github.com/coocood/badger@v1.5.1-0.20200528065104-c02ac3616d04/dir_unix.go (about)

     1  // +build !windows
     2  
     3  /*
     4   * Copyright 2017 Dgraph Labs, Inc. and Contributors
     5   *
     6   * Licensed under the Apache License, Version 2.0 (the "License");
     7   * you may not use this file except in compliance with the License.
     8   * You may obtain a copy of the License at
     9   *
    10   *     http://www.apache.org/licenses/LICENSE-2.0
    11   *
    12   * Unless required by applicable law or agreed to in writing, software
    13   * distributed under the License is distributed on an "AS IS" BASIS,
    14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    15   * See the License for the specific language governing permissions and
    16   * limitations under the License.
    17   */
    18  
    19  package badger
    20  
    21  import (
    22  	"fmt"
    23  	"io/ioutil"
    24  	"os"
    25  	"path/filepath"
    26  
    27  	"github.com/pingcap/errors"
    28  	"golang.org/x/sys/unix"
    29  )
    30  
    31  // directoryLockGuard holds a lock on a directory and a pid file inside.  The pid file isn't part
    32  // of the locking mechanism, it's just advisory.
    33  type directoryLockGuard struct {
    34  	// File handle on the directory, which we've flocked.
    35  	f *os.File
    36  	// The absolute path to our pid file.
    37  	path string
    38  	// Was this a shared lock for a read-only database?
    39  	readOnly bool
    40  }
    41  
    42  // acquireDirectoryLock gets a lock on the directory (using flock). If
    43  // this is not read-only, it will also write our pid to
    44  // dirPath/pidFileName for convenience.
    45  func acquireDirectoryLock(dirPath string, pidFileName string, readOnly bool) (*directoryLockGuard, error) {
    46  	// Convert to absolute path so that Release still works even if we do an unbalanced
    47  	// chdir in the meantime.
    48  	absPidFilePath, err := filepath.Abs(filepath.Join(dirPath, pidFileName))
    49  	if err != nil {
    50  		return nil, errors.Wrap(err, "cannot get absolute path for pid lock file")
    51  	}
    52  	f, err := os.Open(dirPath)
    53  	if err != nil {
    54  		return nil, errors.Wrapf(err, "cannot open directory %q", dirPath)
    55  	}
    56  	opts := unix.LOCK_EX | unix.LOCK_NB
    57  	if readOnly {
    58  		opts = unix.LOCK_SH | unix.LOCK_NB
    59  	}
    60  
    61  	err = unix.Flock(int(f.Fd()), opts)
    62  	if err != nil {
    63  		f.Close()
    64  		return nil, errors.Wrapf(err,
    65  			"Cannot acquire directory lock on %q.  Another process is using this Badger database.",
    66  			dirPath)
    67  	}
    68  
    69  	if !readOnly {
    70  		// Yes, we happily overwrite a pre-existing pid file.  We're the
    71  		// only read-write badger process using this directory.
    72  		err = ioutil.WriteFile(absPidFilePath, []byte(fmt.Sprintf("%d\n", os.Getpid())), 0666)
    73  		if err != nil {
    74  			f.Close()
    75  			return nil, errors.Wrapf(err,
    76  				"Cannot write pid file %q", absPidFilePath)
    77  		}
    78  	}
    79  	return &directoryLockGuard{f, absPidFilePath, readOnly}, nil
    80  }
    81  
    82  // Release deletes the pid file and releases our lock on the directory.
    83  func (guard *directoryLockGuard) release() error {
    84  	var err error
    85  	if !guard.readOnly {
    86  		// It's important that we remove the pid file first.
    87  		err = os.Remove(guard.path)
    88  	}
    89  
    90  	if closeErr := guard.f.Close(); err == nil {
    91  		err = closeErr
    92  	}
    93  	guard.path = ""
    94  	guard.f = nil
    95  
    96  	return err
    97  }
    98  
    99  // openDir opens a directory for syncing.
   100  func openDir(path string) (*os.File, error) { return os.Open(path) }