feat(nodejs): add Scannable primitive for streaming ingestion (#3271)

## **Summary**

This PR adds a **Scannable primitive** to the Node.js bindings, bringing
parity with Python's `PyScannable`.

A `Scannable` wraps a schema, an optional row count hint, a rescannable
flag, and a batch producing callback. On the Rust side it implements
`lancedb::data::scannable::Scannable`. The goal is to give consumers
such as `Table.add`, `createTable`, and `mergeInsert` a way to stream
data without materializing the full dataset in JS memory.

This PR introduces only the primitive. Migrating existing consumers to
use it will come in follow up work.

---

## **Design**

### **Transport**

The transport uses the **Arrow IPC Stream format, one batch at a time**.

The JS side encodes each `RecordBatch` into a self contained IPC Stream
message containing schema, batch, and end of stream. The message is
returned as a `Buffer` through a napi `ThreadsafeFunction`. The Rust
side decodes it using `arrow_ipc::reader::StreamReader`.

Only one batch is active at a time, so JS memory stays bounded by the
batch size. The Node `Buffer` size limit of about 4 GiB therefore does
not constrain the stream as a whole.

I initially evaluated the Arrow C Data Interface, which is the approach
used in Python. I dropped that path after confirming that the
`apache-arrow` npm package does not expose a C Data Interface export in
any supported version from 15 to 18. JavaScript is not listed in Arrow's
C Data Interface implementation table, and the upstream tracking issue
remains open with no scheduled work.

Third party FFI shims would introduce additional dependency risk without
solving the core maintenance problem. Using IPC adds one encode and
decode step per batch, but the cost is predictable and typically
dominated by Lance's write path.

---

### **API**

```ts
class Scannable {
  readonly schema: Schema
  readonly numRows: number | null
  readonly rescannable: boolean

  static fromFactory(schema, factory, opts?)
  static fromTable(table, opts?)
  static fromIterable(schema, iter, opts?)
  static fromRecordBatchReader(reader, opts?)
}
```

The FFI boundary consists of a single callback:

`getNextBatch(isStart: boolean): Promise<Buffer | null>`

`isStart` is `true` on the first call of each new scan and `false` for
every call after it. The JS side uses it to drop any cached iterator and
re-invoke the factory at scan boundaries. This is what makes a
rescannable source restart at batch 0 on every `scan_as_stream` call,
even when a previous scan ended mid stream, for example a retried write
after a network error. Without this signal a retry would resume a stale
iterator and silently skip already emitted batches.

In addition, a schema only IPC buffer is transferred once during
construction.

---

## **Changes**

* `nodejs/src/scannable.rs`
Adds `NapiScannable` and the `LanceScannable` implementation. Implements
`schema()`, `num_rows()`, `rescannable()`, and `scan_as_stream()`.
Includes per batch schema validation against the declared schema, one
shot enforcement for non rescannable sources, and a scan boundary reset
signal (`isStart`) so rescannable sources restart from batch 0 on every
`scan_as_stream` call rather than resuming a stale iterator.

* `nodejs/src/lib.rs`
  Module registration.

* `nodejs/lancedb/scannable.ts`
Defines the `Scannable` class and the four constructors listed above.
Each constructor rejects option combinations it cannot honor, for
example a `rescannable: true` request on a one shot iterable or reader,
and a `numRows` that disagrees with an in memory table's row count.

* `nodejs/lancedb/index.ts`
  Exports the new primitive.

* `nodejs/__test__/scannable.test.ts`
  Test suite for the primitive.

---

## **Validation**

Before implementing the bridge, I ran an end to end harness with a JS
producer feeding a standalone Rust consumer built against the same
`arrow-ipc` version used in the bridge.

The harness covered the following scenarios:

* happy path
* empty stream
* 1,000 small batches
* 10 large batches
* mixed primitive types with nullables
* nested `List<Struct<>>`
* truncated stream error handling
* declared schema mismatch validation
* a 6 GB stress test through the pipe

All scenarios completed with bounded memory usage. The goal of this
harness was to confirm that the IPC Stream transport works correctly end
to end and that Node's `Buffer` size limit does not constrain the
overall stream.

Separately, the rescannable restart contract was verified with a focused
harness. A rescannable source is consumed partially and the scan is
dropped mid stream, then re-scanned. The re-scan replays from batch 0
rather than resuming the stale iterator. The same harness was run with
the `isStart` reset path disabled and the mid stream restart case failed
as expected, confirming the test exercises the real regression.

These harnesses are not meant to replace the full test suite, which is
described below.

---

## **Tests**

`__test__/scannable.test.ts` covers construction, metadata reflection,
per constructor defaults and overrides, construction time validation,
the native handle surface, and schema variety across empty tables,
nested types, `FixedSizeList`, and wide schemas.

