1 //! This module contains specializations that can offload `io::copy()` operations on file descriptor
2 //! containing types (`File`, `TcpStream`, etc.) to more efficient syscalls than `read(2)` and `write(2)`.
4 //! Specialization is only applied to wholly std-owned types so that user code can't observe
5 //! that the `Read` and `Write` traits are not used.
7 //! Since a copy operation involves a reader and writer side where each can consist of different types
8 //! and also involve generic wrappers (e.g. `Take`, `BufReader`) it is not practical to specialize
9 //! a single method on all possible combinations.
11 //! Instead readers and writers are handled separately by the `CopyRead` and `CopyWrite` specialization
12 //! traits and then specialized on by the `Copier::copy` method.
14 //! `Copier` uses the specialization traits to unpack the underlying file descriptors and
15 //! additional prerequisites and constraints imposed by the wrapper types.
17 //! Once it has obtained all necessary pieces and brought any wrapper types into a state where they
18 //! can be safely bypassed it will attempt to use the `copy_file_range(2)`,
19 //! `sendfile(2)` or `splice(2)` syscalls to move data directly between file descriptors.
20 //! Since those syscalls have requirements that cannot be fully checked in advance and
21 //! gathering additional information about file descriptors would require additional syscalls
22 //! anyway it simply attempts to use them one after another (guided by inaccurate hints) to
23 //! figure out which one works and falls back to the generic read-write copy loop if none of them
25 //! Once a working syscall is found for a pair of file descriptors it will be called in a loop
26 //! until the copy operation is completed.
28 //! Advantages of using these syscalls:
30 //! * fewer context switches since reads and writes are coalesced into a single syscall
31 //! and more bytes are transferred per syscall. This translates to higher throughput
32 //! and fewer CPU cycles, at least for sufficiently large transfers to amortize the initial probing.
33 //! * `copy_file_range` creates reflink copies on CoW filesystems, thus moving less data and
34 //! consuming less disk space
35 //! * `sendfile` and `splice` can perform zero-copy IO under some circumstances while
36 //! a naive copy loop would move every byte through the CPU.
40 //! * copy operations smaller than the default buffer size can under some circumstances, especially
41 //! on older kernels, incur more syscalls than the naive approach would. As mentioned above
42 //! the syscall selection is guided by hints to minimize this possibility but they are not perfect.
43 //! * optimizations only apply to std types. If a user adds a custom wrapper type, e.g. to report
44 //! progress, they can hit a performance cliff.
48 use crate::fs::{File, Metadata};
49 use crate::io::copy::generic_copy;
51 BufRead, BufReader, BufWriter, Error, Read, Result, StderrLock, StdinLock, StdoutLock, Take,
54 use crate::mem::ManuallyDrop;
55 use crate::net::TcpStream;
56 use crate::os::unix::fs::FileTypeExt;
57 use crate::os::unix::io::{AsRawFd, FromRawFd, RawFd};
58 use crate::os::unix::net::UnixStream;
59 use crate::process::{ChildStderr, ChildStdin, ChildStdout};
61 use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
63 use crate::sys::weak::syscall;
64 use libc::{EBADF, EINVAL, ENOSYS, EOPNOTSUPP, EOVERFLOW, EPERM, EXDEV};
69 pub(crate) fn copy_spec<R: Read + ?Sized, W: Write + ?Sized>(
73 let copier = Copier { read, write };
74 SpecCopy::copy(copier)
77 /// This type represents either the inferred `FileType` of a `RawFd` based on the source
78 /// type from which it was extracted or the actual metadata
80 /// The methods on this type only provide hints, due to `AsRawFd` and `FromRawFd` the inferred
81 /// type may be wrong.
83 /// We obtained the FD from a type that can contain any type of `FileType` and queried the metadata
84 /// because it is cheaper than probing all possible syscalls (reader side)
88 /// We don't have any metadata, e.g. because the original type was `File` which can represent
89 /// any `FileType` and we did not query the metadata either since it did not seem beneficial
95 fn maybe_fifo(&self) -> bool {
97 FdMeta::Metadata(meta) => meta.file_type().is_fifo(),
98 FdMeta::Socket => false,
100 FdMeta::NoneObtained => true,
104 fn potential_sendfile_source(&self) -> bool {
106 // procfs erroneously shows 0 length on non-empty readable files.
