Use WAL by default.

For #169 / #175.

CHANGES.rst

@@ -11,6 +11,30 @@ Version 1.4
 
 Unreleased
 
+* Use SQLite's `write-ahead logging`_ by default,
+  to increase concurrency and prevent "database is locked" errors.
+  (:issue:`169`, :issue:`175`)
+
+  This changes how the database appears on disk and how it can be used;
+  most importantly:
+
+  * There are additional quasi-persistent ``-wal`` and ``-shm`` files
+    associated with the database.
+    Backing up the database by simply copying it may not be enough
+    (this was also true before this change, to a lesser extent).
+    To make a backup of the database, use the SQLite CLI::
+
+      sqlite3 src.sqlite ".backup 'dst.sqlite'"
+
+  * All processes using the database must be on the same host computer;
+    WAL does not work over a network filesystem.
+
+  *reader* will enable WAL exactly once: at creation for new databases,
+  and when migrating to reader 1.4 for existing databases.
+  After this, it can be disabled again from the SQLite CLI::
+
+    sqlite3 db.sqlite "pragma journal_mode = delete;"
+
 * Do not fail for feeds with incorrectly-declared media types,
   if feedparser can parse the feed;
   this is similar to the current behavior for incorrectly-declared encodings.
@@ -23,6 +47,9 @@ Unreleased
 * Other minor web application improvements.
 
 
+.. _write-ahead logging: https://www.sqlite.org/wal.html
+
+
 Version 1.3
 -----------
 

src/reader/_storage.py

@@ -54,6 +54,9 @@ def create_db(db: sqlite3.Connection) -> None:
     create_feeds(db)
     create_entries(db)
     create_feed_metadata(db)
+    # https://github.com/lemon24/reader/issues/169
+    # https://github.com/lemon24/reader/issues/175
+    db.execute("PRAGMA journal_mode = WAL;")
 
 
 def create_feeds(db: sqlite3.Connection, name: str = 'feeds') -> None:
@@ -142,16 +145,22 @@ def update_from_18_to_19(db: sqlite3.Connection) -> None:  # pragma: no cover
     db.execute("ALTER TABLE feeds ADD COLUMN last_exception TEXT;")
 
 
+def update_from_19_to_20(db: sqlite3.Connection) -> None:  # pragma: no cover
+    # for https://github.com/lemon24/reader/issues/169
+    db.execute("PRAGMA journal_mode = WAL;")
+
+
 def setup_db(db: sqlite3.Connection) -> None:
     return setup_sqlite_db(
         db,
         create=create_db,
-        version=19,
+        version=20,
         migrations={
             # 1-9 removed before 0.1 (last in e4769d8ba77c61ec1fe2fbe99839e1826c17ace7)
             # 10-16 removed before 1.0 (last in 618f158ebc0034eefb724a55a84937d21c93c1a7)
             17: update_from_17_to_18,
             18: update_from_18_to_19,
+            19: update_from_19_to_20,
         },
         # Row value support was added in 3.15.
         minimum_sqlite_version=(3, 15),