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 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::convert::TryInto;
49 use crate::fs::{File, Metadata};
50 use crate::io::copy::generic_copy;
52 BufRead, BufReader, BufWriter, Error, Read, Result, StderrLock, StdinLock, StdoutLock, Take,
55 use crate::mem::ManuallyDrop;
56 use crate::net::TcpStream;
57 use crate::os::unix::fs::FileTypeExt;
58 use crate::os::unix::io::{AsRawFd, FromRawFd, RawFd};
59 use crate::os::unix::net::UnixStream;
60 use crate::process::{ChildStderr, ChildStdin, ChildStdout};
62 use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
64 use crate::sys::weak::syscall;
65 use libc::{EBADF, EINVAL, ENOSYS, EOPNOTSUPP, EOVERFLOW, EPERM, EXDEV};
70 pub(crate) fn copy_spec<R: Read + ?Sized, W: Write + ?Sized>(
74 let copier = Copier { read, write };
75 SpecCopy::copy(copier)
78 /// This type represents either the inferred `FileType` of a `RawFd` based on the source
79 /// type from which it was extracted or the actual metadata
81 /// The methods on this type only provide hints, due to `AsRawFd` and `FromRawFd` the inferred
82 /// type may be wrong.
84 /// We obtained the FD from a type that can contain any type of `FileType` and queried the metadata
85 /// because it is cheaper than probing all possible syscalls (reader side)
89 /// We don't have any metadata, e.g. because the original type was `File` which can represent
90 /// any `FileType` and we did not query the metadata either since it did not seem beneficial
96 fn maybe_fifo(&self) -> bool {
98 FdMeta::Metadata(meta) => meta.file_type().is_fifo(),
99 FdMeta::Socket => false,
100 FdMeta::Pipe => true,
101 FdMeta::NoneObtained => true,
105 fn potential_sendfile_source(&self) -> bool {
107 // procfs erronously shows 0 length on non-empty readable files.
108 // and if a file is truly empty then a `read` syscall will determine that and skip the write syscall
109 // thus there would be benefit from attempting sendfile
110 FdMeta::Metadata(meta)
111 if meta.file_type().is_file() && meta.len() > 0
112 || meta.file_type().is_block_device() =>
120 fn copy_file_range_candidate(&self) -> bool {
122 // copy_file_range will fail on empty procfs files. `read` can determine whether EOF has been reached
123 // without extra cost and skip the write, thus there is no benefit in attempting copy_file_range
124 FdMeta::Metadata(meta) if meta.is_file() && meta.len() > 0 => true,
125 FdMeta::NoneObtained => true,
131 struct CopyParams(FdMeta, Option<RawFd>);
133 struct Copier<'a, 'b, R: Read + ?Sized, W: Write + ?Sized> {
139 fn copy(self) -> Result<u64>;
142 impl<R: Read + ?Sized, W: Write + ?Sized> SpecCopy for Copier<'_, '_, R, W> {
143 default fn copy(self) -> Result<u64> {
144 generic_copy(self.read, self.write)
148 impl<R: CopyRead, W: CopyWrite> SpecCopy for Copier<'_, '_, R, W> {
149 fn copy(self) -> Result<u64> {
150 let (reader, writer) = (self.read, self.write);
151 let r_cfg = reader.properties();
152 let w_cfg = writer.properties();
154 // before direct operations on file descriptors ensure that all source and sink buffers are empty
155 let mut flush = || -> crate::io::Result<u64> {
156 let bytes = reader.drain_to(writer, u64::MAX)?;
157 // BufWriter buffered bytes have already been accounted for in earlier write() calls
162 let mut written = 0u64;
164 if let (CopyParams(input_meta, Some(readfd)), CopyParams(output_meta, Some(writefd))) =
168 let max_write = reader.min_limit();
170 if input_meta.copy_file_range_candidate() && output_meta.copy_file_range_candidate() {
171 let result = copy_regular_files(readfd, writefd, max_write);
172 result.update_take(reader);
175 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
176 CopyResult::Error(e, _) => return Err(e),
177 CopyResult::Fallback(bytes) => written += bytes,
181 // on modern kernels sendfile can copy from any mmapable type (some but not all regular files and block devices)
182 // to any writable file descriptor. On older kernels the writer side can only be a socket.
183 // So we just try and fallback if needed.
184 // If current file offsets + write sizes overflow it may also fail, we do not try to fix that and instead
185 // fall back to the generic copy loop.
