Eph. memory catches invalid target (hi.s. delete)

If hist == nil and mysql database Delete msgid function returns
ErrDBIsNil, we know that the target does not match any channel or user.
Return invalid target error to operator (see #2020)

irc/errors.go

 	errConfusableIdentifier           = errors.New("This identifier is confusable with one already in use")
 	errInsufficientPrivs              = errors.New("Insufficient privileges")
 	errInvalidUsername                = errors.New("Invalid username")
+	errInvalidTarget                  = errors.New("Invalid target")
 	errFeatureDisabled                = errors.New(`That feature is disabled`)
 	errBanned                         = errors.New("IP or nickmask banned")
 	errInvalidParams                  = utils.ErrInvalidParams

irc/mysql/history.go

 
 var (
 	ErrDisallowed = errors.New("disallowed")
+	ErrDBIsNil    = errors.New("db == nil")
 )
 
 const (
 // note that accountName is the unfolded name
 func (mysql *MySQL) DeleteMsgid(msgid, accountName string) (err error) {
 	if mysql.db == nil {
-		return nil
+		return ErrDBIsNil
 	}
 
 	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())

irc/server.go

 package irc
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"net/http"
 
 	if hist == nil {
 		err = server.historyDB.DeleteMsgid(msgid, accountName)
+		if err != nil && errors.Is(err, mysql.ErrDBIsNil) {
+			/*
+				hist == nil, and db == nil. We know that the
+				target was not either a current channel or
+				client, and persistent storage is not used.
+				So this is an invalid target. (see #2020)
+			*/
+			return errInvalidTarget
+		}
 	} else {
 		count := hist.Delete(func(item *history.Item) bool {
 			return item.Message.Msgid == msgid && (accountName == "*" || item.AccountName == accountName)