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    //! This module contains specializations that can offload `io::copy()` operations on file descriptor
    //! containing types (`File`, `TcpStream`, etc.) to more efficient syscalls than `read(2)` and `write(2)`.
    //!
    //! Specialization is only applied to wholly std-owned types so that user code can't observe
    //! that the `Read` and `Write` traits are not used.
    //!
    //! Since a copy operation involves a reader and writer side where each can consist of different types
    //! and also involve generic wrappers (e.g. `Take`, `BufReader`) it is not practical to specialize
    //! a single method on all possible combinations.
    //!
    //! Instead readers and writers are handled separately by the `CopyRead` and `CopyWrite` specialization
    //! traits and then specialized on by the `Copier::copy` method.
    //!
    //! `Copier` uses the specialization traits to unpack the underlying file descriptors and
    //! additional prerequisites and constraints imposed by the wrapper types.
    //!
    //! Once it has obtained all necessary pieces and brought any wrapper types into a state where they
    //! can be safely bypassed it will attempt to use the `copy_file_range(2)`,
    //! `sendfile(2)` or `splice(2)` syscalls to move data directly between file descriptors.
    //! Since those syscalls have requirements that cannot be fully checked in advance and
    //! gathering additional information about file descriptors would require additional syscalls
    //! anyway it simply attempts to use them one after another (guided by inaccurate hints) to
    //! figure out which one works and and falls back to the generic read-write copy loop if none of them
    //! does.
    //! Once a working syscall is found for a pair of file descriptors it will be called in a loop
    //! until the copy operation is completed.
    //!
    //! Advantages of using these syscalls:
    //!
    //! * fewer context switches since reads and writes are coalesced into a single syscall
    //!   and more bytes are transferred per syscall. This translates to higher throughput
    //!   and fewer CPU cycles, at least for sufficiently large transfers to amortize the initial probing.
    //! * `copy_file_range` creates reflink copies on CoW filesystems, thus moving less data and
    //!   consuming less disk space
    //! * `sendfile` and `splice` can perform zero-copy IO under some circumstances while
    //!   a naive copy loop would move every byte through the CPU.
    //!
    //! Drawbacks:
    //!
    //! * copy operations smaller than the default buffer size can under some circumstances, especially
    //!   on older kernels, incur more syscalls than the naive approach would. As mentioned above
    //!   the syscall selection is guided by hints to minimize this possibility but they are not perfect.
    //! * optimizations only apply to std types. If a user adds a custom wrapper type, e.g. to report
    //!   progress, they can hit a performance cliff.
    //! * complexity

    use crate::cmp::min;
    use crate::convert::TryInto;
    use crate::fs::{File, Metadata};
    use crate::io::copy::generic_copy;
    use crate::io::{
        BufRead, BufReader, BufWriter, Error, Read, Result, StderrLock, StdinLock, StdoutLock, Take,
        Write,
    };
    use crate::mem::ManuallyDrop;
    use crate::net::TcpStream;
    use crate::os::unix::fs::FileTypeExt;
    use crate::os::unix::io::{AsRawFd, FromRawFd, RawFd};
    use crate::os::unix::net::UnixStream;
    use crate::process::{ChildStderr, ChildStdin, ChildStdout};
    use crate::ptr;
    use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
    use crate::sys::cvt;
    use libc::{EBADF, EINVAL, ENOSYS, EOPNOTSUPP, EOVERFLOW, EPERM, EXDEV};

    #[cfg(test)]
    mod tests;

    pub(crate) fn copy_spec<R: Read + ?Sized, W: Write + ?Sized>(
        read: &mut R,
        write: &mut W,
    ) -> Result<u64> {
        let copier = Copier { read, write };
        SpecCopy::copy(copier)
    }

