[GitHub] [arrow] pitrou commented on a change in pull request #8316: ARROW-10149: [Rust] Improved support for externally owned memory regions

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

pitrou commented on a change in pull request #8316:
URL: https://github.com/apache/arrow/pull/8316#discussion_r498885291



##########
File path: rust/arrow/src/bytes.rs
##########
@@ -0,0 +1,185 @@
+// 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.
+
+//! This module contains an implementation of a contiguous immutable memory region that knows
+//! how to de-allocate itself, [`Bytes`].
+//! Note that this is a low-level functionality of this crate, and is only required to be used
+//! when implementing FFI.
+
+use core::slice;
+use std::{fmt::Debug, fmt::Formatter, sync::Arc};
+
+use crate::memory;
+
+/// function resposible for de-allocating `Bytes`.
+pub type DropFn = Arc<dyn Fn(&mut Bytes)>;
+
+/// Mode of deallocating memory regions
+pub enum Deallocation {
+    /// Native deallocation, using Rust deallocator with Arrow-specific memory aligment
+    Native(usize),
+    /// Foreign deallocation, using some other form of memory deallocation
+    Foreign(DropFn),
+}
+
+impl Debug for Deallocation {
+    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
+        match self {
+            Deallocation::Native(capacity) => {
+                write!(f, "Deallocation::Native {{ capacity: {} }}", capacity)
+            }
+            Deallocation::Foreign(_) => {
+                write!(f, "Deallocation::Foreign {{ capacity: unknown }}")
+            }
+        }
+    }
+}
+
+/// A continuous, fixed-size, immutable memory region that knows how to de-allocate itself.
+/// This structs' API is inspired by the `bytes::Bytes`, but it is not limited to using rust's
+/// global allocator nor u8 aligmnent.
+///
+/// In the most common case, this buffer is allocated using [`allocate_aligned`](memory::allocate_aligned)
+/// and deallocated accordingly [`free_aligned`](memory::free_aligned).
+/// When the region is allocated by an foreign allocator, [Deallocation::Foreign], this calls the
+/// foreign deallocator to deallocate the region when it is no longer needed.
+pub struct Bytes {
+    /// The raw pointer to be begining of the region
+    ptr: *const u8,
+
+    /// The number of bytes visible to this region. This is always smaller than its capacity (when avaliable).
+    len: usize,
+
+    /// how to deallocate this region
+    deallocation: Deallocation,
+}
+
+impl Bytes {
+    /// Takes ownership of an allocated memory region,
+    ///
+    /// # Arguments
+    ///
+    /// * `ptr` - Pointer to raw parts
+    /// * `len` - Length of raw parts in **bytes**
+    /// * `capacity` - Total allocated memory for the pointer `ptr`, in **bytes**
+    ///
+    /// # Safety
+    ///
+    /// This function is unsafe as there is no guarantee that the given pointer is valid for `len`
+    /// bytes. If the `ptr` and `capacity` come from a `Buffer`, then this is guaranteed.
+    pub unsafe fn new(ptr: *const u8, len: usize, deallocation: Deallocation) -> Bytes {
+        Bytes {
+            ptr,
+            len,
+            deallocation,
+        }
+    }
+
+    #[inline]
+    pub fn as_slice(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.ptr, self.len) }
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
+    #[inline]
+    pub fn raw_data(&self) -> *const u8 {
+        self.ptr
+    }
+
+    #[inline]
+    pub fn raw_data_mut(&mut self) -> *mut u8 {
+        self.ptr as *mut u8
+    }
+
+    pub fn capacity(&self) -> usize {
+        match self.deallocation {
+            Deallocation::Native(capacity) => capacity,
+            // we cannot determine this in general,
+            // and thus we state that this is externally-owned memory
+            Deallocation::Foreign(_) => 0,
+        }
+    }
+}
+
+impl Drop for Bytes {
+    #[inline]
+    fn drop(&mut self) {
+        match &self.deallocation {
+            Deallocation::Native(capacity) => {
+                if !self.ptr.is_null() {
+                    unsafe { memory::free_aligned(self.ptr as *mut u8, *capacity) };
+                }
+            }
+            Deallocation::Foreign(drop) => {
+                (drop.clone())(self);
+            }
+        }
+    }
+}
+
+impl PartialEq for Bytes {
+    fn eq(&self, other: &Bytes) -> bool {
+        self.as_slice() == other.as_slice()
+    }
+}
+
+impl Debug for Bytes {
+    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
+        write!(f, "Bytes {{ ptr: {:?}, len: {}, data: ", self.ptr, self.len,)?;
+
+        f.debug_list().entries(self.as_slice().iter()).finish()?;
+
+        write!(f, " }}")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_dealloc() {
+        let a = Box::new(b"hello");
+
+        let dealloc = Arc::new(|bytes: &mut Bytes| {
+            // println!(""); seems to be the only way to validate that this is actually called

Review comment:
       Perhaps using a closure that mutates a captured variable?
   Apparently you need to use either `FnMut` or a `Cell`:
   https://stackoverflow.com/questions/38677736/passing-a-closure-that-modifies-its-environment-to-a-function-in-rust




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