Merge implementations of read_exact functions/methods

The old functions/methods still exist, so that they continue to raise
the same exceptions as before (which are different depending on
context), but they now use the same implementation internally.

rsrcfork/_io_utils.py

@@ -0,0 +1,18 @@
+"""A collection of utility functions and classes related to IO streams. For internal use only."""
+
+import typing
+
+
+def read_exact(stream: typing.BinaryIO, byte_count: int) -> bytes:
+	"""Read byte_count bytes from the stream and raise an exception if too few bytes are read (i. e. if EOF was hit prematurely).
+	
+	:param stream: The stream to read from.
+	:param byte_count: The number of bytes to read.
+	:return: The read data, which is exactly ``byte_count`` bytes long.
+	:raise EOFError: If not enough data could be read from the stream.
+	"""
+	
+	data = stream.read(byte_count)
+	if len(data) != byte_count:
+		raise EOFError(f"Attempted to read {byte_count} bytes of data, but only got {len(data)} bytes")
+	return data

rsrcfork/api.py

@@ -8,6 +8,7 @@ import types
 import typing
 import warnings
 
+from . import _io_utils
 from . import compress
 
 # The formats of all following structures is as described in the Inside Macintosh book (see module docstring).
@@ -393,10 +394,10 @@ class ResourceFile(typing.Mapping[bytes, typing.Mapping[int, Resource]], typing.
 	def _read_exact(self, byte_count: int) -> bytes:
 		"""Read byte_count bytes from the stream and raise an exception if too few bytes are read (i. e. if EOF was hit prematurely)."""
 		
-		data = self._stream.read(byte_count)
-		if len(data) != byte_count:
-			raise InvalidResourceFileError(f"Attempted to read {byte_count} bytes of data, but only got {len(data)} bytes")
-		return data
+		try:
+			return _io_utils.read_exact(self._stream, byte_count)
+		except EOFError as e:
+			raise InvalidResourceFileError(str(e))
 	
 	def _stream_unpack(self, st: struct.Struct) -> tuple:
 		"""Unpack data from the stream according to the struct st. The number of bytes to read is determined using st.size, so variable-sized structs cannot be used with this method."""

rsrcfork/compress/common.py

@@ -2,6 +2,8 @@ import io
 import struct
 import typing
 
+from .. import _io_utils
+
 
 class DecompressError(Exception):
 	"""Raised when resource data decompression fails, because the data is invalid or the compression type is not supported."""
@@ -182,10 +184,10 @@ def make_peekable(stream: typing.BinaryIO) -> "PeekableIO":
 def read_exact(stream: typing.BinaryIO, byte_count: int) -> bytes:
 	"""Read byte_count bytes from the stream and raise an exception if too few bytes are read (i. e. if EOF was hit prematurely)."""
 	
-	data = stream.read(byte_count)
-	if len(data) != byte_count:
-		raise DecompressError(f"Attempted to read {byte_count} bytes of data, but only got {len(data)} bytes")
-	return data
+	try:
+		return _io_utils.read_exact(stream, byte_count)
+	except EOFError as e:
+		raise DecompressError(str(e))
 
 
 def read_variable_length_integer(stream: typing.BinaryIO) -> int: