tustvold commented on code in PR #3370: URL: https://github.com/apache/arrow-rs/pull/3370#discussion_r1052520521
########## parquet/src/arrow/async_reader/metadata.rs: ########## @@ -0,0 +1,94 @@ +// 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. + +use crate::errors::{ParquetError, Result}; +use crate::file::footer::{decode_footer, decode_metadata}; +use crate::file::metadata::ParquetMetaData; +use bytes::{BufMut, Bytes, BytesMut}; +use std::future::Future; +use std::ops::Range; + +/// Fetches parquet metadata using an async function that can fetch byte ranges +/// +/// The length of the parquet footer, which contains file metadata, is not +/// known up front. Therefore this function will first issue a request to read +/// the last 8 bytes to determine the footer's precise length, before +/// issuing a second request to fetch the metadata bytes +/// +/// If a hint is set, this method will read the specified number of bytes +/// in the first request, instead of 8, and only issue a second request +/// if additional bytes are needed. This can therefore eliminate a +/// potentially costly additional fetch operation +pub async fn fetch_parquet_metadata<F, Fut>( + fetch: F, + file_size: usize, + footer_size_hint: Option<usize>, +) -> Result<ParquetMetaData> +where + F: Fn(Range<usize>) -> Fut, + Fut: Future<Output = Result<Bytes>>, +{ + if file_size < 8 { + return Err(ParquetError::EOF(format!( + "file size of {} is less than footer", + file_size + ))); + } + + // If a size hint is provided, read more than the minimum size + // to try and avoid a second fetch. + let footer_start = if let Some(size_hint) = footer_size_hint { + file_size.saturating_sub(size_hint) + } else { + file_size - 8 + }; + + let suffix = fetch(footer_start..file_size).await?; + let suffix_len = suffix.len(); + + let mut footer = [0; 8]; + footer.copy_from_slice(&suffix[suffix_len - 8..suffix_len]); + + let length = decode_footer(&footer)?; + + if file_size < length + 8 { + return Err(ParquetError::EOF(format!( + "file size of {} is less than footer + metadata {}", + file_size, + length + 8 + ))); + } + + // Did not fetch the entire file metadata in the initial read, need to make a second request + if length > suffix_len - 8 { + let metadata_start = file_size - length - 8; Review Comment: Sadly parquet isn't yet hooked up with logging #1495 -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
