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dir_plan9.go
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/
dir_plan9.go
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/*
* Copyright 2020 Dgraph Labs, Inc. and Contributors
*
* 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 badger
import (
"fmt"
"os"
"path/filepath"
"strings"
"github.com/dgraph-io/badger/v4/y"
)
// directoryLockGuard holds a lock on a directory and a pid file inside. The pid file isn't part
// of the locking mechanism, it's just advisory.
type directoryLockGuard struct {
// File handle on the directory, which we've locked.
f *os.File
// The absolute path to our pid file.
path string
}
// acquireDirectoryLock gets a lock on the directory.
// It will also write our pid to dirPath/pidFileName for convenience.
// readOnly is not supported on Plan 9.
func acquireDirectoryLock(dirPath string, pidFileName string, readOnly bool) (
*directoryLockGuard, error) {
if readOnly {
return nil, ErrPlan9NotSupported
}
// Convert to absolute path so that Release still works even if we do an unbalanced
// chdir in the meantime.
absPidFilePath, err := filepath.Abs(filepath.Join(dirPath, pidFileName))
if err != nil {
return nil, y.Wrap(err, "cannot get absolute path for pid lock file")
}
// If the file was unpacked or created by some other program, it might not
// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
// can be confident that a successful OpenFile implies exclusive use.
//
// OpenFile fails if the file ModeExclusive bit set *and* the file is already open.
// So, if the file is closed when the DB crashed, we're fine. When the process
// that was managing the DB crashes, the OS will close the file for us.
//
// This bit of code is copied from Go's lockedfile internal package:
// https://github.com/golang/go/blob/go1.15rc1/src/cmd/go/internal/lockedfile/lockedfile_plan9.go#L58
if fi, err := os.Stat(absPidFilePath); err == nil {
if fi.Mode()&os.ModeExclusive == 0 {
if err := os.Chmod(absPidFilePath, fi.Mode()|os.ModeExclusive); err != nil {
return nil, y.Wrapf(err, "could not set exclusive mode bit")
}
}
} else if !os.IsNotExist(err) {
return nil, err
}
f, err := os.OpenFile(absPidFilePath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0666|os.ModeExclusive)
if err != nil {
if isLocked(err) {
return nil, y.Wrapf(err,
"Cannot open pid lock file %q. Another process is using this Badger database",
absPidFilePath)
}
return nil, y.Wrapf(err, "Cannot open pid lock file %q", absPidFilePath)
}
if _, err = fmt.Fprintf(f, "%d\n", os.Getpid()); err != nil {
f.Close()
return nil, y.Wrapf(err, "could not write pid")
}
return &directoryLockGuard{f, absPidFilePath}, nil
}
// Release deletes the pid file and releases our lock on the directory.
func (guard *directoryLockGuard) release() error {
// It's important that we remove the pid file first.
err := os.Remove(guard.path)
if closeErr := guard.f.Close(); err == nil {
err = closeErr
}
guard.path = ""
guard.f = nil
return err
}
// openDir opens a directory for syncing.
func openDir(path string) (*os.File, error) { return os.Open(path) }
// When you create or delete a file, you have to ensure the directory entry for the file is synced
// in order to guarantee the file is visible (if the system crashes). (See the man page for fsync,
// or see https://github.com/coreos/etcd/issues/6368 for an example.)
func syncDir(dir string) error {
f, err := openDir(dir)
if err != nil {
return y.Wrapf(err, "While opening directory: %s.", dir)
}
err = f.Sync()
closeErr := f.Close()
if err != nil {
return y.Wrapf(err, "While syncing directory: %s.", dir)
}
return y.Wrapf(closeErr, "While closing directory: %s.", dir)
}
// Opening an exclusive-use file returns an error.
// The expected error strings are:
//
// - "open/create -- file is locked" (cwfs, kfs)
// - "exclusive lock" (fossil)
// - "exclusive use file already open" (ramfs)
//
// See https://github.com/golang/go/blob/go1.15rc1/src/cmd/go/internal/lockedfile/lockedfile_plan9.go#L16
var lockedErrStrings = [...]string{
"file is locked",
"exclusive lock",
"exclusive use file already open",
}
// Even though plan9 doesn't support the Lock/RLock/Unlock functions to
// manipulate already-open files, IsLocked is still meaningful: os.OpenFile
// itself may return errors that indicate that a file with the ModeExclusive bit
// set is already open.
func isLocked(err error) bool {
s := err.Error()
for _, frag := range lockedErrStrings {
if strings.Contains(s, frag) {
return true
}
}
return false
}