feat: Add migration for picture cache db

- Add migration (just delete db and change to new schema) for picture
  cache following the same sort of strategy as the file digest cache
- Rename mtime column to mtime_ns to match file cache for consistency

core/pe/cache_sqlite.py

@@ -15,6 +15,14 @@ from core.pe.cache import bytes_to_colors, colors_to_bytes
 class SqliteCache:
     """A class to cache picture blocks in a sqlite backend."""
 
+    schema_version = 1
+    schema_version_description = "Changed from string to bytes for blocks."
+
+    create_table_query = "CREATE TABLE IF NOT EXISTS pictures(path TEXT, mtime_ns INTEGER, blocks BLOB)"
+    create_index_query = "CREATE INDEX IF NOT EXISTS idx_path on pictures (path)"
+    drop_table_query = "DROP TABLE IF EXISTS pictures"
+    drop_index_query = "DROP INDEX IF EXISTS idx_path"
+
     def __init__(self, db=":memory:", readonly=False):
         # readonly is not used in the sqlite version of the cache
         self.dbname = db
@@ -62,9 +70,9 @@ class SqliteCache:
         else:
             mtime = 0
         if path_str in self:
-            sql = "update pictures set blocks = ?, mtime = ? where path = ?"
+            sql = "update pictures set blocks = ?, mtime_ns = ? where path = ?"
         else:
-            sql = "insert into pictures(blocks,mtime,path) values(?,?,?)"
+            sql = "insert into pictures(blocks,mtime_ns,path) values(?,?,?)"
         try:
             self.con.execute(sql, [blocks, mtime, path_str])
         except sqlite.OperationalError:
@@ -73,18 +81,9 @@ class SqliteCache:
             logging.warning("DatabaseError while setting value for key %r: %s", path_str, str(e))
 
     def _create_con(self, second_try=False):
-        def create_tables():
-            logging.debug("Creating picture cache tables.")
-            self.con.execute("drop table if exists pictures")
-            self.con.execute("drop index if exists idx_path")
-            self.con.execute("create table pictures(path TEXT, mtime INTEGER, blocks BLOB)")
-            self.con.execute("create index idx_path on pictures (path)")
-
-        self.con = sqlite.connect(self.dbname, isolation_level=None)
         try:
-            self.con.execute("select path, mtime, blocks from pictures where 1=2")
-        except sqlite.OperationalError:  # new db
-            create_tables()
+            self.con = sqlite.connect(self.dbname, isolation_level=None)
+            self._check_upgrade()
         except sqlite.DatabaseError as e:  # corrupted db
             if second_try:
                 raise  # Something really strange is happening
@@ -93,6 +92,25 @@ class SqliteCache:
             os.remove(self.dbname)
             self._create_con(second_try=True)
 
+    def _check_upgrade(self) -> None:
+        with self.con as conn:
+            has_schema = conn.execute(
+                "SELECT NAME FROM sqlite_master WHERE type='table' AND name='schema_version'"
+            ).fetchall()
+            version = None
+            if has_schema:
+                version = conn.execute("SELECT version FROM schema_version ORDER BY version DESC").fetchone()[0]
+            else:
+                conn.execute("CREATE TABLE schema_version (version int PRIMARY KEY, description TEXT)")
+            if version != self.schema_version:
+                conn.execute(self.drop_table_query)
+                conn.execute(
+                    "INSERT OR REPLACE INTO schema_version VALUES (:version, :description)",
+                    {"version": self.schema_version, "description": self.schema_version_description},
+                )
+            conn.execute(self.create_table_query)
+            conn.execute(self.create_index_query)
+
     def clear(self):
         self.close()
         if self.dbname != ":memory:":
@@ -129,12 +147,12 @@ class SqliteCache:
         the db.
         """
         todelete = []
-        sql = "select rowid, path, mtime from pictures"
+        sql = "select rowid, path, mtime_ns from pictures"
         cur = self.con.execute(sql)
-        for rowid, path_str, mtime in cur:
-            if mtime and op.exists(path_str):
+        for rowid, path_str, mtime_ns in cur:
+            if mtime_ns and op.exists(path_str):
                 picture_mtime = os.stat(path_str).st_mtime
-                if int(picture_mtime) <= mtime:
+                if int(picture_mtime) <= mtime_ns:
                     # not outdated
                     continue
             todelete.append(rowid)