Detect when a 4.x data file is being read. #886

1 files changed, 14 insertions, 1 deletions

diff --git a/coverage/sqldata.py b/coverage/sqldata.py
index 01f5ce01..613a280c 100644
--- a/coverage/sqldata.py
+++ b/coverage/sqldata.py
@@ -1031,7 +1031,20 @@ class SqliteDb(SimpleReprMixin):
         try:
             return self.con.execute(sql, parameters)
         except sqlite3.Error as exc:
-            raise CoverageException("Couldn't use data file {!r}: {}".format(self.filename, exc))
+            msg = str(exc)
+            try:
+                # `execute` is the first thing we do with the database, so try
+                # hard to provide useful hints if something goes wrong now.
+                with open(self.filename, "rb") as bad_file:
+                    cov4_sig = b"!coverage.py: This is a private format"
+                    if bad_file.read(len(cov4_sig)) == cov4_sig:
+                        msg = (
+                            "Looks like a coverage 4.x data file. "
+                            "Are you mixing versions of coverage?"
+                        )
+            except Exception:
+                pass
+            raise CoverageException("Couldn't use data file {!r}: {}".format(self.filename, msg))
 
     def executemany(self, sql, data):
         """Same as :meth:`python:sqlite3.Connection.executemany`."""