viirya commented on a change in pull request #1384: URL: https://github.com/apache/arrow-rs/pull/1384#discussion_r822063307
########## File path: arrow/src/ffi_stream.rs ########## @@ -0,0 +1,515 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +//! Contains declarations to bind to the [C Stream Interface](https://arrow.apache.org/docs/format/CStreamInterface.html). +//! + +use std::{ + convert::TryFrom, + ffi::CString, + os::raw::{c_char, c_int, c_void}, + sync::Arc, +}; + +use crate::array::Array; +use crate::array::StructArray; +use crate::datatypes::{Schema, SchemaRef}; +use crate::error::ArrowError; +use crate::error::Result; +use crate::ffi::*; +use crate::record_batch::{RecordBatch, RecordBatchReader}; + +const ENOMEM: i32 = 12; +const EIO: i32 = 5; +const EINVAL: i32 = 22; +const ENOSYS: i32 = 78; + +/// ABI-compatible struct for `ArrayStream` from C Stream Interface +/// This interface is experimental +/// See <https://arrow.apache.org/docs/format/CStreamInterface.html#structure-definitions> +/// This was created by bindgen +#[repr(C)] +#[derive(Debug, Clone)] +pub struct FFI_ArrowArrayStream { + pub get_schema: Option< + unsafe extern "C" fn( + arg1: *mut FFI_ArrowArrayStream, + out: *mut FFI_ArrowSchema, + ) -> c_int, + >, + pub get_next: Option< + unsafe extern "C" fn( + arg1: *mut FFI_ArrowArrayStream, + out: *mut FFI_ArrowArray, + ) -> c_int, + >, + pub get_last_error: + Option<unsafe extern "C" fn(arg1: *mut FFI_ArrowArrayStream) -> *const c_char>, + pub release: Option<unsafe extern "C" fn(arg1: *mut FFI_ArrowArrayStream)>, + pub private_data: *mut c_void, +} + +// callback used to drop [FFI_ArrowArrayStream] when it is exported. +unsafe extern "C" fn release_stream(stream: *mut FFI_ArrowArrayStream) { + if stream.is_null() { + return; + } + let stream = &mut *stream; + + stream.get_schema = None; + stream.get_next = None; + stream.get_last_error = None; + + let private_data = Box::from_raw(stream.private_data as *mut StreamPrivateData); + drop(private_data); + + stream.release = None; +} + +struct StreamPrivateData { + batch_reader: Box<dyn RecordBatchReader>, + last_error: String, +} + +// The callback used to get array schema +unsafe extern "C" fn get_schema( + stream: *mut FFI_ArrowArrayStream, + schema: *mut FFI_ArrowSchema, +) -> c_int { + ExportedArrayStream { stream }.get_schema(schema) +} + +// The callback used to get next array +unsafe extern "C" fn get_next( + stream: *mut FFI_ArrowArrayStream, + array: *mut FFI_ArrowArray, +) -> c_int { + ExportedArrayStream { stream }.get_next(array) +} + +// The callback used to get the error from last operation on the `FFI_ArrowArrayStream` +unsafe extern "C" fn get_last_error(stream: *mut FFI_ArrowArrayStream) -> *const c_char { + let last_error = ExportedArrayStream { stream }.get_last_error(); + CString::new(last_error.as_str()).unwrap().into_raw() +} + +impl Drop for FFI_ArrowArrayStream { + fn drop(&mut self) { + match self.release { + None => (), + Some(release) => unsafe { release(self) }, + }; + } +} + +impl FFI_ArrowArrayStream { + /// create a new [`FFI_ArrowArrayStream`]. + pub fn new(batch_reader: Box<dyn RecordBatchReader>) -> Self { + let private_data = Box::new(StreamPrivateData { + batch_reader, + last_error: String::new(), + }); + + Self { + get_schema: Some(get_schema), + get_next: Some(get_next), + get_last_error: Some(get_last_error), + release: Some(release_stream), + private_data: Box::into_raw(private_data) as *mut c_void, + } + } + + pub fn empty() -> Self { + Self { + get_schema: None, + get_next: None, + get_last_error: None, + release: None, + private_data: std::ptr::null_mut(), + } + } + + pub fn to_raw(this: Arc<FFI_ArrowArrayStream>) -> *const FFI_ArrowArrayStream { + Arc::into_raw(this) + } + + /// Get `FFI_ArrowArrayStream` from raw pointer + /// # Safety + /// Assumes that the pointer represents valid C Stream Interfaces, both in memory + /// representation and lifetime via the `release` mechanism. + pub unsafe fn from_raw( + ptr: *const FFI_ArrowArrayStream, + ) -> Arc<FFI_ArrowArrayStream> { + let ffi_stream = (*ptr).clone(); + Arc::new(ffi_stream) + } +} + +struct ExportedArrayStream { + stream: *mut FFI_ArrowArrayStream, +} + +impl ExportedArrayStream { + fn get_private_data(&self) -> Box<StreamPrivateData> { Review comment: `private_data` is kept in the stream as a raw pointer. 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