107 // and if a file is truly empty then a `read` syscall will determine that and skip the write syscall
108 // thus there would be benefit from attempting sendfile
109 FdMeta::Metadata(meta)
110 if meta.file_type().is_file() && meta.len() > 0
111 || meta.file_type().is_block_device() =>
119 fn copy_file_range_candidate(&self) -> bool {
121 // copy_file_range will fail on empty procfs files. `read` can determine whether EOF has been reached
122 // without extra cost and skip the write, thus there is no benefit in attempting copy_file_range
123 FdMeta::Metadata(meta) if meta.is_file() && meta.len() > 0 => true,
124 FdMeta::NoneObtained => true,
130 struct CopyParams(FdMeta, Option<RawFd>);
132 struct Copier<'a, 'b, R: Read + ?Sized, W: Write + ?Sized> {
138 fn copy(self) -> Result<u64>;
141 impl<R: Read + ?Sized, W: Write + ?Sized> SpecCopy for Copier<'_, '_, R, W> {
142 default fn copy(self) -> Result<u64> {
143 generic_copy(self.read, self.write)
147 impl<R: CopyRead, W: CopyWrite> SpecCopy for Copier<'_, '_, R, W> {
148 fn copy(self) -> Result<u64> {
149 let (reader, writer) = (self.read, self.write);
150 let r_cfg = reader.properties();
151 let w_cfg = writer.properties();
153 // before direct operations on file descriptors ensure that all source and sink buffers are empty
154 let mut flush = || -> crate::io::Result<u64> {
155 let bytes = reader.drain_to(writer, u64::MAX)?;
156 // BufWriter buffered bytes have already been accounted for in earlier write() calls
161 let mut written = 0u64;
163 if let (CopyParams(input_meta, Some(readfd)), CopyParams(output_meta, Some(writefd))) =
167 let max_write = reader.min_limit();
169 if input_meta.copy_file_range_candidate() && output_meta.copy_file_range_candidate() {
170 let result = copy_regular_files(readfd, writefd, max_write);
171 result.update_take(reader);
174 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
175 CopyResult::Error(e, _) => return Err(e),
176 CopyResult::Fallback(bytes) => written += bytes,
180 // on modern kernels sendfile can copy from any mmapable type (some but not all regular files and block devices)
181 // to any writable file descriptor. On older kernels the writer side can only be a socket.
182 // So we just try and fallback if needed.
183 // If current file offsets + write sizes overflow it may also fail, we do not try to fix that and instead
184 // fall back to the generic copy loop.
185 if input_meta.potential_sendfile_source() {
186 let result = sendfile_splice(SpliceMode::Sendfile, readfd, writefd, max_write);
187 result.update_take(reader);
190 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
191 CopyResult::Error(e, _) => return Err(e),
192 CopyResult::Fallback(bytes) => written += bytes,
196 if input_meta.maybe_fifo() || output_meta.maybe_fifo() {
197 let result = sendfile_splice(SpliceMode::Splice, readfd, writefd, max_write);
198 result.update_take(reader);
201 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
202 CopyResult::Error(e, _) => return Err(e),
203 CopyResult::Fallback(0) => { /* use the fallback below */ }
204 CopyResult::Fallback(_) => {
205 unreachable!("splice should not return > 0 bytes on the fallback path")
211 // fallback if none of the more specialized syscalls wants to work with these file descriptors
212 match generic_copy(reader, writer) {
213 Ok(bytes) => Ok(bytes + written),
219 #[rustc_specialization_trait]
220 trait CopyRead: Read {
221 /// Implementations that contain buffers (i.e. `BufReader`) must transfer data from their internal
222 /// buffers into `writer` until either the buffers are emptied or `limit` bytes have been
223 /// transferred, whichever occurs sooner.
224 /// If nested buffers are present the outer buffers must be drained first.
226 /// This is necessary to directly bypass the wrapper types while preserving the data order
227 /// when operating directly on the underlying file descriptors.
228 fn drain_to<W: Write>(&mut self, _writer: &mut W, _limit: u64) -> Result<u64> {
232 /// Updates `Take` wrappers to remove the number of bytes copied.
