refactor BackupOperation a bit, make more illegal states explicit

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/CountingOutputStream.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2007 The Guava Authors
+ *
+ * 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 org.sufficientlysecure.keychain.util;
+
+
+import java.io.FilterOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+
+import android.support.annotation.NonNull;
+
+
+public final class CountingOutputStream extends FilterOutputStream {
+
+    private long count;
+
+    /**
+     * Wraps another output stream, counting the number of bytes written.
+     *
+     * @param out
+     *         the output stream to be wrapped
+     */
+    public CountingOutputStream(@NonNull OutputStream out) {
+        super(out);
+    }
+
+    /**
+     * Returns the number of bytes written.
+     */
+    public long getCount() {
+        return count;
+    }
+
+    @Override
+    public void write(@NonNull byte[] b, int off, int len) throws IOException {
+        out.write(b, off, len);
+        count += len;
+    }
+
+    @Override
+    public void write(int b) throws IOException {
+        out.write(b);
+        count++;
+    }
+
+    // Overriding close() because FilterOutputStream's close() method pre-JDK8 has bad behavior:
+    // it silently ignores any exception thrown by flush(). Instead, just close the delegate stream.
+    // It should flush itself if necessary.
+    @Override
+    public void close() throws IOException {
+        out.close();
+    }
+}