    /// This type represents either the inferred `FileType` of a `RawFd` based on the source
    /// type from which it was extracted or the actual metadata
    ///
    /// The methods on this type only provide hints, due to `AsRawFd` and `FromRawFd` the inferred
    /// type may be wrong.
    enum FdMeta {
        /// We obtained the FD from a type that can contain any type of `FileType` and queried the metadata
        /// because it is cheaper than probing all possible syscalls (reader side)
        Metadata(Metadata),
        Socket,
        Pipe,
        /// We don't have any metadata, e.g. because the original type was `File` which can represent
        /// any `FileType` and we did not query the metadata either since it did not seem beneficial
        /// (writer side)
        NoneObtained,
    }

    impl FdMeta {
        fn maybe_fifo(&self) -> bool {
            match self {
                FdMeta::Metadata(meta) => meta.file_type().is_fifo(),
                FdMeta::Socket => false,
                FdMeta::Pipe => true,
                FdMeta::NoneObtained => true,
            }
        }

        fn potential_sendfile_source(&self) -> bool {
            match self {
                // procfs erronously shows 0 length on non-empty readable files.
                // and if a file is truly empty then a `read` syscall will determine that and skip the write syscall
                // thus there would be benefit from attempting sendfile
                FdMeta::Metadata(meta)
                    if meta.file_type().is_file() && meta.len() > 0
                        || meta.file_type().is_block_device() =>
                {
                    true
                }
                _ => false,
            }
        }

        fn copy_file_range_candidate(&self) -> bool {
            match self {
                // copy_file_range will fail on empty procfs files. `read` can determine whether EOF has been reached
                // without extra cost and skip the write, thus there is no benefit in attempting copy_file_range
                FdMeta::Metadata(meta) if meta.is_file() && meta.len() > 0 => true,
                FdMeta::NoneObtained => true,
                _ => false,
            }
        }
    }

    struct CopyParams(FdMeta, Option<RawFd>);

    struct Copier<'a, 'b, R: Read + ?Sized, W: Write + ?Sized> {
        read: &'a mut R,
        write: &'b mut W,
    }

    trait SpecCopy {
        fn copy(self) -> Result<u64>;
    }

    impl<R: Read + ?Sized, W: Write + ?Sized> SpecCopy for Copier<'_, '_, R, W> {
        default fn copy(self) -> Result<u64> {
            generic_copy(self.read, self.write)
        }
    }

    impl<R: CopyRead, W: CopyWrite> SpecCopy for Copier<'_, '_, R, W> {
        fn copy(self) -> Result<u64> {
            let (reader, writer) = (self.read, self.write);
            let r_cfg = reader.properties();
            let w_cfg = writer.properties();

            // before direct operations on file descriptors ensure that all source and sink buffers are empty
            let mut flush = || -> crate::io::Result<u64> {
                let bytes = reader.drain_to(writer, u64::MAX)?;
                // BufWriter buffered bytes have already been accounted for in earlier write() calls
                writer.flush()?;
                Ok(bytes)
            };

            let mut written = 0u64;

            if let (CopyParams(input_meta, Some(readfd)), CopyParams(output_meta, Some(writefd))) =
                (r_cfg, w_cfg)
            {
                written += flush()?;
                let max_write = reader.min_limit();

                if input_meta.copy_file_range_candidate() && output_meta.copy_file_range_candidate() {
                    let result = copy_regular_files(readfd, writefd, max_write);
                    result.update_take(reader);

                    match result {
                        CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
                        CopyResult::Error(e, _) => return Err(e),
                        CopyResult::Fallback(bytes) => written += bytes,
                    }
                }

                // on modern kernels sendfile can copy from any mmapable type (some but not all regular files and block devices)
                // to any writable file descriptor. On older kernels the writer side can only be a socket.
                // So we just try and fallback if needed.
                // If current file offsets + write sizes overflow it may also fail, we do not try to fix that and instead
                // fall back to the generic copy loop.
                if input_meta.potential_sendfile_source() {
                    let result = sendfile_splice(SpliceMode::Sendfile, readfd, writefd, max_write);
                    result.update_take(reader);

                    match result {
                        CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
                        CopyResult::Error(e, _) => return Err(e),
                        CopyResult::Fallback(bytes) => written += bytes,
                    }
                }

                if input_meta.maybe_fifo() || output_meta.maybe_fifo() {
                    let result = sendfile_splice(SpliceMode::Splice, readfd, writefd, max_write);
                    result.update_take(reader);

                    match result {
                        CopyResult::Ended(bytes_copied) => return Ok(bytes_copied + written),
                        CopyResult::Error(e, _) => return Err(e),
                        CopyResult::Fallback(0) => { /* use the fallback below */ }
                        CopyResult::Fallback(_) => {
                            unreachable!("splice should not return > 0 bytes on the fallback path")
                        }
                    }
                }
            }

