perf: the file mapping cache was off by mistake. #1527

Author: nedbat

CHANGES.rst

@@ -20,7 +20,12 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
-Nothing yet.
+- Performance: an internal cache of file names was accidentally disabled,
+  resulting in sometimes drastic reductions in performance.  This is now fixed,
+  closing `issue 1527`_.   Thanks to Ivan Ciuvalschii for the reproducible test
+  case.
+
+.. _issue 1527: https://github.com/nedbat/coveragepy/issues/1527
 
 
 .. _changes_7-0-3:

CONTRIBUTORS.txt

@@ -81,6 +81,7 @@ Hugo van Kemenade
 Ilia Meerovich
 Imri Goldberg
 Ionel Cristian Mărieș
+Ivan Ciuvalschii
 J. M. F. Tsang
 JT Olds
 Jerin Peter George

coverage/collector.py

@@ -433,7 +433,7 @@ def disable_plugin(self, disposition: TFileDisposition) -> None:
         plugin._coverage_enabled = False
         disposition.trace = False
 
-    @functools.lru_cache(maxsize=0)
+    @functools.lru_cache(maxsize=None)
     def cached_mapped_file(self, filename: str) -> str:
         """A locally cached version of file names mapped through file_mapper."""
         return self.file_mapper(filename)