Initial commit

pgp-cert-d-java/src/main/java/pgp/cert_d/FileLockingMechanism.java

@@ -0,0 +1,96 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.nio.channels.FileLock;
+import java.nio.channels.OverlappingFileLockException;
+
+public class FileLockingMechanism implements LockingMechanism {
+
+    private final File lockFile;
+    private RandomAccessFile randomAccessFile;
+    private FileLock fileLock;
+
+    public FileLockingMechanism(File lockFile) {
+        this.lockFile = lockFile;
+    }
+
+    public static FileLockingMechanism defaultDirectoryFileLock(File baseDirectory) {
+        return new FileLockingMechanism(new File(baseDirectory, "writelock"));
+    }
+
+    @Override
+    public synchronized void lockDirectory() throws IOException, InterruptedException {
+        if (randomAccessFile != null) {
+            // we own the lock already. Let's wait...
+            this.wait();
+        }
+
+        try {
+            randomAccessFile = new RandomAccessFile(lockFile, "rw");
+        } catch (FileNotFoundException e) {
+            lockFile.createNewFile();
+            randomAccessFile = new RandomAccessFile(lockFile, "rw");
+        }
+
+        fileLock = randomAccessFile.getChannel().lock();
+    }
+
+    @Override
+    public synchronized boolean tryLockDirectory() throws IOException {
+        if (randomAccessFile != null) {
+            // We already locked the directory for another write operation.
+            // We fail, since we have not yet released the lock from the other operation.
+            return false;
+        }
+
+        try {
+            randomAccessFile = new RandomAccessFile(lockFile, "rw");
+        } catch (FileNotFoundException e) {
+            lockFile.createNewFile();
+            randomAccessFile = new RandomAccessFile(lockFile, "rw");
+        }
+
+        try {
+            fileLock = randomAccessFile.getChannel().tryLock();
+            if (fileLock == null) {
+                // try-lock failed, file is locked by another process.
+                randomAccessFile.close();
+                randomAccessFile = null;
+                return false;
+            }
+        } catch (OverlappingFileLockException e) {
+            // Some other object is holding the lock.
+            randomAccessFile.close();
+            randomAccessFile = null;
+            return false;
+        }
+        return true;
+    }
+
+    @Override
+    public synchronized void releaseDirectory() throws IOException {
+        // unlock file
+        if (fileLock != null) {
+            fileLock.release();
+            fileLock = null;
+        }
+        // close file
+        if (randomAccessFile != null) {
+            randomAccessFile.close();
+            randomAccessFile = null;
+        }
+        // delete file
+        if (lockFile.exists()) {
+            lockFile.delete();
+        }
+        // notify waiters
+        this.notify();
+    }
+}