db: add validation for WALFailover options

options.go

@@ -1418,6 +1418,13 @@ type WALFailoverOptions struct {
 	wal.FailoverOptions
 }
 
+func (o *WALFailoverOptions) Validate() error {
+	if o.Secondary.FS == nil {
+		return errors.New("Secondary.FS is required")
+	}
+	return nil
+}
+
 // ReadaheadConfig controls the use of read-ahead.
 type ReadaheadConfig = objstorageprovider.ReadaheadConfig
 
@@ -2568,6 +2575,12 @@ func (o *Options) Validate() error {
 		}
 	}
 
+	if o.WALFailover != nil {
+		if err := o.WALFailover.Validate(); err != nil {
+			fmt.Fprintf(&buf, "WALFailover validation failed: %v\n", err)
+		}
+	}
+
 	if buf.Len() == 0 {
 		return nil
 	}

options_test.go

@@ -547,6 +547,14 @@ func TestOptionsValidate(t *testing.T) {
 			}
 		})
 	}
+	t.Run("wal-failover", func(t *testing.T) {
+		var opts Options
+		opts.EnsureDefaults()
+		opts.WALFailover = &WALFailoverOptions{}
+		err := opts.Validate()
+		require.Error(t, err)
+		require.Regexp(t, "Secondary.FS is required", err.Error())
+	})
 }
 
 func TestKeyCategories(t *testing.T) {