accounts: return consistent error for duplicate label

In preparation for when we have a SQL DB implementation, we want our
unit tests to run smoothly against all DB backends and have the same
results. To achieve this, we need to turn some errors into global error
variables that can be matched against instead.

In this commit, we do this for the unique constraint violation of the
account label.

Author: ellemouton

accounts/errors.go

@@ -0,0 +1,11 @@
+package accounts
+
+import "errors"
+
+var (
+	// ErrLabelAlreadyExists is returned by the CreateAccount method if the
+	// account label is already used by an existing account.
+	ErrLabelAlreadyExists = errors.New(
+		"account label uniqueness constraint violation",
+	)
+)

accounts/store_kvdb.go

@@ -133,7 +133,8 @@ func (s *BoltStore) NewAccount(ctx context.Context, balance lnwire.MilliSatoshi,
 		for _, account := range accounts {
 			if account.Label == label {
 				return nil, fmt.Errorf("an account with the "+
-					"label '%s' already exists", label)
+					"label '%s' already exists: %w", label,
+					ErrLabelAlreadyExists)
 			}
 		}
 	}

accounts/store_test.go

@@ -29,7 +29,7 @@ func TestAccountStore(t *testing.T) {
 
 	// Make sure we cannot create a second account with the same label.
 	_, err = store.NewAccount(ctx, 123, time.Time{}, "foo")
-	require.ErrorContains(t, err, "account with the label 'foo' already")
+	require.ErrorIs(t, err, ErrLabelAlreadyExists)
 
 	// Make sure we cannot set a label that looks like an account ID.
 	_, err = store.NewAccount(ctx, 123, time.Time{}, "0011223344556677")