core/java/com/android/internal/os/AtomicFile.java

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * 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 com.android.internal.os;

import android.os.FileUtils;
import android.util.Log;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * Helper class for performing atomic operations on a file, by creating a
 * backup file until a write has successfully completed.
 * <p>
 * Atomic file guarantees file integrity by ensuring that a file has
 * been completely written and sync'd to disk before removing its backup.
 * As long as the backup file exists, the original file is considered
 * to be invalid (left over from a previous attempt to write the file).
 * </p><p>
 * Atomic file does not confer any file locking semantics.
 * Do not use this class when the file may be accessed or modified concurrently
 * by multiple threads or processes.  The caller is responsible for ensuring
 * appropriate mutual exclusion invariants whenever it accesses the file.
 * </p>
 */
public final class AtomicFile {
    private final File mBaseName;
    private final File mBackupName;
    
    public AtomicFile(File baseName) {
        mBaseName = baseName;
        mBackupName = new File(baseName.getPath() + ".bak");
    }
    
    public File getBaseFile() {
        return mBaseName;
    }
    
    public FileOutputStream startWrite() throws IOException {
        // Rename the current file so it may be used as a backup during the next read
        if (mBaseName.exists()) {
            if (!mBackupName.exists()) {
                if (!mBaseName.renameTo(mBackupName)) {
                    Log.w("AtomicFile", "Couldn't rename file " + mBaseName
                            + " to backup file " + mBackupName);
                }
            } else {
                mBaseName.delete();
            }
        }
        FileOutputStream str = null;
        try {
            str = new FileOutputStream(mBaseName);
        } catch (FileNotFoundException e) {
            File parent = mBaseName.getParentFile();
            if (!parent.mkdir()) {
                throw new IOException("Couldn't create directory " + mBaseName);
            }
            FileUtils.setPermissions(
                parent.getPath(),
                FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH,
                -1, -1);
            try {
                str = new FileOutputStream(mBaseName);
            } catch (FileNotFoundException e2) {
                throw new IOException("Couldn't create " + mBaseName);
            }
        }
        return str;
    }
    
    public void finishWrite(FileOutputStream str) {
        if (str != null) {
            FileUtils.sync(str);
            try {
                str.close();
                mBackupName.delete();
            } catch (IOException e) {
                Log.w("AtomicFile", "finishWrite: Got exception:", e);
            }
        }
    }
    
    public void failWrite(FileOutputStream str) {
        if (str != null) {
            FileUtils.sync(str);
            try {
                str.close();
                mBaseName.delete();
                mBackupName.renameTo(mBaseName);
            } catch (IOException e) {
                Log.w("AtomicFile", "failWrite: Got exception:", e);
            }
        }
    }
    
    public FileOutputStream openAppend() throws IOException {
        try {
            return new FileOutputStream(mBaseName, true);
        } catch (FileNotFoundException e) {
            throw new IOException("Couldn't append " + mBaseName);
        }
    }
    
    public void truncate() throws IOException {
        try {
            FileOutputStream fos = new FileOutputStream(mBaseName);
            FileUtils.sync(fos);
            fos.close();
        } catch (FileNotFoundException e) {
            throw new IOException("Couldn't append " + mBaseName);
        } catch (IOException e) {
        }
    }

    public boolean exists() {
        return mBaseName.exists() || mBackupName.exists();
    }

    public void delete() {
        mBaseName.delete();
        mBackupName.delete();
    }

    public FileInputStream openRead() throws FileNotFoundException {
        if (mBackupName.exists()) {
            mBaseName.delete();
            mBackupName.renameTo(mBaseName);
        }
        return new FileInputStream(mBaseName);
    }
    
    public byte[] readFully() throws IOException {
        FileInputStream stream = openRead();
        try {
            int pos = 0;
            int avail = stream.available();
            byte[] data = new byte[avail];
            while (true) {
                int amt = stream.read(data, pos, data.length-pos);
                //Log.i("foo", "Read " + amt + " bytes at " + pos
                //        + " of avail " + data.length);
                if (amt <= 0) {
                    //Log.i("foo", "**** FINISHED READING: pos=" + pos
                    //        + " len=" + data.length);
                    return data;
                }
                pos += amt;
                avail = stream.available();
                if (avail > data.length-pos) {
                    byte[] newData = new byte[pos+avail];
                    System.arraycopy(data, 0, newData, 0, pos);
                    data = newData;
                }
            }
        } finally {
            stream.close();
        }
    }
}