Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

gateway/bench.tengo

@@ -0,0 +1,5 @@
+text := import("text")
+if text.re_match("blah",msgText) {
+    msgText="replaced by this"
+    msgUsername="fakeuser"
+}

gateway/gateway.go

@@ -1,6 +1,7 @@
 package gateway
 
 import (
+	"io/ioutil"
 	"os"
 	"regexp"
 	"strings"
@@ -8,6 +9,7 @@ import (
 
 	"github.com/42wim/matterbridge/bridge"
 	"github.com/42wim/matterbridge/bridge/config"
+	"github.com/d5/tengo/script"
 	"github.com/hashicorp/golang-lru"
 	"github.com/peterhellberg/emojilib"
 	"github.com/sirupsen/logrus"
@@ -334,6 +336,10 @@ func (gw *Gateway) modifyAvatar(msg config.Message, dest *bridge.Bridge) string
 }
 
 func (gw *Gateway) modifyMessage(msg *config.Message) {
+	if err := modifyMessageTengo(gw.BridgeValues().General.TengoModifyMessage, msg); err != nil {
+		flog.Errorf("TengoModifyMessage failed: %s", err)
+	}
+
 	// replace :emoji: to unicode
 	msg.Text = emojilib.Replace(msg.Text)
 
@@ -458,3 +464,28 @@ func getProtocol(msg *config.Message) string {
 	p := strings.Split(msg.Account, ".")
 	return p[0]
 }
+
+func modifyMessageTengo(filename string, msg *config.Message) error {
+	if filename == "" {
+		return nil
+	}
+	res, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return err
+	}
+	s := script.New(res)
+	_ = s.Add("msgText", msg.Text)
+	_ = s.Add("msgUsername", msg.Username)
+	_ = s.Add("msgAccount", msg.Account)
+	_ = s.Add("msgChannel", msg.Channel)
+	c, err := s.Compile()
+	if err != nil {
+		return err
+	}
+	if err := c.Run(); err != nil {
+		return err
+	}
+	msg.Text = c.Get("msgText").String()
+	msg.Username = c.Get("msgUsername").String()
+	return nil
+}

gateway/gateway_test.go

@@ -499,3 +499,13 @@ func TestIgnoreNicks(t *testing.T) {
 		assert.Equalf(t, testcase.output, output, "case '%s' failed", testname)
 	}
 }
+
+func BenchmarkTengo(b *testing.B) {
+	msg := &config.Message{Username: "user", Text: "blah testing", Account: "protocol.account", Channel: "mychannel"}
+	for n := 0; n < b.N; n++ {
+		err := modifyMessageTengo("bench.tengo", msg)
+		if err != nil {
+			return
+		}
+	}
+}