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Simplify some filesystem extensions on Windows
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This commit it borne out of CI failures on bytecodealliance/wasmtime#7638.
Investigating this failure has revealed a number of aspects here which
I've attempted to address in this PR. The notable changes here are:

* The current code in this crate was handling the case where
  `FileExt::seek_write` on Windows was leaving intermediate bytes as
  undefined when a write happened beyond the end of a file. I believe
  that this is due to an error in the documentation of the Rust standard
  library which I've submitted rust-lang/rust#120452 to fix.

* Removing handling of "always write zeros" handles the primary failure
  of the PR bytecodealliance/wasmtime#7638 which is that
  `write_vectored_at` was always returning 0 on Windows for writes past
  the end of the file. This is because Windows doesn't have a vectored
  file write so the vector chosen was the first nonempty vector which
  was the one containing zeros to extend the file. That meant that the
  method always returned zero.

* Previously the methods here used file locking which appeared to handle
  the case where the file was calculated and then the write happened.
  Given that this no longer happens I've removed the locking here.

* The `write_all_at` method had a loop around `reopen_write` handling
  the `Interrupted` error but no other methods did, so I opted to remove
  the loop and leave that to the internals of `reopen_write` if
  necessary.

* Other methods related to this are all simplified to directly use
  `seek_write` and avoid handling the case where writes past the end
  need to write zeros (as zeros are guaranteed by Windows).

Overall my hope is to use this to unblock bytecodealliance/wasmtime#7638
to get more platform-agnostic behavior for writing beyond the end of a
file.
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@alexcrichton
alexcrichton committed Jan 28, 2024
1 parent 0c57561 commit 480f135
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Showing 2 changed files with 20 additions and 126 deletions.
145 changes: 19 additions & 126 deletions src/fs/file_io_ext.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,7 @@ use rustix::io::{preadv, pwritev};
use std::io::{self, IoSlice, IoSliceMut, Seek, SeekFrom};
use std::slice;
#[cfg(windows)]
use {cap_fs_ext::Reopen, fd_lock::RwLock, std::fs, std::os::windows::fs::FileExt};
use {cap_fs_ext::Reopen, std::fs, std::os::windows::fs::FileExt};
#[cfg(not(windows))]
use {rustix::fs::tell, rustix::fs::FileExt};

Expand Down Expand Up @@ -703,144 +703,37 @@ impl FileIoExt for std::fs::File {
fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
// Windows' `seek_write` modifies the current position in the file, so
// re-open the file to leave the original open file unmodified.
//
// We take a lock so that we can test for writing past the end of the
// file and implement writing zeros if the offset is past the end.
//
// Windows documentation [says]:
//
// > A write operation increases the size of the file to the file
// > pointer position plus the size of the buffer written, which results
// > in the intervening bytes uninitialized.
//
// [says]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfilepointer#remarks
//
// However, our desired behavior is to fill the intervening bytes with
// zeros, so we do it ourselves.
let reopened = reopen_write(self)?;
let mut reopened = RwLock::new(reopened);
let reopened = reopened.write()?;
let mut seek_offset = offset;
let mut write_buf = buf;
let mut prepend_zeros;
let reopened_size = reopened.metadata()?.len();
let num_zeros = offset.saturating_sub(reopened_size);
let num_zeros: usize = num_zeros
.try_into()
.map_err(|_| io::Error::new(io::ErrorKind::OutOfMemory, "write_all_vectored_at"))?;
if num_zeros > 0 {
prepend_zeros = vec![0_u8; num_zeros];
prepend_zeros.extend_from_slice(buf);
seek_offset = reopened_size;
write_buf = &prepend_zeros;
}
let num_written = reopened
.seek_write(write_buf, seek_offset)?
.saturating_sub(num_zeros);
Ok(num_written)
reopen_write(self)?.seek_write(buf, offset)
}