Runtime scan behavior including `scan_as_stream`, one shot enforcement
on non rescannable sources, schema mismatch detection, IPC decode
failures, and rescannable restart semantics is not exercised here. There
is no in tree JS consumer of `NapiScannable` yet. This mirrors Python's
`PyScannable`, which has no dedicated test file and is covered
transitively through the consumers that accept a Scannable.

Runtime coverage will follow in the consumer migration work.

---

## **Status**

Ready for review.

Closes #3223

---

nodejs/__test__/scannable.test.ts

@@ -0,0 +1,438 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import {
+  Field,
+  Float16,
+  Int32,
+  type RecordBatch,
+  RecordBatchReader,
+  Schema,
+  tableToIPC,
+} from "apache-arrow";
+import { makeArrowTable, makeEmptyTable } from "../lancedb/arrow";
+import { Scannable } from "../lancedb/scannable";
+
+function makeTable() {
+  return makeArrowTable(
+    [
+      { id: 1, name: "a" },
+      { id: 2, name: "b" },
+      { id: 3, name: "c" },
+    ],
+    { vectorColumns: {} },
+  );
+}
+
+async function makeReader(): Promise<RecordBatchReader> {
+  // `RecordBatchReader.from()` returns an unopened reader; `.schema` is only
+  // populated after `.open()`. Opening sync readers is synchronous.
+  const reader = RecordBatchReader.from(tableToIPC(makeTable()));
+  return reader.open() as RecordBatchReader;
+}
+
+describe("Scannable", () => {
+  describe("fromTable", () => {
+    test("reflects schema, numRows, and defaults rescannable=true", async () => {
+      const table = makeTable();
+      const scannable = await Scannable.fromTable(table);
+
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.numRows).toBe(table.numRows);
+      expect(scannable.rescannable).toBe(true);
+    });
+
+    test("throws when opts.numRows does not match table.numRows", async () => {
+      await expect(
+        Scannable.fromTable(makeTable(), { numRows: 42 }),
+      ).rejects.toThrow(/does not match table\.numRows/);
+    });
+
+    test("throws when opts.rescannable is false", async () => {
+      await expect(
+        Scannable.fromTable(makeTable(), { rescannable: false }),
+      ).rejects.toThrow(/always rescannable/);
+    });
+  });
+
+  describe("fromRecordBatchReader", () => {
+    test("reflects schema and defaults numRows=null, rescannable=false", async () => {
+      const reader = await makeReader();
+      const scannable = await Scannable.fromRecordBatchReader(reader);
+
+      expect(scannable.schema).toBe(reader.schema);
+      expect(scannable.numRows).toBeNull();
+      expect(scannable.rescannable).toBe(false);
+    });
+
+    test("honors numRows override", async () => {
+      const scannable = await Scannable.fromRecordBatchReader(
+        await makeReader(),
+        { numRows: 3 },
+      );
+
+      expect(scannable.numRows).toBe(3);
+      expect(scannable.rescannable).toBe(false);
+    });
+
+    test("rescannable: false explicit does not throw", async () => {
+      const reader = await makeReader();
+      const scannable = await Scannable.fromRecordBatchReader(reader, {
+        rescannable: false,
+      });
+      expect(scannable.rescannable).toBe(false);
+    });
+
+    test("throws when opts.rescannable is true", async () => {
+      const reader = await makeReader();
+      await expect(
+        Scannable.fromRecordBatchReader(reader, { rescannable: true }),
+      ).rejects.toThrow(/does not accept rescannable/);
+    });
+
+    test("throws when opts.rescannable is true even alongside numRows", async () => {
+      const reader = await makeReader();
+      await expect(
+        Scannable.fromRecordBatchReader(reader, {
+          numRows: 3,
+          rescannable: true,
+        }),
+      ).rejects.toThrow(/does not accept rescannable/);
+    });
+  });
+
+  describe("fromIterable", () => {
+    test("accepts a sync iterable of batches", async () => {
+      const table = makeTable();
+      const scannable = await Scannable.fromIterable(
+        table.schema,
+        table.batches,
+      );
+
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.numRows).toBeNull();
+      expect(scannable.rescannable).toBe(false);
+    });
+
+    test("accepts an async iterable of batches", async () => {
+      const table = makeTable();
+      async function* generator(): AsyncGenerator<RecordBatch> {
+        for (const batch of table.batches) {
+          yield batch;
+        }
+      }
+
+      const scannable = await Scannable.fromIterable(table.schema, generator());
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.rescannable).toBe(false);
+    });
+
+    describe("rescannable: true detection", () => {
+      // Replayable inputs: [Symbol.iterator]() / [Symbol.asyncIterator]()
+      // returns a fresh iterator each call. Must NOT throw.
+
+      test("Array passes (fresh ArrayIterator each call)", async () => {
+        const table = makeTable();
+        const scannable = await Scannable.fromIterable(
+          table.schema,
+          table.batches,
+          { rescannable: true },
+        );
+        expect(scannable.rescannable).toBe(true);
