read_bytes(): use previous implementation again for small reads

For small reads, the new code that tries to avoid unnecessary reads is
noticeably slower than the previous code that reads unconditionally. In
the worst case (1-byte reads), the new code is 13 times as slow as the
previous implementation. The potential memory/IO savings only become
worth it for larger reads, where the performance difference disappears.

Co-authored-by: Petr Pucil <petr.pucil@seznam.cz>

Author: dgelessus

kaitaistruct.py

@@ -301,9 +301,18 @@ def read_bytes(self, n):
             )
 
         is_satisfiable = True
-        # in Python 2, there is a common error ['file' object has no
-        # attribute 'seekable'], so we need to make sure that seekable() exists
-        if callable(getattr(self._io, 'seekable', None)) and self._io.seekable():
+        # When a large number of bytes is requested, try to check first
+        # that there is indeed enough data left in the stream.
+        # This avoids reading large amounts of data only to notice afterwards
+        # that it's not long enough. For smaller amounts of data, it's faster to
+        # first read the data unconditionally and check the length afterwards.
+        if (
+            n >= 8*1024*1024  # = 8 MiB
+            # in Python 2, there is a common error ['file' object has no
+            # attribute 'seekable'], so we need to make sure that seekable() exists
+            and callable(getattr(self._io, 'seekable', None))
+            and self._io.seekable()
+        ):
             num_bytes_available = self.size() - self.pos()
             is_satisfiable = (n <= num_bytes_available)