rust_rocksdb/

ffi_util.rs

// Copyright 2016 Alex Regueiro
//
// Licensed 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 crate::{Error, ffi};
use libc::{self, c_char, c_void, size_t};
use std::ffi::{CStr, CString};
#[cfg(unix)]
use std::os::unix::ffi::OsStrExt;
use std::path::Path;
use std::ptr;

/// Copies `ptr` into a String, replacing invalid UTF-8 using [`String::from_utf8_lossy`], *without*
/// freeing it. Prefer [`from_cstr_and_free`] to make leaks less likely.
pub(crate) unsafe fn from_cstr_without_free(ptr: *const c_char) -> String {
    let cstr = unsafe { CStr::from_ptr(ptr as *const _) };
    String::from_utf8_lossy(cstr.to_bytes()).into_owned()
}

/// Copies `ptr` into a String, replacing invalid UTF-8 using [`String::from_utf8_lossy`], then
/// frees it using `rocksdb_free`.
pub(crate) unsafe fn from_cstr_and_free(ptr: *const c_char) -> String {
    let cstr = unsafe { CStr::from_ptr(ptr as *const _) };
    let s = String::from_utf8_lossy(cstr.to_bytes()).into_owned();
    unsafe { ffi::rocksdb_free(ptr as *mut c_void) };
    s
}

pub(crate) unsafe fn raw_data(ptr: *const c_char, size: usize) -> Option<Vec<u8>> {
    if ptr.is_null() {
        None
    } else {
        // SAFETY: Caller guarantees `ptr` points to `size` bytes; we immediately copy them.
        Some(unsafe { std::slice::from_raw_parts(ptr.cast::<u8>(), size) }.to_vec())
    }
}

/// Copies `size` bytes out of a buffer that the RocksDB C API allocated with
/// `malloc` (see `CopyString` in `rocksdb/db/c.cc`), then releases the original
/// with `rocksdb_free`.
///
/// The copy is not optional. Handing a `malloc`ed pointer to
/// `Vec::from_raw_parts` makes Rust's global allocator responsible for freeing
/// memory it never allocated, which is undefined behaviour and corrupts the
/// heap whenever the two allocators are not the same one: any downstream
/// `#[global_allocator]` (mimalloc, jemallocator, snmalloc), or Windows, where
/// Rust's `System` allocator uses `HeapAlloc`/`HeapFree` and the C runtime
/// does not.
pub(crate) unsafe fn raw_data_and_free(ptr: *mut c_char, size: usize) -> Option<Vec<u8>> {
    if ptr.is_null() {
        return None;
    }
    // A zero-length value is a legitimate RocksDB value. `malloc(0)` still
    // returns a live pointer that has to be freed, but it is not necessarily
    // valid to read from, so skip the copy rather than calling
    // `from_raw_parts` on it.
    let data = if size == 0 {
        Vec::new()
    } else {
        // SAFETY: Caller guarantees `ptr` points to `size` readable bytes.
        unsafe { std::slice::from_raw_parts(ptr.cast::<u8>(), size) }.to_vec()
    };
    // SAFETY: `ptr` came from `malloc` inside the RocksDB C API, so it must be
    // released by the matching `free`, which is what `rocksdb_free` calls.
    unsafe { ffi::rocksdb_free(ptr.cast::<c_void>()) };
    Some(data)
}

/// Convert a RocksDB error message to an Error and frees it. The argument must not be used after
/// this function is called.
pub fn convert_rocksdb_error(rocksdb_err: *const c_char) -> Error {
    let rocksdb_err_str = unsafe { from_cstr_and_free(rocksdb_err) };
    Error::new(rocksdb_err_str)
}

/// Returns a raw pointer to borrowed bytes, or null if None.
///
/// # Safety
/// - The input must outlive the returned pointer.
/// - Common types: `&str`, `&[u8]`, `&String`, `&Vec<u8>`
pub fn opt_bytes_to_ptr<T: AsRef<[u8]> + ?Sized>(opt: Option<&T>) -> *const c_char {
    match opt {
        Some(v) => v.as_ref().as_ptr() as *const c_char,
        None => ptr::null(),
    }
}

