Show that but-db caches open in memory if migrations can't be applied.

This means that newer caches versions that are incompatible won't be overwritten
or cleared by older clients. They will instead open in memory,
which seems like it's desirable behaviour.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: codex

crates/but-db/src/cache/tests.rs

@@ -53,6 +53,36 @@ mod open_with_migrations_infallible {
         assert!(table_exists(&conn, "foo")?);
         Ok(())
     }
+
+    #[test]
+    fn existing_cache_with_migration_failure_falls_back_to_memory() -> anyhow::Result<()> {
+        let tmp = tempfile::tempdir()?;
+        let url = tmp.path().join("existing-cache.sqlite");
+        let conn = rusqlite::Connection::open(&url)?;
+        conn.execute_batch(
+            "CREATE TABLE `foo`(
+                `id` TEXT NOT NULL PRIMARY KEY
+            );",
+        )?;
+        drop(conn);
+
+        let (conn, actual_url) = open_with_migrations_infallible(&url, migrations());
+        assert_eq!(
+            actual_url, ":memory:",
+            "migration failures on a valid existing cache fall back to memory"
+        );
+        assert!(url.exists(), "valid caches are kept on disk");
+        assert!(table_exists(&conn, "foo")?);
+        assert!(table_exists(&conn, "__diesel_schema_migrations")?);
+
+        let disk_conn = rusqlite::Connection::open(&url)?;
+        assert!(table_exists(&disk_conn, "foo")?);
+        assert!(
+            !table_exists(&disk_conn, "__diesel_schema_migrations")?,
+            "failed migrations leave the existing cache untouched"
+        );
+        Ok(())
+    }
 }
 
 fn migrations() -> impl Iterator<Item = M<'static>> + Clone {