private static int readFromBuffer(ByteBuf buffer) {
        return buffer.isReadable() ? (buffer.readByte() & 0xFF) : -1;
    }
Closes this {@link InputStream} and releases any system resources associated with the stream.
Sets EOS, if it has not been set yet. Drains all the allocated buffers.
core/src/main/java/com/linecorp/armeria/internal/common/stream/ByteBufsInputStream.java