error reporting: fix resource warning for unclosed file (in Py3.2+), cache...

error reporting: fix resource warning for unclosed file (in Py3.2+), cache source lines when we read them more than once

Cython/Compiler/Errors.py

@@ -20,7 +20,7 @@ def context(position):
     assert not (isinstance(source, unicode) or isinstance(source, str)), (
         "Please replace filename strings with Scanning.FileSourceDescriptor instances %r" % source)
     try:
-        F = list(source.get_lines())
+        F = source.get_lines()
     except UnicodeDecodeError:
         # file has an encoding problem
         s = u"[unprintable code]\n"

Cython/Compiler/Scanning.py

@@ -172,13 +172,34 @@ class FileSourceDescriptor(SourceDescriptor):
         self.filename = filename
         self.set_file_type_from_name(filename)
         self._cmp_name = filename
+        self._lines = {}
 
     def get_lines(self, encoding=None, error_handling=None):
-        return Utils.open_source_file(
+        # we cache the lines only the second time this is called, in
+        # order to save memory when they are only used once
+        key = (encoding, error_handling)
+        try:
+            lines = self._lines[key]
+            if lines is not None:
+                return lines
+        except KeyError:
+            pass
+        f = Utils.open_source_file(
             self.filename, encoding=encoding,
             error_handling=error_handling,
             # newline normalisation is costly before Py2.6
             require_normalised_newlines=False)
+        try:
+            lines = list(f)
+        finally:
+            f.close()
+        if key in self._lines:
+            self._lines[key] = lines
+        else:
+            # do not cache the first access, but remember that we
+            # already read it once
+            self._lines[key] = None
+        return lines
 
     def get_description(self):
         return self.path_description