            // fallback if none of the more specialized syscalls wants to work with these file descriptors
            match generic_copy(reader, writer) {
                Ok(bytes) => Ok(bytes + written),
                err => err,
            }
        }
    }

    #[rustc_specialization_trait]
    trait CopyRead: Read {
        /// Implementations that contain buffers (i.e. `BufReader`) must transfer data from their internal
        /// buffers into `writer` until either the buffers are emptied or `limit` bytes have been
        /// transferred, whichever occurs sooner.
        /// If nested buffers are present the outer buffers must be drained first.
        ///
        /// This is necessary to directly bypass the wrapper types while preserving the data order
        /// when operating directly on the underlying file descriptors.
        fn drain_to<W: Write>(&mut self, _writer: &mut W, _limit: u64) -> Result<u64> {
            Ok(0)
        }

        /// Updates `Take` wrappers to remove the number of bytes copied.
        fn taken(&mut self, _bytes: u64) {}

        /// The minimum of the limit of all `Take<_>` wrappers, `u64::MAX` otherwise.
        /// This method does not account for data `BufReader` buffers and would underreport
        /// the limit of a `Take<BufReader<Take<_>>>` type. Thus its result is only valid
        /// after draining the buffers via `drain_to`.
        fn min_limit(&self) -> u64 {
            u64::MAX
        }

        /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
        fn properties(&self) -> CopyParams;
    }

    #[rustc_specialization_trait]
    trait CopyWrite: Write {
        /// Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
        fn properties(&self) -> CopyParams;
    }

    impl<T> CopyRead for &mut T
    where
        T: CopyRead,
    {
        fn drain_to<W: Write>(&mut self, writer: &mut W, limit: u64) -> Result<u64> {
            (**self).drain_to(writer, limit)
        }

        fn taken(&mut self, bytes: u64) {
            (**self).taken(bytes);
        }

        fn min_limit(&self) -> u64 {
            (**self).min_limit()
        }

        fn properties(&self) -> CopyParams {
            (**self).properties()
        }
    }

    impl<T> CopyWrite for &mut T
    where
        T: CopyWrite,
    {
        fn properties(&self) -> CopyParams {
            (**self).properties()
        }
    }

    impl CopyRead for File {
        fn properties(&self) -> CopyParams {
            CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for &File {
        fn properties(&self) -> CopyParams {
            CopyParams(fd_to_meta(*self), Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for File {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for &File {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for TcpStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for &TcpStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for TcpStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for &TcpStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for UnixStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for &UnixStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for UnixStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for &UnixStream {
        fn properties(&self) -> CopyParams {
            // avoid the stat syscall since we can be fairly sure it's a socket
            CopyParams(FdMeta::Socket, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for ChildStdin {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for ChildStdout {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for ChildStderr {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::Pipe, Some(self.as_raw_fd()))
        }
    }

    impl CopyRead for StdinLock<'_> {
        fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
            let buf_reader = self.as_mut_buf();
            let buf = buf_reader.buffer();
            let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
            let bytes_drained = buf.len();
            writer.write_all(buf)?;
            buf_reader.consume(bytes_drained);

            Ok(bytes_drained as u64)
        }

        fn properties(&self) -> CopyParams {
            CopyParams(fd_to_meta(self), Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for StdoutLock<'_> {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
        }
    }

    impl CopyWrite for StderrLock<'_> {
        fn properties(&self) -> CopyParams {
            CopyParams(FdMeta::NoneObtained, Some(self.as_raw_fd()))
        }
    }

    impl<T: CopyRead> CopyRead for Take<T> {
        fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
            let local_limit = self.limit();
            let combined_limit = min(outer_limit, local_limit);
            let bytes_drained = self.get_mut().drain_to(writer, combined_limit)?;
            // update limit since read() was bypassed
            self.set_limit(local_limit - bytes_drained);

            Ok(bytes_drained)
        }

        fn taken(&mut self, bytes: u64) {
            self.set_limit(self.limit() - bytes);
            self.get_mut().taken(bytes);
        }

        fn min_limit(&self) -> u64 {
            min(Take::limit(self), self.get_ref().min_limit())
        }

        fn properties(&self) -> CopyParams {
            self.get_ref().properties()
        }
    }

    impl<T: CopyRead> CopyRead for BufReader<T> {
        fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
            let buf = self.buffer();
            let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
            let bytes = buf.len();
            writer.write_all(buf)?;
            self.consume(bytes);

            let remaining = outer_limit - bytes as u64;

