Limit message size in Slack sender (#279)

* senders: slack: Limit message size

* senders: slack: Add message size limit test coverage

* Adjust for 40k limit

senders/slack/slack.go

@@ -18,6 +18,9 @@ const (
 	nodataEmoji    = ":moira-state-nodata:"
 	exceptionEmoji = ":moira-state-exception:"
 	testEmoji      = ":moira-state-test:"
+
+	messageMaxCharacters          = 40000
+	additionalInfoCharactersCount = 400
 )
 
 var stateEmoji = map[moira.State]string{
@@ -93,13 +96,30 @@ func (sender *Sender) buildMessage(events moira.NotificationEvents, trigger moir
 
 	message.WriteString("\n```")
 
+	var printEventsCount int
+	messageCharsCount := len([]rune(message.String()))
+	messageLimitReached := false
+
 	for _, event := range events {
-		message.WriteString(fmt.Sprintf("\n%s: %s = %s (%s to %s)", event.FormatTimestamp(sender.location), event.Metric, event.GetMetricValue(), event.OldState, event.State))
+		line := fmt.Sprintf("\n%s: %s = %s (%s to %s)", event.FormatTimestamp(sender.location), event.Metric, event.GetMetricValue(), event.OldState, event.State)
 		if len(moira.UseString(event.Message)) > 0 {
-			message.WriteString(fmt.Sprintf(". %s", moira.UseString(event.Message)))
+			line += fmt.Sprintf(". %s", moira.UseString(event.Message))
+		}
+		lineCharsCount := len([]rune(line))
+		if messageCharsCount+lineCharsCount > messageMaxCharacters-additionalInfoCharactersCount {
+			messageLimitReached = true
+			break
 		}
+		message.WriteString(line)
+		messageCharsCount += lineCharsCount
+		printEventsCount++
 	}
 	message.WriteString("```")
+
+	if messageLimitReached {
+		message.WriteString(fmt.Sprintf("\n\n...and %d more events.", len(events)-printEventsCount))
+	}
+
 	if throttled {
 		message.WriteString("\nPlease, *fix your system or tune this trigger* to generate less events.")
 	}

senders/slack/slack_test.go

@@ -2,6 +2,7 @@ package slack
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 	"time"
 
@@ -136,6 +137,17 @@ func TestBuildMessage(t *testing.T) {
 			So(actual, ShouldResemble, expected)
 		})
 
+		events := make([]moira.NotificationEvent, 0)
+		Convey("Print moira message with 1129 events and cutoff", func() {
+			for i := 0; i < 1200; i++ {
+				events = append(events, event)
+			}
+			lines := strings.Repeat("\n02:40: Metric = 123 (OK to NODATA)", 1129)
+			actual := sender.buildMessage(events, trigger, false)
+			expected := fmt.Sprintf("*NODATA* [tag1][tag2] <http://moira.url/trigger/TriggerID|Name>\n```%s```\n\n...and 71 more events.", lines)
+			So(actual, ShouldResemble, expected)
+		})
+
 		Convey("Print moira message with empty triggerID, but with trigger name", func() {
 			actual := sender.buildMessage([]moira.NotificationEvent{event}, moira.TriggerData{Name: "Name"}, false)
 			expected := "*NODATA* Name\n```\n02:40: Metric = 123 (OK to NODATA)```"