[RFC PATCH 06/24] erofs: add alloc_helper in Rust

[Yiyang Wu <toolmanp@tlmp.cc>]

In normal rust, heap related operations are infallible meaning
that they do not throw errors and Rust will panic in usermode instead.
However in kernel, it will throw AllocError this module helps to
bridge the gaps and returns Errno universally.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/rust/erofs_sys.rs              |  1 +
 fs/erofs/rust/erofs_sys/alloc_helper.rs | 35 +++++++++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 fs/erofs/rust/erofs_sys/alloc_helper.rs

diff --git a/fs/erofs/rust/erofs_sys.rs b/fs/erofs/rust/erofs_sys.rs
index 34267ec7772d..c6fd7f78ac97 100644
--- a/fs/erofs/rust/erofs_sys.rs
+++ b/fs/erofs/rust/erofs_sys.rs
@@ -23,6 +23,7 @@
 /// to avoid naming conflicts.
 pub(crate) type PosixResult<T> = Result<T, Errno>;
 
+pub(crate) mod alloc_helper;
 pub(crate) mod errnos;
 pub(crate) mod inode;
 pub(crate) mod superblock;
diff --git a/fs/erofs/rust/erofs_sys/alloc_helper.rs b/fs/erofs/rust/erofs_sys/alloc_helper.rs
new file mode 100644
index 000000000000..05ef2018d379
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/alloc_helper.rs
@@ -0,0 +1,35 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+/// This module provides helper functions for the alloc crate
+/// Note that in linux kernel, the allocation is fallible however in userland it is not.
+/// Since most of the functions depend on infallible allocation, here we provide helper functions
+/// so that most of codes don't need to be changed.
+
+#[cfg(CONFIG_EROFS_FS = "y")]
+use kernel::prelude::*;
+
+#[cfg(not(CONFIG_EROFS_FS = "y"))]
+use alloc::vec;
+
+use super::*;
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+
+pub(crate) fn push_vec<T>(v: &mut Vec<T>, value: T) -> PosixResult<()> {
+    v.push(value, GFP_KERNEL)
+        .map_or_else(|_| Err(Errno::ENOMEM), |_| Ok(()))
+}
+
+pub(crate) fn extend_from_slice<T: Clone>(v: &mut Vec<T>, slice: &[T]) -> PosixResult<()> {
+    v.extend_from_slice(slice, GFP_KERNEL)
+        .map_or_else(|_| Err(Errno::ENOMEM), |_| Ok(()))
+}
+
+pub(crate) fn heap_alloc<T>(value: T) -> PosixResult<Box<T>> {
+    Box::new(value, GFP_KERNEL).map_or_else(|_| Err(Errno::ENOMEM), |v| Ok(v))
+}
+
+pub(crate) fn vec_with_capacity<T: Default + Clone>(capacity: usize) -> PosixResult<Vec<T>> {
+    Vec::with_capacity(capacity, GFP_KERNEL).map_or_else(|_| Err(Errno::ENOMEM), |v| Ok(v))
+}
-- 
2.46.0