+      });
+
+      test("Set passes (fresh SetIterator each call)", async () => {
+        const table = makeTable();
+        const set = new Set<RecordBatch>(table.batches);
+        const scannable = await Scannable.fromIterable(table.schema, set, {
+          rescannable: true,
+        });
+        expect(scannable.rescannable).toBe(true);
+      });
+
+      test("custom Iterable returning a fresh iterator passes", async () => {
+        const table = makeTable();
+        const replayable: Iterable<RecordBatch> = {
+          [Symbol.iterator]() {
+            return table.batches[Symbol.iterator]();
+          },
+        };
+        const scannable = await Scannable.fromIterable(
+          table.schema,
+          replayable,
+          { rescannable: true },
+        );
+        expect(scannable.rescannable).toBe(true);
+      });
+
+      test("object with generator method passes (fresh generator each call)", async () => {
+        const table = makeTable();
+        const replayable: Iterable<RecordBatch> = {
+          *[Symbol.iterator]() {
+            for (const batch of table.batches) yield batch;
+          },
+        };
+        const scannable = await Scannable.fromIterable(
+          table.schema,
+          replayable,
+          { rescannable: true },
+        );
+        expect(scannable.rescannable).toBe(true);
+      });
+
+      test("empty Array passes (replayable degenerate case)", async () => {
+        const schema = makeTable().schema;
+        const scannable = await Scannable.fromIterable(
+          schema,
+          [] as RecordBatch[],
+          { rescannable: true },
+        );
+        expect(scannable.rescannable).toBe(true);
+      });
+
+      // One-shot inputs: [Symbol.iterator]() / [Symbol.asyncIterator]()
+      // returns the same object, or the input is already-an-iterator.
+      // Must throw with a /one-shot/ message.
+
+      test("sync generator throws", async () => {
+        const table = makeTable();
+        function* generator(): Generator<RecordBatch> {
+          for (const batch of table.batches) yield batch;
+        }
+        await expect(
+          Scannable.fromIterable(table.schema, generator(), {
+            rescannable: true,
+          }),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      test("async generator throws", async () => {
+        const table = makeTable();
+        async function* generator(): AsyncGenerator<RecordBatch> {
+          for (const batch of table.batches) yield batch;
+        }
+        await expect(
+          Scannable.fromIterable(table.schema, generator(), {
+            rescannable: true,
+          }),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      test("empty generator throws (one-shot degenerate case)", async () => {
+        const schema = makeTable().schema;
+        function* generator(): Generator<RecordBatch> {
+          // intentionally empty; yields nothing.
+        }
+        await expect(
+          Scannable.fromIterable(schema, generator(), { rescannable: true }),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      test("custom self-iterator throws", async () => {
+        const table = makeTable();
+        const batches = table.batches;
+        let i = 0;
+        const oneShot: Iterable<RecordBatch> & Iterator<RecordBatch> = {
+          [Symbol.iterator]() {
+            return this;
+          },
+          next() {
+            if (i >= batches.length) {
+              return { done: true, value: undefined };
+            }
+            return { done: false, value: batches[i++] };
+          },
+        };
+        await expect(
+          Scannable.fromIterable(table.schema, oneShot, { rescannable: true }),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      test("Array.values() (IterableIterator) throws", async () => {
+        const table = makeTable();
+        const iter = table.batches.values();
+        await expect(
+          Scannable.fromIterable(table.schema, iter, { rescannable: true }),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      test("raw iterator (only `.next`) throws", async () => {
+        const table = makeTable();
+        const batches = table.batches;
+        let i = 0;
+        const rawIter = {
+          next(): IteratorResult<RecordBatch> {
+            if (i >= batches.length) {
+              return { done: true, value: undefined };
+            }
+            return { done: false, value: batches[i++] };
+          },
+        };
+        await expect(
+          Scannable.fromIterable(
+            table.schema,
+            rawIter as unknown as Iterable<RecordBatch>,
+            { rescannable: true },
+          ),
+        ).rejects.toThrow(/one-shot/);
+      });
+
+      // Edge: null/undefined must not crash the detection helper. The
+      // null check belongs to `normalizeIterator` and only fires when a
+      // scan starts.
+
+      test("null input does not crash detection at construction", async () => {
+        const schema = makeTable().schema;
+        await expect(
+          Scannable.fromIterable(
+            schema,