233 fn taken(&mut self, _bytes: u64) {}
235 /// The minimum of the limit of all `Take<_>` wrappers, `u64::MAX` otherwise.
236 /// This method does not account for data `BufReader` buffers and would underreport
237 /// the limit of a `Take<BufReader<Take<_>>>` type. Thus its result is only valid
238 /// after draining the buffers via `drain_to`.
239 fn min_limit(&self) -> u64 {
243 /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
244 fn properties(&self) -> CopyParams;
247 #[rustc_specialization_trait]
248 trait CopyWrite: Write {
249 /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
250 fn properties(&self) -> CopyParams;
253 impl<T> CopyRead for &mut T
257 fn drain_to<W: Write>(&mut self, writer: &mut W, limit: u64) -> Result<u64> {
258 (**self).drain_to(writer, limit)
261 fn taken(&mut self, bytes: u64) {
262 (**self).taken(bytes);
265 fn min_limit(&self) -> u64 {
269 fn properties(&self) -> CopyParams {
270 (**self).properties()
274 impl<T> CopyWrite for &mut T
278 fn properties(&self) -> CopyParams {
279 (**self).properties()
283 impl CopyRead for File {
284 fn properties(&self) -> CopyParams {
285 CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
289 impl CopyRead for &File {
290 fn properties(&self) -> CopyParams {
291 CopyParams(fd_to_meta(*self), Some(self.as_raw_fd()))
295 impl CopyWrite for File {
296 fn properties(&self) -> CopyParams {
297 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
301 impl CopyWrite for &File {
302 fn properties(&self) -> CopyParams {
303 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
307 impl CopyRead for TcpStream {
308 fn properties(&self) -> CopyParams {
309 // avoid the stat syscall since we can be fairly sure it's a socket
310 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
314 impl CopyRead for &TcpStream {
315 fn properties(&self) -> CopyParams {
316 // avoid the stat syscall since we can be fairly sure it's a socket
317 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
321 impl CopyWrite for TcpStream {
322 fn properties(&self) -> CopyParams {
323 // avoid the stat syscall since we can be fairly sure it's a socket
324 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
328 impl CopyWrite for &TcpStream {
329 fn properties(&self) -> CopyParams {
330 // avoid the stat syscall since we can be fairly sure it's a socket
331 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
335 impl CopyRead for UnixStream {
336 fn properties(&self) -> CopyParams {
337 // avoid the stat syscall since we can be fairly sure it's a socket
338 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
342 impl CopyRead for &UnixStream {
343 fn properties(&self) -> CopyParams {
344 // avoid the stat syscall since we can be fairly sure it's a socket
345 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
349 impl CopyWrite for UnixStream {
350 fn properties(&self) -> CopyParams {
351 // avoid the stat syscall since we can be fairly sure it's a socket
352 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
356 impl CopyWrite for &UnixStream {
357 fn properties(&self) -> CopyParams {
358 // avoid the stat syscall since we can be fairly sure it's a socket
359 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
363 impl CopyWrite for ChildStdin {
364 fn properties(&self) -> CopyParams {
365 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
369 impl CopyRead for ChildStdout {
370 fn properties(&self) -> CopyParams {
371 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
375 impl CopyRead for ChildStderr {
376 fn properties(&self) -> CopyParams {
377 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
381 impl CopyRead for StdinLock<'_> {
382 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
383 let buf_reader = self.as_mut_buf();
384 let buf = buf_reader.buffer();
385 let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
386 let bytes_drained = buf.len();
387 writer.write_all(buf)?;
388 buf_reader.consume(bytes_drained);
390 Ok(bytes_drained as u64)
393 fn properties(&self) -> CopyParams {
394 CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
398 impl CopyWrite for StdoutLock<'_> {
399 fn properties(&self) -> CopyParams {
400 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
404 impl CopyWrite for StderrLock<'_> {
405 fn properties(&self) -> CopyParams {
406 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
410 impl<T: CopyRead> CopyRead for Take<T> {
411 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
412 let local_limit = self.limit();
413 let combined_limit = min(outer_limit, local_limit);
