PLT-6581 breakup webhooks greater than 4K into multiple posts (#6530)

app/webhook.go

@@ -150,11 +150,37 @@ func CreateWebhookPost(userId, teamId, channelId, text, overrideUsername, overri
 		}
 	}
 
-	if _, err := CreatePost(post, teamId, false); err != nil {
-		return nil, model.NewLocAppError("CreateWebhookPost", "api.post.create_webhook_post.creating.app_error", nil, "err="+err.Message)
+	splits := make([]string, 0)
+	remainingText := post.Message
+
+	for len(remainingText) > model.POST_MESSAGE_MAX_RUNES {
+		splits = append(splits, remainingText[:model.POST_MESSAGE_MAX_RUNES])
+		remainingText = remainingText[model.POST_MESSAGE_MAX_RUNES:]
 	}
 
-	return post, nil
+	splits = append(splits, remainingText)
+
+	var firstPost *model.Post = nil
+
+	for _, txt := range splits {
+		post.Id = ""
+		post.UpdateAt = 0
+		post.CreateAt = 0
+		post.Message = txt
+		if _, err := CreatePost(post, teamId, false); err != nil {
+			return nil, model.NewLocAppError("CreateWebhookPost", "api.post.create_webhook_post.creating.app_error", nil, "err="+err.Message)
+		}
+
+		if firstPost == nil {
+			if len(splits) > 1 {
+				firstPost = model.PostFromJson(strings.NewReader(post.ToJson()))
+			} else {
+				firstPost = post
+			}
+		}
+	}
+
+	return firstPost, nil
 }
 
 func CreateIncomingWebhookForChannel(creatorId string, channel *model.Channel, hook *model.IncomingWebhook) (*model.IncomingWebhook, *model.AppError) {
@@ -430,11 +456,6 @@ func HandleIncomingWebhook(hookId string, req *model.IncomingWebhookRequest) *mo
 		return model.NewAppError("HandleIncomingWebhook", "web.incoming_webhook.text.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	textSize := utf8.RuneCountInString(text)
-	if textSize > model.POST_MESSAGE_MAX_RUNES {
-		return model.NewAppError("HandleIncomingWebhook", "web.incoming_webhook.text.length.app_error", map[string]interface{}{"Max": model.POST_MESSAGE_MAX_RUNES, "Actual": textSize}, "", http.StatusBadRequest)
-	}
-
 	channelName := req.ChannelName
 	webhookType := req.Type