            // in case of nested bufreaders we also need to drain the ones closer to the source
            let inner_bytes = self.get_mut().drain_to(writer, remaining)?;

            Ok(bytes as u64 + inner_bytes)
        }

        fn taken(&mut self, bytes: u64) {
            self.get_mut().taken(bytes);
        }

        fn min_limit(&self) -> u64 {
            self.get_ref().min_limit()
        }

        fn properties(&self) -> CopyParams {
            self.get_ref().properties()
        }
    }

    impl<T: CopyWrite> CopyWrite for BufWriter<T> {
        fn properties(&self) -> CopyParams {
            self.get_ref().properties()
        }
    }

    fn fd_to_meta<T: AsRawFd>(fd: &T) -> FdMeta {
        let fd = fd.as_raw_fd();
        let file: ManuallyDrop<File> = ManuallyDrop::new(unsafe { File::from_raw_fd(fd) });
        match file.metadata() {
            Ok(meta) => FdMeta::Metadata(meta),
            Err(_) => FdMeta::NoneObtained,
        }
    }

    pub(super) enum CopyResult {
        Ended(u64),
        Error(Error, u64),
        Fallback(u64),
    }

    impl CopyResult {
        fn update_take(&self, reader: &mut impl CopyRead) {
            match *self {
                CopyResult::Fallback(bytes)
                | CopyResult::Ended(bytes)
                | CopyResult::Error(_, bytes) => reader.taken(bytes),
            }
        }
    }

    /// Invalid file descriptor.
    ///
    /// Valid file descriptors are guaranteed to be positive numbers (see `open()` manpage)
    /// while negative values are used to indicate errors.
    /// Thus -1 will never be overlap with a valid open file.
    const INVALID_FD: RawFd = -1;

    /// Linux-specific implementation that will attempt to use copy_file_range for copy offloading.
    /// As the name says, it only works on regular files.
    ///
    /// Callers must handle fallback to a generic copy loop.
    /// `Fallback` may indicate non-zero number of bytes already written
    /// if one of the files' cursor +`max_len` would exceed u64::MAX (`EOVERFLOW`).
    pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) -> CopyResult {
        use crate::cmp;

        const NOT_PROBED: u8 = 0;
        const UNAVAILABLE: u8 = 1;
        const AVAILABLE: u8 = 2;

        // Kernel prior to 4.5 don't have copy_file_range
        // We store the availability in a global to avoid unnecessary syscalls
        static HAS_COPY_FILE_RANGE: AtomicU8 = AtomicU8::new(NOT_PROBED);

        syscall! {
            fn copy_file_range(
                fd_in: libc::c_int,
                off_in: *mut libc::loff_t,
                fd_out: libc::c_int,
                off_out: *mut libc::loff_t,
                len: libc::size_t,
                flags: libc::c_uint
            ) -> libc::ssize_t
        }

        match HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
            NOT_PROBED => {
                // EPERM can indicate seccomp filters or an immutable file.
                // To distinguish these cases we probe with invalid file descriptors which should result in EBADF if the syscall is supported
                // and some other error (ENOSYS or EPERM) if it's not available
                let result = unsafe {
                    cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
                };

                if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(EBADF))) {
                    HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
                } else {
                    HAS_COPY_FILE_RANGE.store(UNAVAILABLE, Ordering::Relaxed);
                    return CopyResult::Fallback(0);
                }
            }
            UNAVAILABLE => return CopyResult::Fallback(0),
            _ => {}
        };

        let mut written = 0u64;
        while written < max_len {
            let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64);
            // cap to 1GB chunks in case u64::MAX is passed as max_len and the file has a non-zero seek position
            // this allows us to copy large chunks without hitting EOVERFLOW,
            // unless someone sets a file offset close to u64::MAX - 1GB, in which case a fallback would be required
            let bytes_to_copy = cmp::min(bytes_to_copy as usize, 0x4000_0000usize);
            let copy_result = unsafe {
                // We actually don't have to adjust the offsets,
                // because copy_file_range adjusts the file offset automatically
                cvt(copy_file_range(reader, ptr::null_mut(), writer, ptr::null_mut(), bytes_to_copy, 0))
            };

