read.go

Documentation: k8s.io/utils/io

     1  /*
     2  Copyright 2017 The Kubernetes Authors.
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8      http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package io
    18  
    19  import (
    20  	"bytes"
    21  	"errors"
    22  	"fmt"
    23  	"io"
    24  	"io/ioutil"
    25  )
    26  
    27  // ErrLimitReached means that the read limit is reached.
    28  var ErrLimitReached = errors.New("the read limit is reached")
    29  
    30  // ConsistentRead repeatedly reads a file until it gets the same content twice.
    31  // This is useful when reading files in /proc that are larger than page size
    32  // and kernel may modify them between individual read() syscalls.
    33  // It returns InconsistentReadError when it cannot get a consistent read in
    34  // given nr. of attempts. Caller should retry, kernel is probably under heavy
    35  // mount/unmount load.
    36  func ConsistentRead(filename string, attempts int) ([]byte, error) {
    37  	return consistentReadSync(filename, attempts, nil)
    38  }
    39  
    40  // consistentReadSync is the main functionality of ConsistentRead but
    41  // introduces a sync callback that can be used by the tests to mutate the file
    42  // from which the test data is being read
    43  func consistentReadSync(filename string, attempts int, sync func(int)) ([]byte, error) {
    44  	oldContent, err := ioutil.ReadFile(filename)
    45  	if err != nil {
    46  		return nil, err
    47  	}
    48  	for i := 0; i < attempts; i++ {
    49  		if sync != nil {
    50  			sync(i)
    51  		}
    52  		newContent, err := ioutil.ReadFile(filename)
    53  		if err != nil {
    54  			return nil, err
    55  		}
    56  		if bytes.Compare(oldContent, newContent) == 0 {
    57  			return newContent, nil
    58  		}
    59  		// Files are different, continue reading
    60  		oldContent = newContent
    61  	}
    62  	return nil, InconsistentReadError{filename, attempts}
    63  }
    64  
    65  // InconsistentReadError is returned from ConsistentRead when it cannot get
    66  // a consistent read in given nr. of attempts. Caller should retry, kernel is
    67  // probably under heavy mount/unmount load.
    68  type InconsistentReadError struct {
    69  	filename string
    70  	attempts int
    71  }
    72  
    73  func (i InconsistentReadError) Error() string {
    74  	return fmt.Sprintf("could not get consistent content of %s after %d attempts", i.filename, i.attempts)
    75  }
    76  
    77  var _ error = InconsistentReadError{}
    78  
    79  func IsInconsistentReadError(err error) bool {
    80  	if _, ok := err.(InconsistentReadError); ok {
    81  		return true
    82  	}
    83  	return false
    84  }
    85  
    86  // ReadAtMost reads up to `limit` bytes from `r`, and reports an error
    87  // when `limit` bytes are read.
    88  func ReadAtMost(r io.Reader, limit int64) ([]byte, error) {
    89  	limitedReader := &io.LimitedReader{R: r, N: limit}
    90  	data, err := ioutil.ReadAll(limitedReader)
    91  	if err != nil {
    92  		return data, err
    93  	}
    94  	if limitedReader.N <= 0 {
    95  		return data, ErrLimitReached
    96  	}
    97  	return data, nil
    98  }
    99
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