Mail assignee when issue/pull request is assigned (#8546)

* Send email to assigned user

* Only send mail if enabled

* Mail also when assigned through API

* Need to refactor functions from models to issue service

* Refer to issue index rather than ID

* Disable email notifications completly at initalization if global disable

* Check of user enbled mail shall be in mail notification function only

* Initialize notifications from routers init function.

* Use the assigned comment when sending assigned mail

* Refactor so that assignees always added as separate step when new issue/pr.

* Check error from AddAssignees

* Check if user can be assiged to issue or pull request

* Missing return

* Refactor of CanBeAssigned check.

CanBeAssigned shall have same check as UI.

* Clarify function names (toggle rather than update/change), and clean up.

* Fix review comments.

* Flash error if assignees was not added when creating issue/pr

* Generate error if assignee users doesn't exist

4 files changed, 20 insertions, 6 deletions

diff --git a/modules/notification/base/notifier.go b/modules/notification/base/notifier.go
index e44f3cc632..c74bb52014 100644
--- a/modules/notification/base/notifier.go
+++ b/modules/notification/base/notifier.go
@@ -21,7 +21,7 @@ type Notifier interface {
 	NotifyNewIssue(*models.Issue)
 	NotifyIssueChangeStatus(*models.User, *models.Issue, bool)
 	NotifyIssueChangeMilestone(doer *models.User, issue *models.Issue)
-	NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, removed bool)
+	NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, assignee *models.User, removed bool, comment *models.Comment)
 	NotifyIssueChangeContent(doer *models.User, issue *models.Issue, oldContent string)
 	NotifyIssueClearLabels(doer *models.User, issue *models.Issue)
 	NotifyIssueChangeTitle(doer *models.User, issue *models.Issue, oldTitle string)
diff --git a/modules/notification/base/null.go b/modules/notification/base/null.go
index 12be1999f9..9fb08884a1 100644
--- a/modules/notification/base/null.go
+++ b/modules/notification/base/null.go
@@ -83,7 +83,7 @@ func (*NullNotifier) NotifyIssueChangeContent(doer *models.User, issue *models.I
 }
 
 // NotifyIssueChangeAssignee places a place holder function
-func (*NullNotifier) NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, removed bool) {
+func (*NullNotifier) NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, assignee *models.User, removed bool, comment *models.Comment) {
 }
 
 // NotifyIssueClearLabels places a place holder function
diff --git a/modules/notification/mail/mail.go b/modules/notification/mail/mail.go
index e1ae391f78..0900c6dcdf 100644
--- a/modules/notification/mail/mail.go
+++ b/modules/notification/mail/mail.go
@@ -5,6 +5,8 @@
 package mail
 
 import (
+	"fmt"
+
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/notification/base"
@@ -88,3 +90,11 @@ func (m *mailNotifier) NotifyPullRequestReview(pr *models.PullRequest, r *models
 		log.Error("MailParticipants: %v", err)
 	}
 }
+
+func (m *mailNotifier) NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, assignee *models.User, removed bool, comment *models.Comment) {
+	// mail only sent to added assignees and not self-assignee
+	if !removed && doer.ID != assignee.ID && assignee.EmailNotifications() == models.EmailNotificationsEnabled {
+		ct := fmt.Sprintf("Assigned #%d.", issue.Index)
+		mailer.SendIssueAssignedMail(issue, doer, ct, comment, []string{assignee.Email})
+	}
+}
diff --git a/modules/notification/notification.go b/modules/notification/notification.go
index 06220ecb04..0f1b63cf67 100644
--- a/modules/notification/notification.go
+++ b/modules/notification/notification.go
@@ -12,6 +12,7 @@ import (
 	"code.gitea.io/gitea/modules/notification/mail"
 	"code.gitea.io/gitea/modules/notification/ui"
 	"code.gitea.io/gitea/modules/notification/webhook"
+	"code.gitea.io/gitea/modules/setting"
 )
 
 var (
@@ -24,9 +25,12 @@ func RegisterNotifier(notifier base.Notifier) {
 	notifiers = append(notifiers, notifier)
 }
 
-func init() {
+// NewContext registers notification handlers
+func NewContext() {
 	RegisterNotifier(ui.NewNotifier())
-	RegisterNotifier(mail.NewNotifier())
+	if setting.Service.EnableNotifyMail {
+		RegisterNotifier(mail.NewNotifier())
+	}
 	RegisterNotifier(indexer.NewNotifier())
 	RegisterNotifier(webhook.NewNotifier())
 }
@@ -138,9 +142,9 @@ func NotifyIssueChangeContent(doer *models.User, issue *models.Issue, oldContent
 }
 
 // NotifyIssueChangeAssignee notifies change content to notifiers
-func NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, removed bool) {
+func NotifyIssueChangeAssignee(doer *models.User, issue *models.Issue, assignee *models.User, removed bool, comment *models.Comment) {
 	for _, notifier := range notifiers {
-		notifier.NotifyIssueChangeAssignee(doer, issue, removed)
+		notifier.NotifyIssueChangeAssignee(doer, issue, assignee, removed, comment)
 	}
 }