Workaround issue with relpath on Windows. nedbat#895

CHANGES.rst

@@ -29,7 +29,11 @@ Unreleased
   process' file with no suffix when using ``--append``.  This is now fixed,
   closing `issue 880`_.
 
+- Fixed a problem on Windows when the current directory is changed to a
+  different drive (`issue 895`_).  Thanks, Olivier Grisel.
+
 .. _issue 880: https://github.com/nedbat/coveragepy/issues/880
+.. _issue 895: https://github.com/nedbat/coveragepy/issues/895
 
 
 .. _changes_50:

coverage/sqldata.py

@@ -979,7 +979,14 @@ def _connect(self):
         # SQLite on Windows on py2 won't open a file if the filename argument
         # has non-ascii characters in it.  Opening a relative file name avoids
         # a problem if the current directory has non-ascii.
-        filename = os.path.relpath(self.filename)
+        try:
+            filename = os.path.relpath(self.filename)
+        except ValueError:
+            # ValueError can be raised under Windows when os.getcwd() returns a
+            # folder from a different drive than the drive of self.filename in
+            # which case we keep the original value of self.filename unchanged,
+            # hoping that we won't face the non-ascii directory problem.
+            filename = self.filename
         # It can happen that Python switches threads while the tracer writes
         # data. The second thread will also try to write to the data,
         # effectively causing a nested context. However, given the idempotent