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main.go
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main.go
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/* This file is part of copyondemand.
*
* Copyright © 2020 Datto, Inc.
* Author: Bryan Ehrlich <behrlich@datto.com>
*
* Licensed under the Apache Software License, Version 2.0
* Fedora-License-Identifier: ASL 2.0
* SPDX-2.0-License-Identifier: Apache-2.0
* SPDX-3.0-License-Identifier: Apache-2.0
*
* copyondemand is free software.
* For more information on the license, see LICENSE.
* For more information on free software, see <https://www.gnu.org/philosophy/free-sw.en.html>.
*
* 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 main
import (
"fmt"
"os"
"os/signal"
"github.com/datto/copyondemand"
)
const (
nbdToUse = "/dev/nbd0"
exampleFileSize = 1024 * 1024 // 1 MB
)
func main() {
// Source files are immutable and we therefore only need to implement "ReadAt"
sourceFile := ©ondemand.SyncSource{
File: &oneFile{},
Size: exampleFileSize,
}
// You could overwrite the backing file behavior as well. This is out of scope for this example.
// For this example we will read files from our non-file source, but write them to a normal on-disk
// file called "backing.bak"
backingOsFile, err := os.OpenFile("./backing.bak", os.O_RDWR|os.O_CREATE, os.FileMode(int(0755)))
backingFile := ©ondemand.SyncFile{
File: backingOsFile,
Size: exampleFileSize,
}
if err != nil {
fmt.Printf("Could not open ./backing.bak: %s\n", err)
}
config := ©ondemand.DriverConfig{
Source: sourceFile,
Backing: backingFile,
NbdFileName: nbdToUse,
EnableBackgroundSync: false,
Resumable: false,
}
driver, err := copyondemand.New(config)
if err != nil {
fmt.Printf("Could not construct driver: %s\n", err)
}
errorChan := make(chan error)
sig := make(chan os.Signal)
signal.Notify(sig, os.Interrupt)
// Connect blocks, so we wrap it in a routine
go func(errorChan chan<- error) {
if err := driver.Connect(); err != nil {
errorChan <- err
} else {
fmt.Println("Driver stopped gracefully.")
}
}(errorChan)
select {
case <-sig:
// Received SIGINT, cancel the sync operation and clean up
break
case err := <-errorChan:
// Recieved an error, cancel the sync operation and clean up
fmt.Printf("Driver stopped with error: %s\n", err)
break
}
fmt.Println("Disconnecting...")
driver.Disconnect()
}
// oneFile implements a copyondemand.ReadOnlyFile that contains only ones
type oneFile struct {
copyondemand.ReadOnlyFile
}
// 0 indicates that this is not a traditional filesystem file, and therefore
// the COD driver shouldn't attempt to do out-of-band file operations.
// Namely - this non-file should not be scanned for sparseness with lseek.
func (f *oneFile) Fd() uintptr {
return 0
}
// Fill the requested buffer with ones, regardless of offset.
// Always report that we read the full buffer size.
func (f *oneFile) ReadAt(p []byte, off int64) (n int, err error) {
requestedReadSize := len(p)
for ix := range p {
p[ix] = 1
}
return requestedReadSize, nil
}