Added an abstract buffer class

What this is missing is a flush method.  It's not implemented because:

a. cppo is all we've got so far and it doesn't need write at all
b. I'm not 100% sure how I'm doing files yet
c. I'm hoping the way I ought to implement this will make itself apparent when
the code gets used.

blocksfree/buffer/buffertype.py

@@ -0,0 +1,115 @@
+# vim: set tabstop=4 shiftwidth=4 noexpandtab filetype=python:
+
+# Copyright (C) 2017  T. Joseph Carter
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by the
+# Free Software Foundation; either version 2 of the License, or (at your
+# option) any later version.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+# for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+from abc import ABCMeta, abstractmethod
+from typing import Optional
+
+# Rationale:
+#
+# The buffers we will work with most commonly represent 140k disk images.  For
+# that, it'd make a lot of sense to simply use a bytearray.  It's built in to
+# the language and pretty efficient for an interpreted OO construct.  But even
+# ProDOS disk images can be 32M in size.
+#
+# Well that's fine, because most systems have 1G or more of RAM, plus swap, you
+# might say.  And that's true, they do have that much RAM.  Only on an embedded
+# system like the Raspberry Pi, they may not have much or any swap.  And while
+# ProDOS volumes will never be larger than 32M, GS/OS supports other
+# filesystems
+#
+# HFS is even commonly used.  It's true that HFS support has not been something
+# you could expect from AppleCommander, to say nothing of cppo, but we ought to
+# consider the possibility for the future.  As of this writing, we cannot
+# really guarantee that your host operating system can handle HFS completely.
+# How do you access resource fork on Linux?  How do you access any of it on
+# Windows?  Can Apple OSes officially carrying the designation "macOS" even
+# open old HFS volumes read-only anymore?  They haven't had read-write access
+# for some time now.
+
+class BufferType(metaclass=ABCMeta):
+	"""Abstract class that describes a BufferType.
+
+	Read-only BufferType subclasses must implement read and __len__, as well as
+	provide an __init__ that establishes the buffer to be read.
+
+	Read-write subclasses will also implement write, locked, and changed.
+
+	If the size of the buffer may be changed (files rather than block devices),
+	subclasses would implement resize.
+	"""
+
+	def __enter__(self) -> 'BufferType':
+		return self
+
+	def __exit__(self, exc_type, exc_value, traceback) -> None:
+		pass
+
+	@abstractmethod
+	def __len__(self) -> int:
+		"""Return len(self)"""
+		pass
+
+	@property
+	def changed(self):
+		"""Returns True if buffer has been altered
+
+		Always returns false for read-only buffers
+		"""
+		return False
+
+	@abstractmethod
+	def read(
+			self,
+			start: int = 0,
+			count: Optional[int] = None,
+			limit: bool = True
+			) -> bytearray:
+		"""Return bytearray of count bytes from buffer beginning at start
+
+		Should raise IndexError if an attempt to read past the end of the
+		buffer is made.
+		"""
+		pass
+
+	def write(
+			self,
+			buf: bytes,
+			start: int,
+			count: Optional[int] = None,
+			limit: bool = True
+			) -> None:
+		"""Writes bytes to buffer beginning at start
+
+		Raises NotImplementedError unless implemented by subclass
+		"""
+		raise NotImplementedError('buffer does not support writing')
+
+	def resize(self, size: int) -> None:
+		"""Resize buffer to size
+
+		Raises NotImplementedError unless implemented by subclass
+		"""
+		raise NotImplementedError('buffer does not support writing')
+
+	@property
+	def locked(self) -> bool:
+		"""Returns True for read-only buffers.
+
+		Returns True unless writing is implemented by subclass
+		"""
+		return True