Test read_string_from_stream-performance

There is a saftey margin of a factor of 10 in both directions,
so the test should be fairly stable. Tests py-pdf#1350.

tests/test_generic.py

@@ -1,6 +1,7 @@
 import os
 from io import BytesIO
 from pathlib import Path
+import time
 from unittest.mock import patch
 
 import pytest
@@ -170,6 +171,19 @@ def test_readStringFromStream_excape_digit2():
  assert read_string_from_stream(stream) == "hello \x01\x02\x03\x04"
 
 
+def test_readStringFromStream_performance():
+ """
+ This test simulates reading an embedded base64 image of 1 megabyte. 
+ It should be faster than a second, even on ancient machines. 
+ Runs in 200ms on a 2019 notebook. Takes 30 seconds prior to #1350. 
+ """
+ stream = BytesIO(b"(" + b"".join([b"x"]*1024*1024) + b")")
+ start = time.time()
+ assert read_string_from_stream(stream)
+ end = time.time()
+ assert end-start < 2, test_readStringFromStream_performance.__doc__
+
+
 def test_NameObject(caplog):
  stream = BytesIO(b"x")
  with pytest.raises(PdfReadError) as exc: