Merge pull request juju#18747 from manadart/dqlite-empty-relation-state

juju#18747

This fixes a panic where unit relation state is non-nil, but empty. 

We were attempting to prepare a Dqlite statement with the zero index of an empty slice. Now we just skip the insert operation altogether.

This is code slated for rework under a new `hook` domain, to make it transactional along with ports, secrets etc. This patch constitutes a quality-of life improvement in the interim.

## QA steps

Bootstrap, add a model, deploy a unit.

domain/unitstate/state/state.go

@@ -182,19 +182,21 @@ func (st *State) SetUnitStateRelation(ctx domain.AtomicContext, uuid string, sta
 
 	keyVals := makeUnitRelationStateKeyVals(uuid, state)
 
-	q = "INSERT INTO unit_state_relation(*) VALUES ($unitRelationStateKeyVal.*)"
-	iStmt, err := st.Prepare(q, keyVals[0])
-	if err != nil {
-		return errors.Errorf("preparing relation state insert query: %w", err)
-	}
-
 	return domain.Run(ctx, func(ctx context.Context, tx *sqlair.TX) error {
 		if err := tx.Query(ctx, dStmt, id).Run(); err != nil {
 			return errors.Errorf("deleting unit relation state: %w", err)
 		}
 
-		if err := tx.Query(ctx, iStmt, keyVals).Run(); err != nil {
-			return errors.Errorf("setting unit relation state: %w", err)
+		if len(keyVals) != 0 {
+			q = "INSERT INTO unit_state_relation(*) VALUES ($unitRelationStateKeyVal.*)"
+			iStmt, err := st.Prepare(q, keyVals[0])
+			if err != nil {
+				return errors.Errorf("preparing relation state insert query: %w", err)
+			}
+
+			if err := tx.Query(ctx, iStmt, keyVals).Run(); err != nil {
+				return errors.Errorf("setting unit relation state: %w", err)
+			}
 		}
 		return nil
 	})

domain/unitstate/state/state_test.go

@@ -271,6 +271,42 @@ func (s *stateSuite) TestUpdateUnitStateRelation(c *gc.C) {
 	c.Check(gotState, gc.DeepEquals, expState)
 }
 
+func (s *stateSuite) TestUpdateUnitStateRelationEmptyMap(c *gc.C) {
+	st := NewState(s.TxnRunnerFactory())
+	ctx := context.Background()
+
+	// Set some initial state. This should be overwritten.
+	err := s.TxnRunner().StdTxn(ctx, func(ctx context.Context, tx *sql.Tx) error {
+		q := "INSERT INTO unit_state_relation VALUES (?, 1, 'one-val')"
+		_, err := tx.ExecContext(ctx, q, s.unitUUID)
+		return err
+	})
+	c.Assert(err, jc.ErrorIsNil)
+
+	err = st.RunAtomic(ctx, func(ctx domain.AtomicContext) error {
+		return st.SetUnitStateRelation(ctx, s.unitUUID, map[int]string{})
+	})
+	c.Assert(err, jc.ErrorIsNil)
+
+	var rowCount int
+	err = s.TxnRunner().StdTxn(ctx, func(ctx context.Context, tx *sql.Tx) error {
+		q := "SELECT key, value FROM unit_state_relation WHERE unit_uuid = ?"
+		rows, err := tx.QueryContext(ctx, q, s.unitUUID)
+		if err != nil {
+			return err
+		}
+		defer func() { _ = rows.Close() }()
+
+		for rows.Next() {
+			rowCount++
+		}
+		return rows.Err()
+	})
+	c.Assert(err, jc.ErrorIsNil)
+
+	c.Check(rowCount, gc.DeepEquals, 0)
+}
+
 func (s *stateSuite) TestGetUnitState(c *gc.C) {
 	st := NewState(s.TxnRunnerFactory())