            match copy_result {
                Ok(0) if written == 0 => {
                    // fallback to work around several kernel bugs where copy_file_range will fail to
                    // copy any bytes and return 0 instead of an error if
                    // - reading virtual files from the proc filesystem which appear to have 0 size
                    //   but are not empty. noted in coreutils to affect kernels at least up to 5.6.19.
                    // - copying from an overlay filesystem in docker. reported to occur on fedora 32.
                    return CopyResult::Fallback(0);
                }
                Ok(0) => return CopyResult::Ended(written), // reached EOF
                Ok(ret) => written += ret as u64,
                Err(err) => {
                    return match err.raw_os_error() {
                        // when file offset + max_length > u64::MAX
                        Some(EOVERFLOW) => CopyResult::Fallback(written),
                        Some(ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF) => {
                            // Try fallback io::copy if either:
                            // - Kernel version is < 4.5 (ENOSYS¹)
                            // - Files are mounted on different fs (EXDEV)
                            // - copy_file_range is broken in various ways on RHEL/CentOS 7 (EOPNOTSUPP)
                            // - copy_file_range file is immutable or syscall is blocked by seccomp¹ (EPERM)
                            // - copy_file_range cannot be used with pipes or device nodes (EINVAL)
                            // - the writer fd was opened with O_APPEND (EBADF²)
                            //
                            // ¹ these cases should be detected by the initial probe but we handle them here
                            //   anyway in case syscall interception changes during runtime
                            // ² actually invalid file descriptors would cause this too, but in that case
                            //   the fallback code path is expected to encounter the same error again
                            assert_eq!(written, 0);
                            CopyResult::Fallback(0)
                        }
                        _ => CopyResult::Error(err, written),
                    };
                }
            }
        }
        CopyResult::Ended(written)
    }

    #[derive(PartialEq)]
    enum SpliceMode {
        Sendfile,
        Splice,
    }

    /// performs splice or sendfile between file descriptors
    /// Does _not_ fall back to a generic copy loop.
    fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) -> CopyResult {
        static HAS_SENDFILE: AtomicBool = AtomicBool::new(true);
        static HAS_SPLICE: AtomicBool = AtomicBool::new(true);

        syscall! {
            fn splice(
                srcfd: libc::c_int,
                src_offset: *const i64,
                dstfd: libc::c_int,
                dst_offset: *const i64,
                len: libc::size_t,
                flags: libc::c_int
            ) -> libc::ssize_t
        }

        match mode {
            SpliceMode::Sendfile if !HAS_SENDFILE.load(Ordering::Relaxed) => {
                return CopyResult::Fallback(0);
            }
            SpliceMode::Splice if !HAS_SPLICE.load(Ordering::Relaxed) => {
                return CopyResult::Fallback(0);
            }
            _ => (),
        }

        let mut written = 0u64;
        while written < len {
            // according to its manpage that's the maximum size sendfile() will copy per invocation
            let chunk_size = crate::cmp::min(len - written, 0x7ffff000_u64) as usize;

            let result = match mode {
                SpliceMode::Sendfile => {
                    cvt(unsafe { libc::sendfile(writer, reader, ptr::null_mut(), chunk_size) })
                }
                SpliceMode::Splice => cvt(unsafe {
                    splice(reader, ptr::null_mut(), writer, ptr::null_mut(), chunk_size, 0)
                }),
            };

            match result {
                Ok(0) => break, // EOF
                Ok(ret) => written += ret as u64,
                Err(err) => {
                    return match err.raw_os_error() {
                        Some(ENOSYS | EPERM) => {
                            // syscall not supported (ENOSYS)
                            // syscall is disallowed, e.g. by seccomp (EPERM)
                            match mode {
                                SpliceMode::Sendfile => HAS_SENDFILE.store(false, Ordering::Relaxed),
                                SpliceMode::Splice => HAS_SPLICE.store(false, Ordering::Relaxed),
                            }
                            assert_eq!(written, 0);
                            CopyResult::Fallback(0)
                        }
                        Some(EINVAL) => {
                            // splice/sendfile do not support this particular file descriptor (EINVAL)
                            assert_eq!(written, 0);
                            CopyResult::Fallback(0)
                        }
                        Some(os_err) if mode == SpliceMode::Sendfile && os_err == EOVERFLOW => {
                            CopyResult::Fallback(written)
                        }
                        _ => CopyResult::Error(err, written),
                    };
                }
            }
        }
        CopyResult::Ended(written)
    }