[GitHub] [arrow] alamb commented on a change in pull request #8300: ARROW-10135: [Rust] [Parquet] Refactor file module to help adding sources

[GitBox <gi...@apache.org>]

alamb commented on a change in pull request #8300:
URL: https://github.com/apache/arrow/pull/8300#discussion_r500903617



##########
File path: rust/parquet/src/util/cursor.rs
##########
@@ -0,0 +1,203 @@
+// 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 std::cmp;
+use std::io::{self, Error, ErrorKind, Read, Seek, SeekFrom};
+use std::rc::Rc;
+
+/// This is object to use if your file is already in memory.
+/// The sliceable cursor is similar to std::io::Cursor, except that it makes it easy to create "cursor slices".
+/// To achieve this, it uses Rc instead of shared references. Indeed reference fields are painfull

Review comment:
       I would personally suggest moving to `Arc` here rather than `Rc` across the board in the Parquet reader. I suggest `Arc` to "future proof" the code as `Arc` almost always is needed with `async` and multi-threading (as `Rc` doesn't implement `Send`)

##########
File path: rust/parquet/src/file/reader.rs
##########
@@ -19,34 +19,48 @@
 //! readers to read individual column chunks, or access record iterator.
 
 use std::{
-    convert::TryFrom,
-    fs::File,
-    io::{BufReader, Cursor, Read, Seek, SeekFrom},
-    path::Path,
+    boxed::Box,
+    io::{Read, Seek},
     rc::Rc,
 };
 
-use byteorder::{ByteOrder, LittleEndian};
-use parquet_format::{
-    ColumnOrder as TColumnOrder, FileMetaData as TFileMetaData, PageHeader, PageType,
-};
-use thrift::protocol::TCompactInputProtocol;
-
-use crate::basic::{ColumnOrder, Compression, Encoding, Type};
 use crate::column::page::PageIterator;
-use crate::column::{
-    page::{Page, PageReader},
-    reader::{ColumnReader, ColumnReaderImpl},
-};
-use crate::compression::{create_codec, Codec};
+use crate::column::{page::PageReader, reader::ColumnReader};
 use crate::errors::{ParquetError, Result};
-use crate::file::{metadata::*, statistics, FOOTER_SIZE, PARQUET_MAGIC};
+use crate::file::metadata::*;
+pub use crate::file::serialized_reader::{SerializedFileReader, SerializedPageReader};
 use crate::record::reader::RowIter;
-use crate::record::Row;
-use crate::schema::types::{
-    self, ColumnDescPtr, SchemaDescPtr, SchemaDescriptor, Type as SchemaType,
-};
-use crate::util::{io::FileSource, memory::ByteBufferPtr};
+use crate::schema::types::{ColumnDescPtr, SchemaDescPtr, Type as SchemaType};
+
+use crate::basic::Type;
+
+use crate::column::reader::ColumnReaderImpl;
+
+/// Length should return the amount of bytes that implementor contains.
+/// It's mainly used to read the metadata, which is at the end of the source.
+#[allow(clippy::len_without_is_empty)]
+pub trait Length {
+    /// Returns the amount of bytes of the inner source.
+    fn len(&self) -> u64;
+}
+
+pub trait ReadChunck: Read + Length {}
+pub trait ReadSeekChunck: ReadChunck + Seek {}
+impl<T: Read + Length> ReadChunck for T {}

Review comment:
       I like this pattern (of automatically implementing the combined trait for anything that implements `Read` + `Length` (I struggled doing that with FileReader when working with the Parquet API earlier in my Rust days)

##########
File path: rust/parquet/src/util/io.rs
##########
@@ -17,11 +17,31 @@
 
 use std::{cell::RefCell, cmp, io::*};
 
-use crate::file::{reader::ParquetReader, writer::ParquetWriter};
+use crate::file::{reader::Length, writer::ParquetWriter};
 
 const DEFAULT_BUF_SIZE: usize = 8 * 1024;
 
 // ----------------------------------------------------------------------
+
+/// TryClone tries to clone the type and should maintain the `Seek` position of the given

Review comment:
       👍  for comments




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