fact: add protection for aliases

* No circular aliases may be created
* Must have a resolvable destination

plugins/fact/factoid.go

@@ -38,7 +38,7 @@ type alias struct {
 	Next string
 }
 
-func (a *alias) resolve(db *sqlx.DB) *factoid {
+func (a *alias) resolve(db *sqlx.DB) (*factoid, error) {
 	// perform DB query to fill the To field
 	q := `select fact, next from factoid_alias where fact=?`
 	var next alias
@@ -47,9 +47,10 @@ func (a *alias) resolve(db *sqlx.DB) *factoid {
 		// we hit the end of the chain, get a factoid named Next
 		fact, err := getSingleFact(db, a.Next)
 		if err != nil {
-			log.Printf("Error resolvig alias %v: %v", a, err)
+			err := fmt.Errorf("Error resolvig alias %v: %v", a, err)
+			return nil, err
 		}
-		return fact
+		return fact, nil
 	}
 	return next.resolve(db)
 }
@@ -61,12 +62,21 @@ func findAlias(db *sqlx.DB, fact string) (bool, *factoid) {
 	if err != nil {
 		return false, nil
 	}
-	return true, a.resolve(db)
+	f, err := a.resolve(db)
+	return err == nil, f
 }
 
 func (a *alias) save(db *sqlx.DB) error {
+	ok, _ := findAlias(db, a.Fact)
+	if ok {
+		return fmt.Errorf("DANGER: an opposite alias already exists")
+	}
+	_, err := a.resolve(db)
+	if err != nil {
+		return fmt.Errorf("there is no fact at that destination")
+	}
 	q := `insert or replace into factoid_alias (fact, next) values (?, ?)`
-	_, err := db.Exec(q, a.Fact, a.Next)
+	_, err = db.Exec(q, a.Fact, a.Next)
 	if err != nil {
 		return err
 	}
@@ -77,10 +87,6 @@ func aliasFromStrings(from, to string) *alias {
 	return &alias{from, to}
 }
 
-func factoidFromAlias(db *sqlx.DB, a alias) *factoid {
-	return a.resolve(db)
-}
-
 func (f *factoid) save(db *sqlx.DB) error {
 	var err error
 	if f.id.Valid {
@@ -611,8 +617,11 @@ func (p *Factoid) Message(message msg.Message) bool {
 			return true
 		}
 		a := aliasFromStrings(strings.TrimSpace(parts[1]), strings.TrimSpace(parts[0]))
-		a.save(p.db)
+		if err := a.save(p.db); err != nil {
+			p.Bot.SendMessage(message.Channel, err.Error())
+		} else {
 			p.Bot.SendAction(message.Channel, "learns a new synonym")
+		}
 		return true
 	}
 
@@ -620,9 +629,8 @@ func (p *Factoid) Message(message msg.Message) bool {
 		if fact := p.randomFact(); fact != nil {
 			p.sayFact(message, *fact)
 			return true
-		} else {
-			log.Println("Got a nil fact.")
 		}
+		log.Println("Got a nil fact.")
 	}
 
 	if strings.ToLower(message.Body) == "forget that" {