EmilyMatt commented on code in PR #8930:
URL: https://github.com/apache/arrow-rs/pull/8930#discussion_r2702281708
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arrow-avro/src/reader/async_reader/mod.rs:
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@@ -0,0 +1,1157 @@
+use crate::compression::CompressionCodec;
+use crate::reader::Decoder;
+use crate::reader::block::BlockDecoder;
+use crate::reader::vlq::VLQDecoder;
+use arrow_array::RecordBatch;
+use arrow_schema::ArrowError;
+use bytes::Bytes;
+use futures::future::BoxFuture;
+use futures::{FutureExt, Stream};
+use std::mem;
+use std::ops::Range;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+mod async_file_reader;
+mod builder;
+
+pub use async_file_reader::AsyncFileReader;
+pub use builder::AsyncAvroFileReaderBuilder;
+
+#[cfg(feature = "object_store")]
+mod store;
+
+#[cfg(feature = "object_store")]
+pub use store::AvroObjectReader;
+
+enum ReaderState<R: AsyncFileReader> {
+ Idle {
+ reader: R,
+ },
+ FirstFetch {
+ future: BoxFuture<'static, Result<(R, Bytes), ArrowError>>,
+ },
+ Limbo,
+ DecodingBlock {
+ data: Bytes,
+ reader: R,
+ },
+ ReadingBatches {
+ data: Bytes,
+ block_data: Bytes,
+ remaining_in_block: usize,
+ reader: R,
+ },
+ ReadingFinalBlock {
+ current_data: Bytes,
+ future: BoxFuture<'static, Result<(R, Bytes), ArrowError>>,
+ },
+ Finished,
+}
+
+/// An asynchronous Avro file reader that implements `Stream<Item =
Result<RecordBatch, ArrowError>>`.
+/// This uses an [`AsyncFileReader`] to fetch data ranges as needed, starting
with fetching the header,
+/// then reading all the blocks in the provided range where:
+/// 1. Reads and decodes data until the header is fully decoded.
+/// 2. Searching from `range.start` for the first sync marker, and starting
with the following block.
+/// (If `range.start` is less than the header length, we start at the
header length minus the sync marker bytes)
+/// 3. Reading blocks sequentially, decoding them into RecordBatches.
+/// 4. If a block is incomplete (due to range ending mid-block), fetching the
remaining bytes from the [`AsyncFileReader`].
+/// 5. If no range was originally provided, reads the full file.
+/// 6. If the range is 0, file_size is 0, or `range.end` is less than the
header length, finish immediately.
+pub struct AsyncAvroFileReader<R: AsyncFileReader> {
+ // Members required to fetch data
+ range: Range<u64>,
+ file_size: u64,
+
+ // Members required to actually decode and read data
+ decoder: Decoder,
+ block_decoder: BlockDecoder,
+ codec: Option<CompressionCodec>,
+ sync_marker: [u8; 16],
+
+ // Members keeping the current state of the reader
+ reader_state: ReaderState<R>,
+ finishing_partial_block: bool,
+}
+
+impl<R: AsyncFileReader + Unpin + 'static> AsyncAvroFileReader<R> {
+ /// Returns a builder for a new [`Self`], allowing some optional
parameters.
+ pub fn builder(reader: R, file_size: u64, batch_size: usize) ->
AsyncAvroFileReaderBuilder<R> {
+ AsyncAvroFileReaderBuilder::new(reader, file_size, batch_size)
+ }
+
+ fn new(
+ range: Range<u64>,
+ file_size: u64,
+ decoder: Decoder,
+ codec: Option<CompressionCodec>,
+ sync_marker: [u8; 16],
+ reader_state: ReaderState<R>,
+ ) -> Self {
+ Self {
+ range,
+ file_size,
+
+ decoder,
+ block_decoder: Default::default(),
+ codec,
+ sync_marker,
+
+ reader_state,
+ finishing_partial_block: false,
+ }
+ }
+
+ fn read_next(&mut self, cx: &mut Context<'_>) ->
Poll<Option<Result<RecordBatch, ArrowError>>> {
+ loop {
+ match mem::replace(&mut self.reader_state, ReaderState::Limbo) {
+ ReaderState::Idle { mut reader } => {
+ let range = self.range.clone();
+ if range.start >= range.end || range.end > self.file_size {
+ return
Poll::Ready(Some(Err(ArrowError::AvroError(format!(
+ "Invalid range specified for Avro file: start {}
>= end {}, file_size: {}",
+ range.start, range.end, self.file_size
+ )))));
+ }
+
+ let future = async move {
+ let data = reader.get_bytes(range).await?;
+ Ok((reader, data))
+ }
+ .boxed();
+
+ self.reader_state = ReaderState::FirstFetch { future };
+ }
+ ReaderState::FirstFetch { mut future } => {
+ let (reader, data_chunk) = match future.poll_unpin(cx) {
+ Poll::Ready(Ok(data)) => data,
+ Poll::Ready(Err(e)) => {
+ return Poll::Ready(Some(Err(e)));
+ }
+ Poll::Pending => {
+ self.reader_state = ReaderState::FirstFetch {
future };
+ return Poll::Pending;
+ }
+ };
+
+ let sync_marker_pos = data_chunk
+ .windows(16)
+ .position(|slice| slice == self.sync_marker);
+ let block_start = match sync_marker_pos {
+ Some(pos) => pos + 16, // Move past the sync marker
+ None => {
+ // Sync marker not found, this is actually valid
if we arbitrarily split the file at its end.
+ self.reader_state = ReaderState::Finished;
+ return Poll::Ready(None);
+ }
+ };
+
+ // This is the first time we read data, so try and find
the sync marker.
+ self.reader_state = ReaderState::DecodingBlock {
+ reader,
+ data: data_chunk.slice(block_start..),
+ };
+ }
+ ReaderState::Limbo => {
+ unreachable!("ReaderState::Limbo should never be
observed");
+ }
Review Comment:
A user will never know they did not actually finish writing the file, they
will think they've just reached the end, this is in my opinion times of
magnitude more severe than crashing.
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