libsql: Fail sync_db_if_needed() if database exists but metadata is missing

This is an inconsistent state because there is no way for us to know
what generation the database file is tracking. For all we know, it could
be a totally unrelated database file than the one we have remotely.
Therefore, return an error.

Author: penberg

libsql/src/sync.rs

@@ -658,16 +658,14 @@ impl SyncContext {
                 self.sync_db(generation).await
             }
             (false, true) => {
-                // kinda inconsistent state: DB exists but metadata missing
-                // however, this generally not an issue. For a fresh db, a user might do writes
-                // locally and then try to do sync later. So in this case, we will not
-                // bootstrap the db file and let the user proceed. If it is not a fresh db, the
-                // push will fail anyways later.
-                // if metadata file does not exist, then generation should be zero
-                assert_eq!(self.durable_generation, 0);
-                // lets initialise it to first generation
-                self.durable_generation = 1;
-                Ok(())
+                // inconsistent state: DB exists but metadata missing
+                tracing::error!(
+                    "local state is incorrect, db file exists but metadata file does not"
+                );
+                Err(SyncError::InvalidLocalState(
+                    "db file exists but metadata file does not".to_string(),
+                )
+                .into())
             }
             (true, false) => {
                 // inconsistent state: Metadata exists but DB missing