private int readFromAppBuffer(ByteBuffer dst) {
        appReadBuffer.flip();
        int remaining = Math.min(appReadBuffer.remaining(), dst.remaining());
        if (remaining > 0) {
            int limit = appReadBuffer.limit();
            appReadBuffer.limit(appReadBuffer.position() + remaining);
            dst.put(appReadBuffer);
            appReadBuffer.limit(limit);
        }
        appReadBuffer.compact();
        return remaining;
    }
transfers appReadBuffer contents (decrypted data) into dst bytebuffer
@param dst ByteBuffer
clients/src/main/java/org/apache/kafka/common/network/SslTransportLayer.java