Added support to mmap IPC format

.github/workflows/test.yml

@@ -114,6 +114,25 @@ jobs:
       - name: Run
         run: MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test --tests --features io_ipc,io_json_integration io::ipc::write::write_sliced_list
 
+  miri-checks-mmap:
+    name: MIRI on IO IPC mmaping
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly-2022-07-12
+          override: true
+      - uses: Swatinem/rust-cache@v1
+        with:
+          key: key1
+      - name: Install Miri
+        run: |
+          rustup component add miri
+          cargo miri setup
+      - name: Run
+        run: cargo miri test --tests --features io_ipc io::ipc::mmap
+
   feature-compilation:
     name: Feature coverage
     runs-on: ubuntu-latest

examples/ipc_file_mmap.rs

@@ -0,0 +1,35 @@
+//! Example showing how to memory map an Arrow IPC file into a [`Chunk`].
+use std::sync::Arc;
+
+use arrow2::error::Result;
+use arrow2::io::ipc::read;
+use arrow2::mmap::{mmap_dictionaries_unchecked, mmap_unchecked};
+
+// Arrow2 requires a struct that implements `Clone + AsRef<[u8]>`, which
+// usually `Arc<Mmap>` supports. Here we mock it
+#[derive(Clone)]
+struct Mmap(Arc<Vec<u8>>);
+
+impl AsRef<[u8]> for Mmap {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.0.as_ref()
+    }
+}
+
+fn main() -> Result<()> {
+    // given a mmap
+    let mmap = Mmap(Arc::new(vec![]));
+
+    // read the metadata
+    let metadata = read::read_file_metadata(&mut std::io::Cursor::new(mmap.as_ref()))?;
+
+    // mmap the dictionaries
+    let dictionaries = unsafe { mmap_dictionaries_unchecked(&metadata, mmap.clone())? };
+
+    // and finally mmap a chunk (0 in this case).
+    let chunk = unsafe { mmap_unchecked(&metadata, &dictionaries, mmap, 0) }?;
+
+    println!("{chunk:?}");
+    Ok(())
+}

guide/src/SUMMARY.md

@@ -14,6 +14,7 @@
     - [Read Parquet](./io/parquet_read.md)
     - [Write Parquet](./io/parquet_write.md)
     - [Read Arrow](./io/ipc_read.md)
+    - [Memory map Arrow](./io/ipc_mmap.md)
     - [Read Arrow stream](./io/ipc_stream_read.md)
     - [Write Arrow](./io/ipc_write.md)
     - [Read Avro](./io/avro_read.md)

guide/src/io/ipc_mmap.md

@@ -0,0 +1,10 @@
+# Read Arrow
+
+When compiled with feature `io_ipc`, this crate can be used to memory map IPC Arrow files
+into arrays.
+
+The example below shows how to memory map an IPC Arrow file into `Chunk`es:
+
+```rust
+{{#include ../../../examples/ipc_file_mmap.rs}}
+```

src/array/list/mod.rs

@@ -324,7 +324,7 @@ impl<O: Offset> ListArray<O> {
     /// Returns a the inner [`Field`]
     /// # Errors
     /// Panics iff the logical type is not consistent with this struct.
-    fn try_get_child(data_type: &DataType) -> Result<&Field, Error> {
+    pub fn try_get_child(data_type: &DataType) -> Result<&Field, Error> {
         if O::IS_LARGE {
             match data_type.to_logical_type() {
                 DataType::LargeList(child) => Ok(child.as_ref()),

src/ffi/array.rs

@@ -493,7 +493,7 @@ impl InternalArrowArray {
     }
 }
 
-impl ArrowArrayRef for Box<InternalArrowArray> {
+impl ArrowArrayRef for InternalArrowArray {
     /// the data_type as declared in the schema
     fn data_type(&self) -> &DataType {
         &self.data_type