Possible example of a readonly cache.

aaa.py

@@ -0,0 +1,14 @@
+import diskcache
+
+a = diskcache.Cache('/tmp/abcde', sqlite_query_only=True)
+print(f'Query-only: ', a.sqlite_query_only)
+
+o = [1, 2, 3, 4]
+# a['qq'] = o
+
+for k in a:
+    print(k)
+
+o = [1, 2, 3, 4]
+a['qqe'] = o
+

diskcache/core.py

@@ -80,12 +80,15 @@ def __repr__(self):
 MODE_TEXT = 3
 MODE_PICKLE = 4
 
+# settings starting with sqlite_ are used as PRAGMAs in sqlite
+# use with care
 DEFAULT_SETTINGS = {
     u'statistics': 0,  # False
     u'tag_index': 0,   # False
     u'eviction_policy': u'least-recently-stored',
     u'size_limit': 2 ** 30,  # 1gb
     u'cull_limit': 10,
+    u'sqlite_query_only': False,
     u'sqlite_auto_vacuum': 1,        # FULL
     u'sqlite_cache_size': 2 ** 13,   # 8,192 pages
     u'sqlite_journal_mode': u'wal',
@@ -439,6 +442,9 @@ def __init__(self, directory=None, timeout=60, disk=Disk, **settings):
                         ' and could not be created' % self._directory
                     )
 
+        # this must be processed now because it is used in Cache._con
+        self.open_in_query_only = settings.get('sqlite_query_only', False)
+
         sql = self._sql_retry
 
         # Setup Settings table.
@@ -479,14 +485,16 @@ def __init__(self, directory=None, timeout=60, disk=Disk, **settings):
         # Set cached attributes: updates settings and sets pragmas.
 
         for key, value in sets.items():
-            query = 'INSERT OR REPLACE INTO Settings VALUES (?, ?)'
-            sql(query, (key, value))
-            self.reset(key, value)
+            if not self.sqlite_query_only:
+                query = 'INSERT OR REPLACE INTO Settings VALUES (?, ?)'
+                sql(query, (key, value))
+            self.reset(key, value, update=not self.sqlite_query_only)
 
         for key, value in METADATA.items():
-            query = 'INSERT OR IGNORE INTO Settings VALUES (?, ?)'
-            sql(query, (key, value))
-            self.reset(key)
+            if not self.sqlite_query_only:
+                query = 'INSERT OR IGNORE INTO Settings VALUES (?, ?)'
+                sql(query, (key, value))
+            self.reset(key, update=not self.sqlite_query_only)
 
         (self._page_size,), = sql('PRAGMA page_size').fetchall()
 
@@ -555,10 +563,11 @@ def __init__(self, directory=None, timeout=60, disk=Disk, **settings):
 
         # Create tag index if requested.
 
-        if self.tag_index:  # pylint: disable=no-member
-            self.create_tag_index()
-        else:
-            self.drop_tag_index()
+        if not self.sqlite_query_only:
+            if self.tag_index:  # pylint: disable=no-member
+                self.create_tag_index()
+            else:
+                self.drop_tag_index()
 
         # Close and re-open database connection with given timeout.
 
@@ -621,6 +630,11 @@ def _con(self):
                     if key.startswith('sqlite_'):
                         self.reset(key, value, update=False)
 
+            # the settings read fro the DB never contain query_only
+            # so we manually force it here, if it has been passed as a parameter
+            if self.open_in_query_only:
+                self.reset('sqlite_query_only', 1, update=False)
+
         return con
 
 
@@ -706,8 +720,9 @@ def _transact(self, retry=False, filename=None):
                     begin = True
                     self._txn_id = tid
                     break
-                except sqlite3.OperationalError:
-                    if retry:
+                except sqlite3.OperationalError as e:
+                    # TODO: this is potentially an infinite loop anyway
+                    if retry and 'readonly' not in str(e):
                         continue
                     if filename is not None:
                         _disk_remove(filename)