+
     // Avoid iterator_nextv() call if end was already reached
     if ((this[kState][0] & STATE_ENDED) !== 0) {
       return []
diff --git a/test/iterator-test.js b/test/iterator-test.js
index fe764d3b..fd2e2b93 100644
--- a/test/iterator-test.js
+++ b/test/iterator-test.js
@@ -37,3 +37,37 @@ test('iterator optimized for seek', async function (t) {
   await ite.close()
   return db.close()
 })
+
+// TODO: move to abstract-level
+for (const slice of [false, true]) {
+  test(`nextv() after next() respects cache (slice=${slice})`, async function (t) {
+    const db = testCommon.factory()
+
+    await db.open()
+    await db.batch()
+      .put('a', 'value')
+      .put('b', 'value')
+      .put('c', 'value')
+      .put('d', 'value')
+      .write()
+
+    const it = db.iterator()
+
+    // Two called are needed to populate the cache
+    t.same(await it.next(), ['a', 'value'], 'entry a ok')
+    t.same(await it.next(), ['b', 'value'], 'entry b ok')
+    t.is(it.cached, 2)
+
+    if (slice) {
+      t.same(await it.nextv(1), [['c', 'value']], 'entries ok (1)')
+      t.same(await it.nextv(1), [['d', 'value']], 'entries ok (2)')
+      t.same(await it.nextv(1), [], 'empty')
+    } else {
+      t.same(await it.nextv(10), [['c', 'value'], ['d', 'value']], 'entries ok')
+      t.same(await it.nextv(10), [], 'empty')
+    }
+
+    await it.close()
+    return db.close()
+  })
+}