[RFC PATCH 07/24] erofs: add data abstraction in Rust

[Yiyang Wu <toolmanp@tlmp.cc>]

Introduce Buffer, Source, Backend traits.

Implement Uncompressed Backend and RefBuffer to be
used in future data operations.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/rust/erofs_sys.rs                    |  1 +
 fs/erofs/rust/erofs_sys/data.rs               | 62 +++++++++++++++++++
 fs/erofs/rust/erofs_sys/data/backends.rs      |  4 ++
 .../erofs_sys/data/backends/uncompressed.rs   | 39 ++++++++++++
 4 files changed, 106 insertions(+)
 create mode 100644 fs/erofs/rust/erofs_sys/data.rs
 create mode 100644 fs/erofs/rust/erofs_sys/data/backends.rs
 create mode 100644 fs/erofs/rust/erofs_sys/data/backends/uncompressed.rs

diff --git a/fs/erofs/rust/erofs_sys.rs b/fs/erofs/rust/erofs_sys.rs
index c6fd7f78ac97..8cca2cd9b75f 100644
--- a/fs/erofs/rust/erofs_sys.rs
+++ b/fs/erofs/rust/erofs_sys.rs
@@ -24,6 +24,7 @@
 pub(crate) type PosixResult<T> = Result<T, Errno>;
 
 pub(crate) mod alloc_helper;
+pub(crate) mod data;
 pub(crate) mod errnos;
 pub(crate) mod inode;
 pub(crate) mod superblock;
diff --git a/fs/erofs/rust/erofs_sys/data.rs b/fs/erofs/rust/erofs_sys/data.rs
new file mode 100644
index 000000000000..284c8b1f3bd4
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/data.rs
@@ -0,0 +1,62 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+pub(crate) mod backends;
+use super::*;
+
+/// Represent some sort of generic data source. This cound be file, memory or even network.
+/// Note that users should never use this directly please use backends instead.
+pub(crate) trait Source {
+    fn fill(&self, data: &mut [u8], offset: Off) -> PosixResult<u64>;
+    fn as_buf<'a>(&'a self, offset: Off, len: Off) -> PosixResult<RefBuffer<'a>>;
+}
+
+/// Represents a generic data access backend that is backed by some sort of data source.
+/// This often has temporary buffers to decompress the data from the data source.
+/// The method signatures are the same as those of the Source trait.
+pub(crate) trait Backend {
+    fn fill(&self, data: &mut [u8], offset: Off) -> PosixResult<u64>;
+    fn as_buf<'a>(&'a self, offset: Off, len: Off) -> PosixResult<RefBuffer<'a>>;
+}
+
+/// Represents a buffer trait which can yield its internal reference or be casted as an iterator of
+/// DirEntries.
+pub(crate) trait Buffer {
+    fn content(&self) -> &[u8];
+}
+
+/// Represents a buffer that holds a reference to a slice of data that
+/// is borrowed from the thin air.
+pub(crate) struct RefBuffer<'a> {
+    buf: &'a [u8],
+    start: usize,
+    len: usize,
+    put_buf: fn(*mut core::ffi::c_void),
+}
+
+impl<'a> Buffer for RefBuffer<'a> {
+    fn content(&self) -> &[u8] {
+        &self.buf[self.start..self.start + self.len]
+    }
+}
+
+impl<'a> RefBuffer<'a> {
+    pub(crate) fn new(
+        buf: &'a [u8],
+        start: usize,
+        len: usize,
+        put_buf: fn(*mut core::ffi::c_void),
+    ) -> Self {
+        Self {
+            buf,
+            start,
+            len,
+            put_buf,
+        }
+    }
+}
+
+impl<'a> Drop for RefBuffer<'a> {
+    fn drop(&mut self) {
+        (self.put_buf)(self.buf.as_ptr() as *mut core::ffi::c_void)
+    }
+}
diff --git a/fs/erofs/rust/erofs_sys/data/backends.rs b/fs/erofs/rust/erofs_sys/data/backends.rs
new file mode 100644
index 000000000000..3249f1af8be7
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/data/backends.rs
@@ -0,0 +1,4 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+pub(crate) mod uncompressed;
diff --git a/fs/erofs/rust/erofs_sys/data/backends/uncompressed.rs b/fs/erofs/rust/erofs_sys/data/backends/uncompressed.rs
new file mode 100644
index 000000000000..c1b1a60258f8
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/data/backends/uncompressed.rs
@@ -0,0 +1,39 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+use super::super::*;
+
+pub(crate) struct UncompressedBackend<T>
+where
+    T: Source,
+{
+    source: T,
+}
+
+impl<T> Backend for UncompressedBackend<T>
+where
+    T: Source,
+{
+    fn fill(&self, data: &mut [u8], offset: Off) -> PosixResult<u64> {
+        self.source.fill(data, offset)
+    }
+
+    fn as_buf<'a>(&'a self, offset: Off, len: Off) -> PosixResult<RefBuffer<'a>> {
+        self.source.as_buf(offset, len)
+    }
+}
+
+impl<T: Source> UncompressedBackend<T> {
+    pub(crate) fn new(source: T) -> Self {
+        Self { source }
+    }
+}
+
+impl<T> From<T> for UncompressedBackend<T>
+where
+    T: Source,
+{
+    fn from(value: T) -> Self {
+        Self::new(value)
+    }
+}
-- 
2.46.0