414 let bytes_drained = self.get_mut().drain_to(writer, combined_limit)?;
415 // update limit since read() was bypassed
416 self.set_limit(local_limit - bytes_drained);
421 fn taken(&mut self, bytes: u64) {
422 self.set_limit(self.limit() - bytes);
423 self.get_mut().taken(bytes);
426 fn min_limit(&self) -> u64 {
427 min(Take::limit(self), self.get_ref().min_limit())
430 fn properties(&self) -> CopyParams {
431 self.get_ref().properties()
435 impl<T: CopyRead> CopyRead for BufReader<T> {
436 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
437 let buf = self.buffer();
438 let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
439 let bytes = buf.len();
440 writer.write_all(buf)?;
443 let remaining = outer_limit - bytes as u64;
445 // in case of nested bufreaders we also need to drain the ones closer to the source
446 let inner_bytes = self.get_mut().drain_to(writer, remaining)?;
448 Ok(bytes as u64 + inner_bytes)
451 fn taken(&mut self, bytes: u64) {
452 self.get_mut().taken(bytes);
455 fn min_limit(&self) -> u64 {
456 self.get_ref().min_limit()
459 fn properties(&self) -> CopyParams {
460 self.get_ref().properties()
464 impl<T: CopyWrite> CopyWrite for BufWriter<T> {
465 fn properties(&self) -> CopyParams {
466 self.get_ref().properties()
470 fn fd_to_meta<T: AsRawFd>(fd: &T) -> FdMeta {
471 let fd = fd.as_raw_fd();
472 let file: ManuallyDrop<File> = ManuallyDrop::new(unsafe { File::from_raw_fd(fd) });
473 match file.metadata() {
474 Ok(meta) => FdMeta::Metadata(meta),
475 Err(_) => FdMeta::NoneObtained,
479 pub(super) enum CopyResult {
486 fn update_take(&self, reader: &mut impl CopyRead) {
488 CopyResult::Fallback(bytes)
489 | CopyResult::Ended(bytes)
490 | CopyResult::Error(_, bytes) => reader.taken(bytes),
495 /// Invalid file descriptor.
497 /// Valid file descriptors are guaranteed to be positive numbers (see `open()` manpage)
498 /// while negative values are used to indicate errors.
499 /// Thus -1 will never be overlap with a valid open file.
500 const INVALID_FD: RawFd = -1;
502 /// Linux-specific implementation that will attempt to use copy_file_range for copy offloading.
503 /// As the name says, it only works on regular files.
505 /// Callers must handle fallback to a generic copy loop.
506 /// `Fallback` may indicate non-zero number of bytes already written
507 /// if one of the files' cursor +`max_len` would exceed u64::MAX (`EOVERFLOW`).
508 pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) -> CopyResult {
511 const NOT_PROBED: u8 = 0;
512 const UNAVAILABLE: u8 = 1;
513 const AVAILABLE: u8 = 2;
515 // Kernel prior to 4.5 don't have copy_file_range
516 // We store the availability in a global to avoid unnecessary syscalls
517 static HAS_COPY_FILE_RANGE: AtomicU8 = AtomicU8::new(NOT_PROBED);
522 off_in: *mut libc::loff_t,
524 off_out: *mut libc::loff_t,
530 match HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
532 // EPERM can indicate seccomp filters or an immutable file.
533 // To distinguish these cases we probe with invalid file descriptors which should result in EBADF if the syscall is supported
534 // and some other error (ENOSYS or EPERM) if it's not available
535 let result = unsafe {
536 cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
539 if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(EBADF))) {
540 HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
542 HAS_COPY_FILE_RANGE.store(UNAVAILABLE, Ordering::Relaxed);
543 return CopyResult::Fallback(0);
546 UNAVAILABLE => return CopyResult::Fallback(0),
550 let mut written = 0u64;
551 while written < max_len {
552 let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64);
553 // cap to 1GB chunks in case u64::MAX is passed as max_len and the file has a non-zero seek position
554 // this allows us to copy large chunks without hitting EOVERFLOW,
555 // unless someone sets a file offset close to u64::MAX - 1GB, in which case a fallback would be required
556 let bytes_to_copy = cmp::min(bytes_to_copy as usize, 0x4000_0000usize);
557 let copy_result = unsafe {
558 // We actually don't have to adjust the offsets,
559 // because copy_file_range adjusts the file offset automatically
560 cvt(copy_file_range(reader, ptr::null_mut(), writer, ptr::null_mut(), bytes_to_copy, 0))
564 Ok(0) if written == 0 => {
565 // fallback to work around several kernel bugs where copy_file_range will fail to
566 // copy any bytes and return 0 instead of an error if
567 // - reading virtual files from the proc filesystem which appear to have 0 size
568 // but are not empty. noted in coreutils to affect kernels at least up to 5.6.19.