+            null as unknown as Iterable<RecordBatch>,
+            {
+              rescannable: true,
+            },
+          ),
+        ).resolves.toBeDefined();
+      });
+
+      test("undefined input does not crash detection at construction", async () => {
+        const schema = makeTable().schema;
+        await expect(
+          Scannable.fromIterable(
+            schema,
+            undefined as unknown as Iterable<RecordBatch>,
+            { rescannable: true },
+          ),
+        ).resolves.toBeDefined();
+      });
+
+      // Default (rescannable omitted) skips the check entirely, so even
+      // pathological inputs construct without throwing here.
+
+      test("rescannable omitted skips detection entirely (generator passes)", async () => {
+        const table = makeTable();
+        function* generator(): Generator<RecordBatch> {
+          for (const batch of table.batches) yield batch;
+        }
+        const scannable = await Scannable.fromIterable(
+          table.schema,
+          generator(),
+        );
+        expect(scannable.rescannable).toBe(false);
+      });
+
+      test("rescannable: false explicit skips detection entirely (generator passes)", async () => {
+        const table = makeTable();
+        function* generator(): Generator<RecordBatch> {
+          for (const batch of table.batches) yield batch;
+        }
+        const scannable = await Scannable.fromIterable(
+          table.schema,
+          generator(),
+          { rescannable: false },
+        );
+        expect(scannable.rescannable).toBe(false);
+      });
+    });
+  });
+
+  describe("fromFactory", () => {
+    test("defaults rescannable=true and does not invoke the factory eagerly", async () => {
+      const table = makeTable();
+      const factory = jest.fn(() => table.batches);
+
+      const scannable = await Scannable.fromFactory(table.schema, factory);
+
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.rescannable).toBe(true);
+      expect(factory).not.toHaveBeenCalled();
+    });
+
+    test("honors rescannable and numRows overrides", async () => {
+      const table = makeTable();
+      const scannable = await Scannable.fromFactory(
+        table.schema,
+        () => table.batches,
+        { numRows: 7, rescannable: false },
+      );
+
+      expect(scannable.numRows).toBe(7);
+      expect(scannable.rescannable).toBe(false);
+    });
+  });
+
+  describe("validation", () => {
+    test("throws when numRows is negative", async () => {
+      await expect(
+        Scannable.fromFactory(makeTable().schema, () => [], { numRows: -1 }),
+      ).rejects.toThrow(/non-negative/);
+    });
+
+    test("throws when numRows is not an integer", async () => {
+      await expect(
+        Scannable.fromFactory(makeTable().schema, () => [], { numRows: 3.5 }),
+      ).rejects.toThrow(/integer/);
+    });
+  });
+
+  describe("native handle", () => {
+    test("exposes a native handle via inner", async () => {
+      const scannable = await Scannable.fromTable(makeTable());
+      expect(scannable.inner).toBeDefined();
+      expect(typeof scannable.inner).toBe("object");
+      expect(scannable.inner).not.toBeNull();
+    });
+  });
+
+  // Schema-variety construction tests. Each asserts that construction
+  // succeeds against a richer Arrow schema, which transitively exercises
+  // schema serialization and the Rust-side `ipc_file_to_schema` for types
+  // beyond flat primitives.
+  describe("schema variety", () => {
+    test("accepts an empty table", async () => {
+      const schema = new Schema([new Field("id", new Int32(), true)]);
+      const table = makeEmptyTable(schema);
+      const scannable = await Scannable.fromTable(table);
+
+      expect(scannable.numRows).toBe(0);
+      expect(scannable.schema).toBe(table.schema);
+    });
+
+    test("accepts nested struct and list columns", async () => {
+      const table = makeArrowTable(
+        [
+          { id: 1, point: { x: 0, y: 0 }, tags: ["a", "b"] },
+          { id: 2, point: { x: 1, y: 2 }, tags: ["c"] },
+        ],
+        { vectorColumns: {} },
+      );
+      const scannable = await Scannable.fromTable(table);
+
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.numRows).toBe(2);
+    });
+
+    test("accepts a FixedSizeList (vector) column", async () => {
+      const table = makeArrowTable(
+        [
+          { id: 1, vec: [1, 2, 3] },
+          { id: 2, vec: [4, 5, 6] },
+        ],
+        { vectorColumns: { vec: { type: new Float16() } } },
+      );
+      const scannable = await Scannable.fromTable(table);
+
+      expect(scannable.schema).toBe(table.schema);
+      expect(scannable.numRows).toBe(2);
+    });
+
+    test("accepts a table with many columns", async () => {
+      const row: Record<string, number> = {};
+      for (let i = 0; i < 50; i++) row[`c${i}`] = i;
+      const table = makeArrowTable([row, row], { vectorColumns: {} });
+      const scannable = await Scannable.fromTable(table);
+
+      expect(scannable.schema.fields.length).toBe(50);
+      expect(scannable.numRows).toBe(2);
+    });
+  });
+});