186 if input_meta.potential_sendfile_source() {
187 let result = sendfile_splice(SpliceMode::Sendfile, readfd, writefd, max_write);
188 result.update_take(reader);
191 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
192 CopyResult::Error(e, _) => return Err(e),
193 CopyResult::Fallback(bytes) => written += bytes,
197 if input_meta.maybe_fifo() || output_meta.maybe_fifo() {
198 let result = sendfile_splice(SpliceMode::Splice, readfd, writefd, max_write);
199 result.update_take(reader);
202 CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
203 CopyResult::Error(e, _) => return Err(e),
204 CopyResult::Fallback(0) => { /* use the fallback below */ }
205 CopyResult::Fallback(_) => {
206 unreachable!("splice should not return > 0 bytes on the fallback path")
212 // fallback if none of the more specialized syscalls wants to work with these file descriptors
213 match generic_copy(reader, writer) {
214 Ok(bytes) => Ok(bytes + written),
220 #[rustc_specialization_trait]
221 trait CopyRead: Read {
222 /// Implementations that contain buffers (i.e. `BufReader`) must transfer data from their internal
223 /// buffers into `writer` until either the buffers are emptied or `limit` bytes have been
224 /// transferred, whichever occurs sooner.
225 /// If nested buffers are present the outer buffers must be drained first.
227 /// This is necessary to directly bypass the wrapper types while preserving the data order
228 /// when operating directly on the underlying file descriptors.
229 fn drain_to<W: Write>(&mut self, _writer: &mut W, _limit: u64) -> Result<u64> {
233 /// Updates `Take` wrappers to remove the number of bytes copied.
234 fn taken(&mut self, _bytes: u64) {}
236 /// The minimum of the limit of all `Take<_>` wrappers, `u64::MAX` otherwise.
237 /// This method does not account for data `BufReader` buffers and would underreport
238 /// the limit of a `Take<BufReader<Take<_>>>` type. Thus its result is only valid
239 /// after draining the buffers via `drain_to`.
240 fn min_limit(&self) -> u64 {
244 /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
245 fn properties(&self) -> CopyParams;
248 #[rustc_specialization_trait]
249 trait CopyWrite: Write {
250 /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
251 fn properties(&self) -> CopyParams;
254 impl<T> CopyRead for &mut T
258 fn drain_to<W: Write>(&mut self, writer: &mut W, limit: u64) -> Result<u64> {
259 (**self).drain_to(writer, limit)
262 fn taken(&mut self, bytes: u64) {
263 (**self).taken(bytes);
266 fn min_limit(&self) -> u64 {
270 fn properties(&self) -> CopyParams {
271 (**self).properties()
275 impl<T> CopyWrite for &mut T
279 fn properties(&self) -> CopyParams {
280 (**self).properties()
284 impl CopyRead for File {
285 fn properties(&self) -> CopyParams {
286 CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
290 impl CopyRead for &File {
291 fn properties(&self) -> CopyParams {
292 CopyParams(fd_to_meta(*self), Some(self.as_raw_fd()))
296 impl CopyWrite for File {
297 fn properties(&self) -> CopyParams {
298 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
302 impl CopyWrite for &File {
303 fn properties(&self) -> CopyParams {
304 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
308 impl CopyRead for TcpStream {
309 fn properties(&self) -> CopyParams {
310 // avoid the stat syscall since we can be fairly sure it's a socket
311 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
315 impl CopyRead for &TcpStream {
316 fn properties(&self) -> CopyParams {
317 // avoid the stat syscall since we can be fairly sure it's a socket
318 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
322 impl CopyWrite for TcpStream {
323 fn properties(&self) -> CopyParams {
324 // avoid the stat syscall since we can be fairly sure it's a socket
325 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
329 impl CopyWrite for &TcpStream {
330 fn properties(&self) -> CopyParams {
331 // avoid the stat syscall since we can be fairly sure it's a socket
332 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
336 impl CopyRead for UnixStream {
337 fn properties(&self) -> CopyParams {
338 // avoid the stat syscall since we can be fairly sure it's a socket
339 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
343 impl CopyRead for &UnixStream {
344 fn properties(&self) -> CopyParams {
345 // avoid the stat syscall since we can be fairly sure it's a socket
346 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
350 impl CopyWrite for UnixStream {
351 fn properties(&self) -> CopyParams {
352 // avoid the stat syscall since we can be fairly sure it's a socket
