feat: hook up new writer for insert (#3029)

This hooks up a new writer implementation for the `add()` method. The
main immediate benefit is it allows streaming requests to remote tables,
and at the same time allowing retries for most inputs.

In NodeJS, we always convert the data to `Vec<RecordBatch>`, so it's
always retry-able.

For Python, all are retry-able, except `Iterator` and
`pa.RecordBatchReader`, which can only be consumed once. Some, like
`pa.datasets.Dataset` are retry-able *and* streaming.

A lot of the changes here are to make the new DataFusion write pipeline
maintain the same behavior as the existing Python-based preprocessing,
such as:

* casting input data to target schema
* rejecting NaN values if `on_bad_vectors="error"`
* applying embedding functions.

In future PRs, we'll enhance these by moving the embedding calls into
DataFusion and making sure we parallelize them. See:
https://github.com/lancedb/lancedb/issues/3048

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

python/src/table/scannable.rs

@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+use std::sync::Arc;
+
+use arrow::{
+    datatypes::{Schema, SchemaRef},
+    ffi_stream::ArrowArrayStreamReader,
+    pyarrow::{FromPyArrow, PyArrowType},
+};
+use futures::StreamExt;
+use lancedb::{
+    arrow::{SendableRecordBatchStream, SimpleRecordBatchStream},
+    data::scannable::Scannable,
+    Error,
+};
+use pyo3::{types::PyAnyMethods, FromPyObject, Py, PyAny, Python};
+
+/// Adapter that implements Scannable for a Python reader factory callable.
+///
+/// This holds a Python callable that returns a RecordBatchReader when called.
+/// For rescannable sources, the callable can be invoked multiple times to
+/// get fresh readers.
+pub struct PyScannable {
+    /// Python callable that returns a RecordBatchReader
+    reader_factory: Py<PyAny>,
+    schema: SchemaRef,
+    num_rows: Option<usize>,
+    rescannable: bool,
+}
+
+impl std::fmt::Debug for PyScannable {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("PyScannable")
+            .field("schema", &self.schema)
+            .field("num_rows", &self.num_rows)
+            .field("rescannable", &self.rescannable)
+            .finish()
+    }
+}
+
+impl Scannable for PyScannable {
+    fn schema(&self) -> SchemaRef {
+        self.schema.clone()
+    }
+
+    fn scan_as_stream(&mut self) -> SendableRecordBatchStream {
+        let reader: Result<ArrowArrayStreamReader, Error> = {
+            Python::attach(|py| {
+                let result =
+                    self.reader_factory
+                        .call0(py)
+                        .map_err(|e| lancedb::Error::Runtime {
+                            message: format!("Python reader factory failed: {}", e),
+                        })?;
+                ArrowArrayStreamReader::from_pyarrow_bound(result.bind(py)).map_err(|e| {
+                    lancedb::Error::Runtime {
+                        message: format!("Failed to create Arrow reader from Python: {}", e),
+                    }
+                })
+            })
+        };
+
+        // Reader is blocking but stream is non-blocking, so we need to spawn a task to pull.
+        let (tx, rx) = tokio::sync::mpsc::channel(1);
+
+        let join_handle = tokio::task::spawn_blocking(move || {
+            let reader = match reader {
+                Ok(reader) => reader,
+                Err(e) => {
+                    let _ = tx.blocking_send(Err(e));
+                    return;
+                }
+            };
+            for batch in reader {
+                match batch {
+                    Ok(batch) => {
+                        if tx.blocking_send(Ok(batch)).is_err() {
+                            // Receiver dropped, stop processing
+                            break;
+                        }
+                    }
+                    Err(source) => {
+                        let _ = tx.blocking_send(Err(Error::Arrow { source }));
+                        break;
+                    }
+                }
+            }
+        });
+
+        let schema = self.schema.clone();
+        let stream = futures::stream::unfold(
+            (rx, Some(join_handle)),
+            |(mut rx, join_handle)| async move {
+                match rx.recv().await {
+                    Some(Ok(batch)) => Some((Ok(batch), (rx, join_handle))),
+                    Some(Err(e)) => Some((Err(e), (rx, join_handle))),
+                    None => {
+                        // Channel closed. Check if the task panicked — a panic
+                        // drops the sender without sending an error, so without
+                        // this check we'd silently return a truncated stream.
+                        if let Some(handle) = join_handle {
+                            if let Err(join_err) = handle.await {
+                                return Some((
+                                    Err(Error::Runtime {
+                                        message: format!("Reader task panicked: {}", join_err),
+                                    }),
+                                    (rx, None),
+                                ));
+                            }
+                        }
+                        None
+                    }
+                }
+            },
+        );
+        Box::pin(SimpleRecordBatchStream::new(stream.fuse(), schema))
+    }
+
+    fn num_rows(&self) -> Option<usize> {
+        self.num_rows
+    }
+
+    fn rescannable(&self) -> bool {
+        self.rescannable
+    }
+}
+
+impl<'py> FromPyObject<'py> for PyScannable {
+    fn extract_bound(ob: &pyo3::Bound<'py, PyAny>) -> pyo3::PyResult<Self> {
+        // Convert from Scannable dataclass.
+        let schema: PyArrowType<Schema> = ob.getattr("schema")?.extract()?;
+        let schema = Arc::new(schema.0);
+        let num_rows: Option<usize> = ob.getattr("num_rows")?.extract()?;
+        let rescannable: bool = ob.getattr("rescannable")?.extract()?;
+        let reader_factory: Py<PyAny> = ob.getattr("reader")?.unbind();
+
+        Ok(Self {
+            schema,
+            reader_factory,
+            num_rows,
+            rescannable,
+        })
+    }
+}