[MM-32543] - Add ability to send email to admin that users are trying to invite others (#16882)

* [MM-32543] - Add ability to send email to admin that users are trying to join

* Update email template

* Feedback impl-1

* Fix test

* Feedback impl-2

* Fix error

* Feedback impl

* Use 413 status code

* make i18n-extract

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/cloud.go

@@ -4,6 +4,9 @@
 package app
 
 import (
+	"fmt"
+	"net/http"
+
 	"github.com/mattermost/mattermost-server/v5/mlog"
 	"github.com/mattermost/mattermost-server/v5/model"
 )
@@ -18,6 +21,59 @@ func (a *App) getSysAdminsEmailRecipients() ([]*model.User, *model.AppError) {
 	return a.GetUsers(userOptions)
 }
 
+// SendAdminUpgradeRequestEmail takes the username of user trying to alert admins and then applies rate limit of n (number of admins) emails per user per day
+// before sending the emails.
+func (a *App) SendAdminUpgradeRequestEmail(username string, subscription *model.Subscription) *model.AppError {
+	if a.Srv().License() == nil || (a.Srv().License() != nil && !*a.Srv().License().Features.Cloud) {
+		return nil
+	}
+
+	if subscription != nil && subscription.IsPaidTier == "true" {
+		return nil
+	}
+
+	if a.Srv().EmailService.PerDayEmailRateLimiter == nil {
+		return model.NewAppError("app.SendAdminUpgradeRequestEmail", "app.email.no_rate_limiter.app_error", nil, fmt.Sprintf("for username=%s", username), http.StatusInternalServerError)
+	}
+
+	// rate limit based on username as key
+	rateLimited, result, err := a.Srv().EmailService.PerDayEmailRateLimiter.RateLimit(username, 1)
+	if err != nil {
+		return model.NewAppError("app.SendAdminUpgradeRequestEmail", "app.email.setup_rate_limiter.app_error", nil, fmt.Sprintf("username=%s, error=%v", username, err), http.StatusInternalServerError)
+	}
+
+	if rateLimited {
+		return model.NewAppError("app.SendAdminUpgradeRequestEmail",
+			"app.email.rate_limit_exceeded.app_error", map[string]interface{}{"RetryAfter": result.RetryAfter.String(), "ResetAfter": result.ResetAfter.String()},
+			fmt.Sprintf("username=%s, retry_after_secs=%f, reset_after_secs=%f",
+				username, result.RetryAfter.Seconds(), result.ResetAfter.Seconds()),
+			http.StatusRequestEntityTooLarge)
+	}
+
+	sysAdmins, e := a.getSysAdminsEmailRecipients()
+	if e != nil {
+		return e
+	}
+
+	// we want to at least have one email sent out to an admin
+	countNotOks := 0
+
+	for admin := range sysAdmins {
+		ok, err := a.Srv().EmailService.SendUpgradeEmail(username, sysAdmins[admin].Email, sysAdmins[admin].Locale, *a.Config().ServiceSettings.SiteURL)
+		if !ok || err != nil {
+			a.Log().Error("Error sending upgrade request email", mlog.Err(err))
+			countNotOks++
+		}
+	}
+
+	// if not even one admin got an email, we consider that this operation errored
+	if countNotOks == len(sysAdmins) {
+		return model.NewAppError("app.SendAdminUpgradeRequestEmail", "app.user.send_emails.app_error", nil, "", http.StatusInternalServerError)
+	}
+
+	return nil
+}
+
 func (a *App) CheckAndSendUserLimitWarningEmails() *model.AppError {
 	if a.Srv().License() == nil || (a.Srv().License() != nil && !*a.Srv().License().Features.Cloud) {
 		// Not cloud instance, do nothing