353 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
357 impl CopyWrite for &UnixStream {
358 fn properties(&self) -> CopyParams {
359 // avoid the stat syscall since we can be fairly sure it's a socket
360 CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
364 impl CopyWrite for ChildStdin {
365 fn properties(&self) -> CopyParams {
366 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
370 impl CopyRead for ChildStdout {
371 fn properties(&self) -> CopyParams {
372 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
376 impl CopyRead for ChildStderr {
377 fn properties(&self) -> CopyParams {
378 CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
382 impl CopyRead for StdinLock<'_> {
383 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
384 let buf_reader = self.as_mut_buf();
385 let buf = buf_reader.buffer();
386 let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
387 let bytes_drained = buf.len();
388 writer.write_all(buf)?;
389 buf_reader.consume(bytes_drained);
391 Ok(bytes_drained as u64)
394 fn properties(&self) -> CopyParams {
395 CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
399 impl CopyWrite for StdoutLock<'_> {
400 fn properties(&self) -> CopyParams {
401 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
405 impl CopyWrite for StderrLock<'_> {
406 fn properties(&self) -> CopyParams {
407 CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
411 impl<T: CopyRead> CopyRead for Take<T> {
412 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
413 let local_limit = self.limit();
414 let combined_limit = min(outer_limit, local_limit);
415 let bytes_drained = self.get_mut().drain_to(writer, combined_limit)?;
416 // update limit since read() was bypassed
417 self.set_limit(local_limit - bytes_drained);
422 fn taken(&mut self, bytes: u64) {
423 self.set_limit(self.limit() - bytes);
424 self.get_mut().taken(bytes);
427 fn min_limit(&self) -> u64 {
428 min(Take::limit(self), self.get_ref().min_limit())
431 fn properties(&self) -> CopyParams {
432 self.get_ref().properties()
436 impl<T: CopyRead> CopyRead for BufReader<T> {
437 fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
438 let buf = self.buffer();
439 let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
440 let bytes = buf.len();
441 writer.write_all(buf)?;
444 let remaining = outer_limit - bytes as u64;
446 // in case of nested bufreaders we also need to drain the ones closer to the source
447 let inner_bytes = self.get_mut().drain_to(writer, remaining)?;
449 Ok(bytes as u64 + inner_bytes)
452 fn taken(&mut self, bytes: u64) {
453 self.get_mut().taken(bytes);
456 fn min_limit(&self) -> u64 {
457 self.get_ref().min_limit()
460 fn properties(&self) -> CopyParams {
461 self.get_ref().properties()
465 impl<T: CopyWrite> CopyWrite for BufWriter<T> {
466 fn properties(&self) -> CopyParams {
467 self.get_ref().properties()
471 fn fd_to_meta<T: AsRawFd>(fd: &T) -> FdMeta {
472 let fd = fd.as_raw_fd();
473 let file: ManuallyDrop<File> = ManuallyDrop::new(unsafe { File::from_raw_fd(fd) });
474 match file.metadata() {
475 Ok(meta) => FdMeta::Metadata(meta),
476 Err(_) => FdMeta::NoneObtained,
480 pub(super) enum CopyResult {
487 fn update_take(&self, reader: &mut impl CopyRead) {
489 CopyResult::Fallback(bytes)
490 | CopyResult::Ended(bytes)
491 | CopyResult::Error(_, bytes) => reader.taken(bytes),
496 /// Invalid file descriptor.
498 /// Valid file descriptors are guaranteed to be positive numbers (see `open()` manpage)
499 /// while negative values are used to indicate errors.
500 /// Thus -1 will never be overlap with a valid open file.
501 const INVALID_FD: RawFd = -1;
503 /// Linux-specific implementation that will attempt to use copy_file_range for copy offloading.
504 /// As the name says, it only works on regular files.
506 /// Callers must handle fallback to a generic copy loop.
507 /// `Fallback` may indicate non-zero number of bytes already written
508 /// if one of the files' cursor +`max_len` would exceed u64::MAX (`EOVERFLOW`).
509 pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) -> CopyResult {
512 const NOT_PROBED: u8 = 0;
513 const UNAVAILABLE: u8 = 1;
514 const AVAILABLE: u8 = 2;
516 // Kernel prior to 4.5 don't have copy_file_range
517 // We store the availability in a global to avoid unnecessary syscalls
518 static HAS_COPY_FILE_RANGE: AtomicU8 = AtomicU8::new(NOT_PROBED);
523 off_in: *mut libc::loff_t,
525 off_out: *mut libc::loff_t,
531 match HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
533 // EPERM can indicate seccomp filters or an immutable file.
