fix: race condition on data file shouldn't break combining. #1522

CHANGES.rst

@@ -20,7 +20,14 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
-Nothing yet.
+- Fix: when using pytest-cov or pytest-xdist, or perhaps both, the combining
+  step could fail with ``assert row is not None`` using 7.0.2.  This was due to
+  a race condition that has always been possible and is still possible. In
+  7.0.1 and before, the error was silently swallowed by the combining code.
+  Now it will produce a message "Couldn't combine data file" and ignore the
+  data file as it used to do before 7.0.2.  Closes `issue 1522`_.
+
+.. _issue 1522: https://github.com/nedbat/coveragepy/issues/1522
 
 
 .. _changes_7-0-2:

coverage/data.py

@@ -161,6 +161,8 @@ def combine_parallel_data(
                     # The CoverageException has the file name in it, so just
                     # use the message as the warning.
                     data._warn(str(exc))
+                if message:
+                    message(f"Couldn't combine data file {rel_file_name}: {exc}")
                 delete_this_one = False
             else:
                 data.update(new_data, aliases=aliases)

coverage/sqldata.py

@@ -289,6 +289,7 @@ def _read_db(self) -> None:
         with self._dbs[threading.get_ident()] as db:
             try:
                 row = db.execute_one("select version from coverage_schema")
+                assert row is not None
             except Exception as exc:
                 if "no such table: coverage_schema" in str(exc):
                     self._init_db(db)
@@ -299,7 +300,6 @@ def _read_db(self) -> None:
                         )
                     ) from exc
             else:
-                assert row is not None
                 schema_version = row[0]
                 if schema_version != SCHEMA_VERSION:
                     raise DataError(