#[inline]
fn write_all_at(&self, buf: &[u8], offset: u64) -> io::Result<()> {
// Similar to `read_exact_at`, re-open the file so that we can do a seek and
// leave the original file unmodified.
//
// Similar to `write_at`, we take a lock to do this.
let reopened = loop {
match reopen_write(self) {
Ok(file) => break file,
Err(err) if err.kind() == io::ErrorKind::Interrupted => continue,
Err(err) => return Err(err),
}
};
let mut reopened = RwLock::new(reopened);
let reopened = reopened.write()?;
let mut seek_offset = offset;
let mut write_buf = buf;
let mut prepend_zeros;
let reopened_size = reopened.metadata()?.len();
let num_zeros = offset.saturating_sub(reopened_size);
let num_zeros: usize = num_zeros
.try_into()
.map_err(|_| io::Error::new(io::ErrorKind::OutOfMemory, "write_all_vectored_at"))?;
if num_zeros > 0 {
prepend_zeros = vec![0_u8; num_zeros];
prepend_zeros.extend_from_slice(buf);
seek_offset = reopened_size;
write_buf = &prepend_zeros;
}
loop {
match reopened.seek(SeekFrom::Start(seek_offset)) {
Ok(_) => break,
Err(err) if err.kind() == io::ErrorKind::Interrupted => continue,
Err(err) => return Err(err),
}
fn write_all_at(&self, mut buf: &[u8], mut offset: u64) -> io::Result<()> {
// Similar to `read_exact_at`, re-open the file so that we can do a seek
// and leave the original file unmodified.
let reopened = reopen_write(self)?;
while buf.len() > 0 {
let n = reopened.seek_write(buf, offset)?;
offset += u64::try_from(n).unwrap();
buf = &buf[n..];
}
reopened.write_all(write_buf)?;
Ok(())
}

#[inline]
fn write_vectored_at(&self, bufs: &[IoSlice], offset: u64) -> io::Result<usize> {
// Similar to `read_vectored_at`, re-open the file to avoid adjusting
// the current position of the already-open file.
//
// Similar to `write_at`, we take a lock to do this.
let reopened = reopen_write(self)?;
let mut reopened = RwLock::new(reopened);
let reopened = reopened.write()?;
let mut seek_offset = offset;
let mut write_bufs = bufs;
let zeros;
let mut prepend_zeros;
let reopened_size = reopened.metadata()?.len();
let num_zeros = offset.saturating_sub(reopened_size);
let num_zeros: usize = num_zeros
.try_into()
.map_err(|_| io::Error::new(io::ErrorKind::OutOfMemory, "write_vectored_at"))?;
if num_zeros > 0 {
zeros = vec![0_u8; num_zeros];
prepend_zeros = vec![IoSlice::new(&zeros)];
prepend_zeros.extend_from_slice(bufs);
seek_offset = reopened_size;
write_bufs = &prepend_zeros;
// Windows doesn't have a vectored write for files, so pick the first
// non-empty slice and write that.
match bufs.iter().find(|p| p.len() > 0) {
Some(buf) => self.write_at(buf, offset),
None => Ok(0),
}
reopened.seek(SeekFrom::Start(seek_offset))?;
let num_written = reopened
.write_vectored(write_bufs)?
.saturating_sub(num_zeros);
Ok(num_written)
}

#[inline]
fn write_all_vectored_at(&self, bufs: &mut [IoSlice], offset: u64) -> io::Result<()> {
// Similar to `read_vectored_at`, re-open the file to avoid adjusting
// the current position of the already-open file.
//
// Similar to `write_at`, we take a lock to do this.
let reopened = loop {
match reopen_write(self) {
Ok(file) => break file,
Err(err) if err.kind() == io::ErrorKind::Interrupted => continue,
Err(err) => return Err(err),
}
};
let mut reopened = RwLock::new(reopened);
let reopened = reopened.write()?;
let reopened_size = reopened.metadata()?.len();
let num_zeros = offset.saturating_sub(reopened_size);
let num_zeros: usize = num_zeros
.try_into()
.map_err(|_| io::Error::new(io::ErrorKind::OutOfMemory, "write_vectored_at"))?;
if num_zeros > 0 {
let zeros = vec![0_u8; num_zeros];
let mut prepend_zeros = vec![IoSlice::new(&zeros)];
prepend_zeros.extend_from_slice(bufs);
reopened.seek(SeekFrom::Start(reopened_size))?;
reopened.write_all_vectored(&mut prepend_zeros)?;
} else {
reopened.seek(SeekFrom::Start(offset))?;
reopened.write_all_vectored(bufs)?;
fn write_all_vectored_at(&self, bufs: &mut [IoSlice], mut offset: u64) -> io::Result<()> {
for buf in bufs {
self.write_all_at(buf, offset)?;
offset += u64::try_from(buf.len()).unwrap();
}
Ok(())
}
Expand Down
1 change: 1 addition & 0 deletions tests/vectored_at.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -412,6 +412,7 @@ fn write_vectored_after_end() {
let buf1 = b"MNOPQRST".to_vec();
let bufs = vec![IoSlice::new(&buf0), IoSlice::new(&buf1)];
let nwritten = check!(file.write_vectored_at(&bufs, 32));
assert!(nwritten > 0);
assert_eq!(check!(file.stream_position()), 26);
let mut back = String::new();
check!(file.seek(std::io::SeekFrom::Start(0)));
Expand Down

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