569 // - copying from an overlay filesystem in docker. reported to occur on fedora 32.
570 return CopyResult::Fallback(0);
572 Ok(0) => return CopyResult::Ended(written), // reached EOF
573 Ok(ret) => written += ret as u64,
575 return match err.raw_os_error() {
576 // when file offset + max_length > u64::MAX
577 Some(EOVERFLOW) => CopyResult::Fallback(written),
578 Some(ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF) if written == 0 => {
579 // Try fallback io::copy if either:
580 // - Kernel version is < 4.5 (ENOSYS¹)
581 // - Files are mounted on different fs (EXDEV)
582 // - copy_file_range is broken in various ways on RHEL/CentOS 7 (EOPNOTSUPP)
583 // - copy_file_range file is immutable or syscall is blocked by seccomp¹ (EPERM)
584 // - copy_file_range cannot be used with pipes or device nodes (EINVAL)
585 // - the writer fd was opened with O_APPEND (EBADF²)
586 // and no bytes were written successfully yet. (All these errnos should
587 // not be returned if something was already written, but they happen in
588 // the wild, see #91152.)
590 // ¹ these cases should be detected by the initial probe but we handle them here
591 // anyway in case syscall interception changes during runtime
592 // ² actually invalid file descriptors would cause this too, but in that case
593 // the fallback code path is expected to encounter the same error again
594 CopyResult::Fallback(0)
596 _ => CopyResult::Error(err, written),
601 CopyResult::Ended(written)
610 /// performs splice or sendfile between file descriptors
611 /// Does _not_ fall back to a generic copy loop.
612 fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) -> CopyResult {
613 static HAS_SENDFILE: AtomicBool = AtomicBool::new(true);
614 static HAS_SPLICE: AtomicBool = AtomicBool::new(true);
616 // Android builds use feature level 14, but the libc wrapper for splice is
617 // gated on feature level 21+, so we have to invoke the syscall directly.
618 #[cfg(target_os = "android")]
622 src_offset: *const i64,
624 dst_offset: *const i64,
630 #[cfg(target_os = "linux")]
634 SpliceMode::Sendfile if !HAS_SENDFILE.load(Ordering::Relaxed) => {
635 return CopyResult::Fallback(0);
637 SpliceMode::Splice if !HAS_SPLICE.load(Ordering::Relaxed) => {
638 return CopyResult::Fallback(0);
643 let mut written = 0u64;
644 while written < len {
645 // according to its manpage that's the maximum size sendfile() will copy per invocation
646 let chunk_size = crate::cmp::min(len - written, 0x7ffff000_u64) as usize;
648 let result = match mode {
649 SpliceMode::Sendfile => {
650 cvt(unsafe { libc::sendfile(writer, reader, ptr::null_mut(), chunk_size) })
652 SpliceMode::Splice => cvt(unsafe {
653 splice(reader, ptr::null_mut(), writer, ptr::null_mut(), chunk_size, 0)
658 Ok(0) => break, // EOF
659 Ok(ret) => written += ret as u64,
661 return match err.raw_os_error() {
662 Some(ENOSYS | EPERM) => {
663 // syscall not supported (ENOSYS)
664 // syscall is disallowed, e.g. by seccomp (EPERM)
666 SpliceMode::Sendfile => HAS_SENDFILE.store(false, Ordering::Relaxed),
667 SpliceMode::Splice => HAS_SPLICE.store(false, Ordering::Relaxed),
669 assert_eq!(written, 0);
670 CopyResult::Fallback(0)
673 // splice/sendfile do not support this particular file descriptor (EINVAL)
674 assert_eq!(written, 0);
675 CopyResult::Fallback(0)
677 Some(os_err) if mode == SpliceMode::Sendfile && os_err == EOVERFLOW => {
678 CopyResult::Fallback(written)
680 _ => CopyResult::Error(err, written),
685 CopyResult::Ended(written)