consistentread.go

/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package fsctl

import (
	"bytes"
	"fmt"
	"io/ioutil"
)

// ConsistentRead repeatedly reads a file until it gets the same content twice.
// This is useful when reading files in /proc that are larger than page size
// and kernel may modify them between individual read() syscalls.
func (f Fs) ConsistentRead(filename string, attempts int) ([]byte, error) {
	return consistentReadSync(filename, attempts, nil)
}

// consistentReadSync is the main functionality of ConsistentRead but
// introduces a sync callback that can be used by the tests to mutate the file
// from which the test data is being read
func consistentReadSync(filename string, attempts int, sync func(int)) ([]byte, error) {
	oldContent, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}
	for i := 0; i < attempts; i++ {
		if sync != nil {
			sync(i)
		}
		newContent, err := ioutil.ReadFile(filename)
		if err != nil {
			return nil, err
		}
		if bytes.Compare(oldContent, newContent) == 0 {
			return newContent, nil
		}
		// Files are different, continue reading
		oldContent = newContent
	}
	return nil, fmt.Errorf("could not get consistent content of %s after %d attempts", filename, attempts)
}