Fix upper bound detection in RangeIterator prefetch

lib/ranges.js

@@ -119,6 +119,7 @@ class RangeIterator {
     // TODO: dbl check this for off-by-ones with the offset and keys and children
     let limit = this.limit
 
+    // TODO: if limit === -1, don't return early here
     if (limit < this.tree.context.minKeys) return
 
     const parent = this.stack[this.stack.length - 1]
@@ -128,17 +129,21 @@ class RangeIterator {
     this.prefetching = pv
 
     for (let i = parent.offset >> 1; i < pv.children.length; i++) {
-      const k = pv.keys.get(i)
+      // If the preceding key in parent is beyond upper bound,
+      // stop fetching child nodes.
+      if (i > 0) {
+        const k = pv.keys.get(i - 1)
 
-      const cmp = this.reverse
-        ? this.start
-          ? b4a.compare(this.start, k.key)
-          : -1
-        : this.end
-          ? b4a.compare(k.key, this.end)
-          : -1
+        const cmp = this.reverse
+          ? this.start
+            ? b4a.compare(this.start, k.key)
+            : -1
+          : this.end
+            ? b4a.compare(k.key, this.end)
+            : -1
 
-      if (cmp > this.compare) break
+        if (cmp > this.compare) return
+      }
 
       const c = pv.children.get(i)
       if (!c.value) this.tree.inflate(c, this.config).catch(noop)

test/basic.js

@@ -556,3 +556,35 @@ test('lock to avoid building concurrent batches', async function (t) {
 
   t.alike((await db.get(name)).value, b4a.from('WORLD!'))
 })
+
+test('RangeIterator.prefetchNext with upper bound', async function (t) {
+  const db = await create(t)
+
+  function encodeUint32(n) {
+    const buf = new ArrayBuffer(4)
+    const view = new DataView(buf)
+    view.setUint32(0, n, false)
+    return b4a.from(buf)
+  }
+
+  const ENTRIES = 256
+
+  const w = db.write()
+  for (let i = 0; i < ENTRIES; i++) {
+    w.tryPut(encodeUint32(i), encodeUint32(i))
+  }
+  await w.flush()
+
+  const opt = {
+    prefetch: true,
+    lt: encodeUint32(ENTRIES),
+    // Needs a limit > minKeys to avoid early exit in prefetchNext
+    // (separate bug that can hide this one)
+    limit: ENTRIES * 2
+  }
+  let count = 0
+  for await (const _ of db.createReadStream(opt)) {
+    count++
+  }
+  t.alike(count, ENTRIES)
+})