534 // To distinguish these cases we probe with invalid file descriptors which should result in EBADF if the syscall is supported
535 // and some other error (ENOSYS or EPERM) if it's not available
536 let result = unsafe {
537 cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
540 if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(EBADF))) {
541 HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
543 HAS_COPY_FILE_RANGE.store(UNAVAILABLE, Ordering::Relaxed);
544 return CopyResult::Fallback(0);
547 UNAVAILABLE => return CopyResult::Fallback(0),
551 let mut written = 0u64;
552 while written < max_len {
553 let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64);
554 // cap to 1GB chunks in case u64::MAX is passed as max_len and the file has a non-zero seek position
555 // this allows us to copy large chunks without hitting EOVERFLOW,
556 // unless someone sets a file offset close to u64::MAX - 1GB, in which case a fallback would be required
557 let bytes_to_copy = cmp::min(bytes_to_copy as usize, 0x4000_0000usize);
558 let copy_result = unsafe {
559 // We actually don't have to adjust the offsets,
560 // because copy_file_range adjusts the file offset automatically
561 cvt(copy_file_range(reader, ptr::null_mut(), writer, ptr::null_mut(), bytes_to_copy, 0))
565 Ok(0) if written == 0 => {
566 // fallback to work around several kernel bugs where copy_file_range will fail to
567 // copy any bytes and return 0 instead of an error if
568 // - reading virtual files from the proc filesystem which appear to have 0 size
569 // but are not empty. noted in coreutils to affect kernels at least up to 5.6.19.
570 // - copying from an overlay filesystem in docker. reported to occur on fedora 32.
571 return CopyResult::Fallback(0);
573 Ok(0) => return CopyResult::Ended(written), // reached EOF
574 Ok(ret) => written += ret as u64,
576 return match err.raw_os_error() {
577 // when file offset + max_length > u64::MAX
578 Some(EOVERFLOW) => CopyResult::Fallback(written),
579 Some(ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF) if written == 0 => {
580 // Try fallback io::copy if either:
581 // - Kernel version is < 4.5 (ENOSYS¹)
582 // - Files are mounted on different fs (EXDEV)
583 // - copy_file_range is broken in various ways on RHEL/CentOS 7 (EOPNOTSUPP)
584 // - copy_file_range file is immutable or syscall is blocked by seccomp¹ (EPERM)
585 // - copy_file_range cannot be used with pipes or device nodes (EINVAL)
586 // - the writer fd was opened with O_APPEND (EBADF²)
587 // and no bytes were written successfully yet. (All these errnos should
588 // not be returned if something was already written, but they happen in
589 // the wild, see #91152.)
591 // ¹ these cases should be detected by the initial probe but we handle them here
592 // anyway in case syscall interception changes during runtime
593 // ² actually invalid file descriptors would cause this too, but in that case
594 // the fallback code path is expected to encounter the same error again
595 CopyResult::Fallback(0)
597 _ => CopyResult::Error(err, written),
602 CopyResult::Ended(written)
611 /// performs splice or sendfile between file descriptors
612 /// Does _not_ fall back to a generic copy loop.
613 fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) -> CopyResult {
614 static HAS_SENDFILE: AtomicBool = AtomicBool::new(true);
615 static HAS_SPLICE: AtomicBool = AtomicBool::new(true);
617 // Android builds use feature level 14, but the libc wrapper for splice is
618 // gated on feature level 21+, so we have to invoke the syscall directly.
619 #[cfg(target_os = "android")]
623 src_offset: *const i64,
625 dst_offset: *const i64,
631 #[cfg(target_os = "linux")]
635 SpliceMode::Sendfile if !HAS_SENDFILE.load(Ordering::Relaxed) => {
636 return CopyResult::Fallback(0);
638 SpliceMode::Splice if !HAS_SPLICE.load(Ordering::Relaxed) => {
639 return CopyResult::Fallback(0);
644 let mut written = 0u64;
645 while written < len {
646 // according to its manpage that's the maximum size sendfile() will copy per invocation
647 let chunk_size = crate::cmp::min(len - written, 0x7ffff000_u64) as usize;
649 let result = match mode {
650 SpliceMode::Sendfile => {
651 cvt(unsafe { libc::sendfile(writer, reader, ptr::null_mut(), chunk_size) })
653 SpliceMode::Splice => cvt(unsafe {
654 splice(reader, ptr::null_mut(), writer, ptr::null_mut(), chunk_size, 0)
659 Ok(0) => break, // EOF
660 Ok(ret) => written += ret as u64,
662 return match err.raw_os_error() {
663 Some(ENOSYS | EPERM) => {
664 // syscall not supported (ENOSYS)
665 // syscall is disallowed, e.g. by seccomp (EPERM)
667 SpliceMode::Sendfile => HAS_SENDFILE.store(false, Ordering::Relaxed),
668 SpliceMode::Splice => HAS_SPLICE.store(false, Ordering::Relaxed),
670 assert_eq!(written, 0);
671 CopyResult::Fallback(0)
674 // splice/sendfile do not support this particular file descriptor (EINVAL)
675 assert_eq!(written, 0);
676 CopyResult::Fallback(0)
678 Some(os_err) if mode == SpliceMode::Sendfile && os_err == EOVERFLOW => {
679 CopyResult::Fallback(written)
681 _ => CopyResult::Error(err, written),
686 CopyResult::Ended(written)