F cache version

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src/caosadvancedtools/cache.py

@@ -50,6 +50,8 @@ def get_pretty_xml(cont):
     return etree.tounicode(cont.to_xml(
         local_serialization=True), pretty_print=True)
 
+# Increase this, when changes to the cache tables are made:
+CACHE_SCHEMA_VERSION = 2
 
 class Cache(object):
     """
@@ -59,20 +61,67 @@ class Cache(object):
     without querying.
     """
 
-    def __init__(self, db_file=None):
+    def __init__(self, db_file=None, force_creation=False):
+        """
+        db_file: The path of the database file.
+        
+        if force_creation is set to True, the file will be created
+        regardless of a file at the same path already exists.
+        """
         if db_file is None:
             self.db_file = "cache.db"
         else:
             self.db_file = db_file
 
+        if force_creation:
+            self.create_cache()
+        else:
+            self.check_cache()
+
+    def check_cache(self):
+        """
+        Check whether the cache in db file self.db_file exists and conforms
+        to the latest database schema.
+
+        If it does not exist, it will be created using the newest database schema.
+
+        If it exists, but the schema is outdated, an exception will be raised.
+        """
         if not os.path.exists(self.db_file):
             self.create_cache()
+        else:
+            try:
+                current_schema = self.get_cache_version()
+            except sqlite3.OperationalError:
+                current_schema = 1
+            # TODO: Write unit tests for too old, too new and non-existent version of cache.
+
+            if current_schema > CACHE_SCHEMA_VERSION:
+                raise RuntimeError("Cache is corrupt or was created with a future version of this program.")
+            elif current_schema < CACHE_SCHEMA_VERSION:
+                raise RuntimeError("Cache version too old.")
+
+    def get_cache_version(self):
+        try:
+            conn = sqlite3.connect(self.db_file)
+            c = conn.cursor()
+            c.execute("SELECT schema FROM version")
+            version_row = c.fetchall()
+            if len(version_row) != 1:
+                raise RuntimeError("Cache version table broken.")
+            return version_row[0][0]
+        finally:
+            conn.close()
 
     def create_cache(self):
         conn = sqlite3.connect(self.db_file)
         c = conn.cursor()
         c.execute(
-            '''CREATE TABLE identifiables (digest text primary key, caosdb_id integer)''')
+            '''CREATE TABLE identifiables (digest TEXT PRIMARY KEY, caosdb_id INTEGER, caosdb_version TEXT)''')
+        c.execute(
+            '''CREATE TABLE version (schema INTEGER)''')
+        c.execute("INSERT INTO version VALUES (?)", (CACHE_SCHEMA_VERSION,))
+        
         conn.commit()
         conn.close()