/*
 * 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.

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).

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.

/**
 * 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();
        }
    }
}