#[cfg(unix)]
pub(crate) fn to_cpath<P: AsRef<Path>>(path: P) -> Result<CString, Error> {
    CString::new(path.as_ref().as_os_str().as_bytes())
        .map_err(|e| Error::new(format!("Failed to convert path to CString: {e}")))
}

#[cfg(not(unix))]
pub(crate) fn to_cpath<P: AsRef<Path>>(path: P) -> Result<CString, Error> {
    match CString::new(path.as_ref().to_string_lossy().as_bytes()) {
        Ok(c) => Ok(c),
        Err(e) => Err(Error::new(format!(
            "Failed to convert path to CString: {e}"
        ))),
    }
}

/// Calls a RocksDB C API function that returns an error as a pointer to a C string as the last
/// argument. The C function result is converted into `Result<T, Error>`. This ensures the error
/// message pointer is not leaked. See [`convert_rocksdb_error`] for details.
macro_rules! ffi_try {
    ( $($function:ident)::*() ) => {
        ffi_try_impl!($($function)::*())
    };

    ( $($function:ident)::*( $arg1:expr $(, $arg:expr)* $(,)? ) ) => {
        ffi_try_impl!($($function)::*($arg1 $(, $arg)* ,))
    };
}

macro_rules! ffi_try_impl {
    ( $($function:ident)::*( $($arg:expr,)*) ) => {{
        let mut err: *mut ::libc::c_char = ::std::ptr::null_mut();
        let result = $($function)::*($($arg,)* &mut err);
        if !err.is_null() {
            return Err($crate::ffi_util::convert_rocksdb_error(err));
        }
        result
    }};
}

/// Value which can be converted into a C string.
///
/// The trait is used as argument to functions which wish to accept either
/// [`&str`] or [`&CStr`](CStr) arguments while internally need to interact with
/// C APIs.  Accepting [`&str`] may be more convenient for users but requires
/// conversion into [`CString`] internally which requires allocation.  With this
/// trait, latency-conscious users may choose to prepare [`CStr`] in advance and
/// then pass it directly without having to incur the conversion cost.
///
/// To use the trait, function should accept `impl CStrLike` and after baking
/// the argument (with [`CStrLike::bake`] method) it can use it as a [`&CStr`](CStr)
/// (since the baked result dereferences into [`CStr`]).
///
/// # Example
///
/// ```
/// use std::ffi::{CStr, CString};
/// use rust_rocksdb::CStrLike;
///
/// fn strlen(arg: impl CStrLike) -> std::result::Result<usize, String> {
///     let baked = arg.bake().map_err(|err| err.to_string())?;
///     Ok(unsafe { libc::strlen(baked.as_ptr()) })
/// }
///
/// const FOO: &str = "foo";
/// const BAR: &CStr = unsafe { CStr::from_bytes_with_nul_unchecked(b"bar\0") };
///
/// assert_eq!(Ok(3), strlen(FOO));
/// assert_eq!(Ok(3), strlen(BAR));
/// ```
pub trait CStrLike {
    type Baked: std::ops::Deref<Target = CStr>;
    type Error: std::fmt::Debug + std::fmt::Display;

    /// Bakes self into value which can be freely converted into [`&CStr`](CStr).
    ///
    /// This may require allocation and may fail if `self` has invalid value.
    fn bake(self) -> Result<Self::Baked, Self::Error>;

    /// Consumers and converts value into an owned [`CString`].
    ///
    /// If `Self` is already a `CString` simply returns it; if it’s a reference
    /// to a `CString` then the value is cloned.  In other cases this may
    /// require allocation and may fail if `self` has invalid value.
    fn into_c_string(self) -> Result<CString, Self::Error>;
}

impl CStrLike for &str {
    type Baked = CString;
    type Error = std::ffi::NulError;

    fn bake(self) -> Result<Self::Baked, Self::Error> {
        CString::new(self)
    }
    fn into_c_string(self) -> Result<CString, Self::Error> {
        CString::new(self)
    }
}

