diff --git a/python/subunit/v2.py b/python/subunit/v2.py
index c2c63f66..e8a31d69 100644
--- a/python/subunit/v2.py
+++ b/python/subunit/v2.py
@@ -72,13 +72,32 @@ def has_nul(buffer_or_bytes):
         return NUL_ELEMENT in buffer_or_bytes
 
 
+def read_exactly(stream, size):
+    """Read exactly size bytes from stream.
+
+    :param stream: A file like object to read bytes from. Must support
+        read(<count>) and return bytes.
+    :param size: The number of bytes to retrieve.
+    """
+    data = b''
+    remaining = size
+    while remaining:
+        read = stream.read(remaining)
+        if len(read) == 0:
+            raise ParseError('Short read - got %d bytes, wanted %d bytes' % (
+                len(data), size))
+        data += read
+        remaining -= len(read)
+    return data
+
+
 class ParseError(Exception):
     """Used to pass error messages within the parser."""
 
 
 class StreamResultToBytes(object):
     """Convert StreamResult API calls to bytes.
-    
+
     The StreamResult API is defined by testtools.StreamResult.
     """
 
@@ -276,7 +295,7 @@ def __init__(self, source, non_subunit_name=None):
 
     def run(self, result):
         """Parse source and emit events to result.
-        
+
         This is a blocking call: it will run until EOF is detected on source.
         """
         self.codec.reset()
@@ -406,21 +425,12 @@ def _parse_varint(self, data, pos, max_3_bytes=False):
 
     def _parse(self, packet, result):
         # 2 bytes flags, at most 3 bytes length.
-        packet.append(self.source.read(5))
-        if len(packet[-1]) != 5:
-            raise ParseError(
-                'Short read - got %d bytes, wanted 5' % len(packet[-1]))
-
-        flag_bytes = packet[-1][:2]
-        flags = struct.unpack(FMT_16, flag_bytes)[0]
-        length, consumed = self._parse_varint(
-            packet[-1], 2, max_3_bytes=True)
-        remainder = self.source.read(length - 6)
-        if len(remainder) != length - 6:
-            raise ParseError(
-                'Short read - got %d bytes, wanted %d bytes' % (
-                    len(remainder), length - 6))
+        header = read_exactly(self.source, 5)
+        packet.append(header)
+        flags = struct.unpack(FMT_16, header[:2])[0]
+        length, consumed = self._parse_varint(header, 2, max_3_bytes=True)
 
+        remainder = read_exactly(self.source, length - 6)
         if consumed != 3:
             # Avoid having to parse torn values
             packet[-1] += remainder
@@ -533,4 +543,3 @@ def _read_utf8(self, buf, pos):
             return utf8, length+pos
         except UnicodeDecodeError:
             raise ParseError('UTF8 string at offset %d is not UTF8' % (pos-2,))
-