copy_file_range_unix.go

Documentation: internal/poll

     1  // Copyright 2024 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build freebsd || linux
     6  
     7  package poll
     8  
     9  import "internal/syscall/unix"
    10  
    11  // CopyFileRange copies at most remain bytes of data from src to dst, using
    12  // the copy_file_range system call. dst and src must refer to regular files.
    13  func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
    14  	if !supportCopyFileRange() {
    15  		return 0, false, nil
    16  	}
    17  
    18  	for remain > 0 {
    19  		max := remain
    20  		if max > maxCopyFileRangeRound {
    21  			max = maxCopyFileRangeRound
    22  		}
    23  		n, e := copyFileRange(dst, src, int(max))
    24  		if n > 0 {
    25  			remain -= n
    26  			written += n
    27  		}
    28  		handled, err = handleCopyFileRangeErr(e, n, written)
    29  		if n == 0 || !handled || err != nil {
    30  			return
    31  		}
    32  	}
    33  
    34  	return written, true, nil
    35  }
    36  
    37  // copyFileRange performs one round of copy_file_range(2).
    38  func copyFileRange(dst, src *FD, max int) (written int64, err error) {
    39  	// For Linux, the signature of copy_file_range(2) is:
    40  	//
    41  	// ssize_t copy_file_range(int fd_in, loff_t *off_in,
    42  	//                         int fd_out, loff_t *off_out,
    43  	//                         size_t len, unsigned int flags);
    44  	//
    45  	// For FreeBSD, the signature of copy_file_range(2) is:
    46  	//
    47  	// ssize_t
    48  	// copy_file_range(int infd, off_t *inoffp, int outfd, off_t *outoffp,
    49  	//                 size_t len, unsigned int flags);
    50  	//
    51  	// Note that in the call to unix.CopyFileRange below, we use nil
    52  	// values for off_in/off_out and inoffp/outoffp, which means "the file
    53  	// offset for infd(fd_in) or outfd(fd_out) respectively will be used and
    54  	// updated by the number of bytes copied".
    55  	//
    56  	// That is why we must acquire locks for both file descriptors (and why
    57  	// this whole machinery is in the internal/poll package to begin with).
    58  	if err := dst.writeLock(); err != nil {
    59  		return 0, err
    60  	}
    61  	defer dst.writeUnlock()
    62  	if err := src.readLock(); err != nil {
    63  		return 0, err
    64  	}
    65  	defer src.readUnlock()
    66  	return ignoringEINTR2(func() (int64, error) {
    67  		n, err := unix.CopyFileRange(src.Sysfd, nil, dst.Sysfd, nil, max, 0)
    68  		return int64(n), err
    69  	})
    70  }
    71
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