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use crate::Mmap;
use anyhow::{Context, Result};
use std::fs::File;
use std::ops::{Deref, DerefMut, Range};
use std::path::Path;
use std::sync::Arc;
/// A type akin to `Vec<u8>`, but backed by `mmap` and able to be split.
///
/// This type is a non-growable owned list of bytes. It can be segmented into
/// disjoint separately owned views akin to the `split_at` method on slices in
/// Rust. An `MmapVec` is backed by an OS-level memory allocation and is not
/// suitable for lots of small allocation (since it works at the page
/// granularity).
///
/// An `MmapVec` is an owned value which means that owners have the ability to
/// get exclusive access to the underlying bytes, enabling mutation.
pub struct MmapVec {
mmap: Arc<Mmap>,
range: Range<usize>,
}
impl MmapVec {
/// Consumes an existing `mmap` and wraps it up into an `MmapVec`.
///
/// The returned `MmapVec` will have the `size` specified, which can be
/// smaller than the region mapped by the `Mmap`. The returned `MmapVec`
/// will only have at most `size` bytes accessible.
pub fn new(mmap: Mmap, size: usize) -> MmapVec {
assert!(size <= mmap.len());
MmapVec {
mmap: Arc::new(mmap),
range: 0..size,
}
}
/// Creates a new zero-initialized `MmapVec` with the given `size`.
///
/// This commit will return a new `MmapVec` suitably sized to hold `size`
/// bytes. All bytes will be initialized to zero since this is a fresh OS
/// page allocation.
pub fn with_capacity(size: usize) -> Result<MmapVec> {
Ok(MmapVec::new(Mmap::with_at_least(size)?, size))
}
/// Creates a new `MmapVec` from the contents of an existing `slice`.
///
/// A new `MmapVec` is allocated to hold the contents of `slice` and then
/// `slice` is copied into the new mmap. It's recommended to avoid this
/// method if possible to avoid the need to copy data around.
pub fn from_slice(slice: &[u8]) -> Result<MmapVec> {
let mut result = MmapVec::with_capacity(slice.len())?;
result.copy_from_slice(slice);
Ok(result)
}
/// Creates a new `MmapVec` which is the `path` specified mmap'd into
/// memory.
///
/// This function will attempt to open the file located at `path` and will
/// then use that file to learn about its size and map the full contents
/// into memory. This will return an error if the file doesn't exist or if
/// it's too large to be fully mapped into memory.
pub fn from_file(path: &Path) -> Result<MmapVec> {
let mmap = Mmap::from_file(path)
.with_context(|| format!("failed to create mmap for file: {}", path.display()))?;
let len = mmap.len();
Ok(MmapVec::new(mmap, len))
}
/// Returns whether the original mmap was created from a readonly mapping.
pub fn is_readonly(&self) -> bool {
self.mmap.is_readonly()
}
/// Splits the collection into two at the given index.
///
/// Returns a separate `MmapVec` which shares the underlying mapping, but
/// only has access to elements in the range `[at, len)`. After the call,
/// the original `MmapVec` will be left with access to the elements in the
/// range `[0, at)`.
///
/// This is an `O(1)` operation which does not involve copies.
pub fn split_off(&mut self, at: usize) -> MmapVec {
assert!(at <= self.range.len());
// Create a new `MmapVec` which refers to the same underlying mmap, but
// has a disjoint range from ours. Our own range is adjusted to be
// disjoint just after `ret` is created.
let ret = MmapVec {
mmap: self.mmap.clone(),
range: at..self.range.end,
};
self.range.end = self.range.start + at;
return ret;
}
/// Makes the specified `range` within this `mmap` to be read/write.
pub unsafe fn make_writable(&self, range: Range<usize>) -> Result<()> {
self.mmap
.make_writable(range.start + self.range.start..range.end + self.range.start)
}
/// Makes the specified `range` within this `mmap` to be read/execute.
pub unsafe fn make_executable(
&self,
range: Range<usize>,
enable_branch_protection: bool,
) -> Result<()> {
self.mmap.make_executable(
range.start + self.range.start..range.end + self.range.start,
enable_branch_protection,
)
}
/// Returns the underlying file that this mmap is mapping, if present.
pub fn original_file(&self) -> Option<&Arc<File>> {
self.mmap.original_file()
}
/// Returns the offset within the original mmap that this `MmapVec` is
/// created from.
pub fn original_offset(&self) -> usize {
self.range.start
}
}
impl Deref for MmapVec {
type Target = [u8];
fn deref(&self) -> &[u8] {
&self.mmap.as_slice()[self.range.clone()]
}
}
impl DerefMut for MmapVec {
fn deref_mut(&mut self) -> &mut [u8] {
debug_assert!(!self.is_readonly());
// SAFETY: The underlying mmap is protected behind an `Arc` which means
// there there can be many references to it. We are guaranteed, though,
// that each reference to the underlying `mmap` has a disjoint `range`
// listed that it can access. This means that despite having shared
// access to the mmap itself we have exclusive ownership of the bytes
// specified in `self.range`. This should allow us to safely hand out
// mutable access to these bytes if so desired.
unsafe {
let slice = std::slice::from_raw_parts_mut(self.mmap.as_mut_ptr(), self.mmap.len());
&mut slice[self.range.clone()]
}
}
}
#[cfg(test)]
mod tests {
use super::MmapVec;
#[test]
fn smoke() {
let mut mmap = MmapVec::with_capacity(10).unwrap();
assert_eq!(mmap.len(), 10);
assert_eq!(&mmap[..], &[0; 10]);
mmap[0] = 1;
mmap[2] = 3;
assert!(mmap.get(10).is_none());
assert_eq!(mmap[0], 1);
assert_eq!(mmap[2], 3);
}
#[test]
fn split_off() {
let mut vec = Vec::from([1, 2, 3, 4]);
let mut mmap = MmapVec::from_slice(&vec).unwrap();
assert_eq!(&mmap[..], &vec[..]);
// remove nothing; vec length remains 4
assert_eq!(&mmap.split_off(4)[..], &vec.split_off(4)[..]);
assert_eq!(&mmap[..], &vec[..]);
// remove 1 element; vec length is now 3
assert_eq!(&mmap.split_off(3)[..], &vec.split_off(3)[..]);
assert_eq!(&mmap[..], &vec[..]);
// remove 2 elements; vec length is now 1
assert_eq!(&mmap.split_off(1)[..], &vec.split_off(1)[..]);
assert_eq!(&mmap[..], &vec[..]);
// remove last element; vec length is now 0
assert_eq!(&mmap.split_off(0)[..], &vec.split_off(0)[..]);
assert_eq!(&mmap[..], &vec[..]);
// nothing left to remove, but that's okay
assert_eq!(&mmap.split_off(0)[..], &vec.split_off(0)[..]);
assert_eq!(&mmap[..], &vec[..]);
}
}