// This is redundant for the most part and exists so that `foo(&string)` (where
// `string: String` works just as if `foo` took `arg: &str` argument.
impl CStrLike for &String {
    type Baked = CString;
    type Error = std::ffi::NulError;

    fn bake(self) -> Result<Self::Baked, Self::Error> {
        CString::new(self.as_bytes())
    }
    fn into_c_string(self) -> Result<CString, Self::Error> {
        CString::new(self.as_bytes())
    }
}

impl CStrLike for &CStr {
    type Baked = Self;
    type Error = std::convert::Infallible;

    fn bake(self) -> Result<Self::Baked, Self::Error> {
        Ok(self)
    }
    fn into_c_string(self) -> Result<CString, Self::Error> {
        Ok(self.to_owned())
    }
}

// This exists so that if caller constructs a `CString` they can pass it into
// the function accepting `CStrLike` argument.  Some of such functions may take
// the argument whereas otherwise they would need to allocated a new owned
// object.
impl CStrLike for CString {
    type Baked = CString;
    type Error = std::convert::Infallible;

    fn bake(self) -> Result<Self::Baked, Self::Error> {
        Ok(self)
    }
    fn into_c_string(self) -> Result<CString, Self::Error> {
        Ok(self)
    }
}

// This is redundant for the most part and exists so that `foo(&cstring)` (where
// `string: CString` works just as if `foo` took `arg: &CStr` argument.
impl<'a> CStrLike for &'a CString {
    type Baked = &'a CStr;
    type Error = std::convert::Infallible;

    fn bake(self) -> Result<Self::Baked, Self::Error> {
        Ok(self)
    }
    fn into_c_string(self) -> Result<CString, Self::Error> {
        Ok(self.clone())
    }
}

/// Owned malloc-allocated memory slice.
/// Do not derive `Clone` for this because it will cause double-free.
pub struct CSlice {
    data: *const c_char,
    len: size_t,
}

impl CSlice {
    /// Constructing such a slice may be unsafe.
    ///
    /// # Safety
    /// The caller must ensure that the pointer and length are valid.
    /// Moreover, `CSlice` takes the ownership of the memory and will free it
    /// using `rocksdb_free`. The caller must ensure that the memory is
    /// allocated by `malloc` in RocksDB and will not be freed by any other
    /// means.
    pub(crate) unsafe fn from_raw_parts(data: *const c_char, len: size_t) -> Self {
        Self { data, len }
    }
}

impl AsRef<[u8]> for CSlice {
    #[inline]
    fn as_ref(&self) -> &[u8] {
        if self.len == 0 {
            // `CopyString` returns `malloc(0)` for an empty value, and
            // `malloc(0)` is allowed to return a pointer that must not be
            // dereferenced. `slice::from_raw_parts(ptr, 0)` requires a
            // dereferenceable pointer, so short-circuit instead.
            return &[];
        }
        unsafe { std::slice::from_raw_parts(self.data.cast::<u8>(), self.len) }
    }
}

impl Drop for CSlice {
    fn drop(&mut self) {
        unsafe {
            ffi::rocksdb_free(self.data as *mut c_void);
        }
    }
}

#[test]
fn test_c_str_like_bake() {
    fn test<S: CStrLike>(value: S) -> Result<usize, S::Error> {
        value.bake().map(|value| value.count_bytes())
    }

    assert_eq!(Ok(3), test("foo")); // &str
    assert_eq!(Ok(3), test(&String::from("foo"))); // String
    assert_eq!(Ok(3), test(CString::new("foo").unwrap().as_ref())); // &CStr
    assert_eq!(Ok(3), test(&CString::new("foo").unwrap())); // &CString
    assert_eq!(Ok(3), test(CString::new("foo").unwrap())); // CString

    assert_eq!(3, test("foo\0bar").err().unwrap().nul_position());
}

#[test]
fn test_c_str_like_into() {
    fn test<S: CStrLike>(value: S) -> Result<CString, S::Error> {
        value.into_c_string()
    }

    let want = CString::new("foo").unwrap();

    assert_eq!(Ok(want.clone()), test("foo")); // &str
    assert_eq!(Ok(want.clone()), test(&String::from("foo"))); // &String
    assert_eq!(
        Ok(want.clone()),
        test(CString::new("foo").unwrap().as_ref())
    ); // &CStr
    assert_eq!(Ok(want.clone()), test(&CString::new("foo").unwrap())); // &CString
    assert_eq!(Ok(want), test(CString::new("foo").unwrap())); // CString

    assert_eq!(3, test("foo\0bar").err().unwrap().nul_position());
}