dtype_linux.go

Documentation: github.com/containerd/continuity/fs

     1  //go:build linux
     2  // +build linux
     3  
     4  /*
     5     Copyright The containerd Authors.
     6  
     7     Licensed under the Apache License, Version 2.0 (the "License");
     8     you may not use this file except in compliance with the License.
     9     You may obtain a copy of the License at
    10  
    11         http://www.apache.org/licenses/LICENSE-2.0
    12  
    13     Unless required by applicable law or agreed to in writing, software
    14     distributed under the License is distributed on an "AS IS" BASIS,
    15     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    16     See the License for the specific language governing permissions and
    17     limitations under the License.
    18  */
    19  
    20  package fs
    21  
    22  import (
    23  	"fmt"
    24  	"os"
    25  	"syscall"
    26  	"unsafe"
    27  )
    28  
    29  func locateDummyIfEmpty(path string) (string, error) {
    30  	children, err := os.ReadDir(path)
    31  	if err != nil {
    32  		return "", err
    33  	}
    34  	if len(children) != 0 {
    35  		return "", nil
    36  	}
    37  	dummyFile, err := os.CreateTemp(path, "fsutils-dummy")
    38  	if err != nil {
    39  		return "", err
    40  	}
    41  	name := dummyFile.Name()
    42  	err = dummyFile.Close()
    43  	return name, err
    44  }
    45  
    46  // SupportsDType returns whether the filesystem mounted on path supports d_type
    47  func SupportsDType(path string) (bool, error) {
    48  	// locate dummy so that we have at least one dirent
    49  	dummy, err := locateDummyIfEmpty(path)
    50  	if err != nil {
    51  		return false, err
    52  	}
    53  	if dummy != "" {
    54  		defer os.Remove(dummy)
    55  	}
    56  
    57  	visited := 0
    58  	supportsDType := true
    59  	fn := func(ent *syscall.Dirent) bool {
    60  		visited++
    61  		if ent.Type == syscall.DT_UNKNOWN {
    62  			supportsDType = false
    63  			// stop iteration
    64  			return true
    65  		}
    66  		// continue iteration
    67  		return false
    68  	}
    69  	if err = iterateReadDir(path, fn); err != nil {
    70  		return false, err
    71  	}
    72  	if visited == 0 {
    73  		return false, fmt.Errorf("did not hit any dirent during iteration %s", path)
    74  	}
    75  	return supportsDType, nil
    76  }
    77  
    78  func iterateReadDir(path string, fn func(*syscall.Dirent) bool) error {
    79  	d, err := os.Open(path)
    80  	if err != nil {
    81  		return err
    82  	}
    83  	defer d.Close()
    84  	fd := int(d.Fd())
    85  	buf := make([]byte, 4096)
    86  	for {
    87  		nbytes, err := syscall.ReadDirent(fd, buf)
    88  		if err != nil {
    89  			return err
    90  		}
    91  		if nbytes == 0 {
    92  			break
    93  		}
    94  		for off := 0; off < nbytes; {
    95  			ent := (*syscall.Dirent)(unsafe.Pointer(&buf[off]))
    96  			if stop := fn(ent); stop {
    97  				return nil
    98  			}
    99  			off += int(ent.Reclen)
   100  		}
   101  	}
   102  	return nil
   103  }
   104
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