feat: unprivileged read-only open of SQLite DB

If the state SQLite database is configured to use a write-ahead-log, it
creates WAL files in the state directory.

When the state SQLite database is closed by the `nix-daemon` after
builds, those files are removed.

When an unprivileged user would like to open _in read only_ that
database, they cannot do so because they would need to create those WAL
files and they do not have the permission to do so.

For this, SQLite offers a "persistent WAL" feature [1] to leave the WAL
files around, even after closing the database.

This CL enable the persistent WAL mode.

Fixes: https://github.com/NixOS/nix/issues/10300
[1]: https://www.sqlite.org/wal.html

Change-Id: Id8ae534d7d2290457af28782e5215222ae051fe5
Signed-off-by: Raito Bezarius <raito@lix.systems>

src/libstore/local-store.cc

@@ -548,6 +548,15 @@ void LocalStore::openDB(State & state, bool create)
         sqlite3_exec(db, ("pragma main.journal_mode = " + mode + ";").c_str(), 0, 0, 0) != SQLITE_OK)
         SQLiteError::throw_(db, "setting journal mode");
 
+    if (mode == "wal" ) {
+         /* persist the WAL files when the DB connection is closed.
+         * This allows for read-only connections without any write permissions
+         * on the state directory to succeed on a closed database. */
+        int enable = 1;
+        if (sqlite3_file_control(db, NULL, SQLITE_FCNTL_PERSIST_WAL, &enable) != SQLITE_OK)
+            SQLiteError::throw_(db, "setting persistent WAL mode");
+    }
+
     /* Increase the auto-checkpoint interval to 40000 pages.  This
        seems enough to ensure that instantiating the NixOS system
        derivation is done in a single fsync(). */