[feature] Allow admins to send test emails (#1620)

* [feature] Allow admins to send test emails

* implement unwrap on new error type

* add + use gtserror types

* GoToSocial Email Test -> GoToSocial Test Email

* add + use getInstance db call

* removed unused "unknown" error type

internal/processing/admin/email.go

@@ -0,0 +1,61 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/email"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+// EmailTest sends a generic test email to the given toAddress (which
+// should be a valid email address). To help callers differentiate between
+// proper errors and the smtp errors they're likely fishing for, will return
+// 422 + help text on an SMTP error, or error 500 otherwise.
+func (p *Processor) EmailTest(ctx context.Context, account *gtsmodel.Account, toAddress string) gtserror.WithCode {
+	// Pull our instance entry from the database,
+	// so we can greet the email recipient nicely.
+	instance, err := p.state.DB.GetInstance(ctx, config.GetHost())
+	if err != nil {
+		err = fmt.Errorf("SendConfirmEmail: error getting instance: %s", err)
+		return gtserror.NewErrorInternalError(err)
+	}
+
+	testData := email.TestData{
+		SendingUsername: account.Username,
+		InstanceURL:     instance.URI,
+		InstanceName:    instance.Title,
+	}
+
+	if err := p.emailSender.SendTestEmail(toAddress, testData); err != nil {
+		if errorType := gtserror.Type(err); errorType == gtserror.TypeSMTP {
+			// An error occurred during the SMTP part.
+			// We should indicate this to the caller, as
+			// it will likely help them debug the issue.
+			return gtserror.NewErrorUnprocessableEntity(err, err.Error())
+		}
+		// An actual error has occurred.
+		return gtserror.NewErrorInternalError(err)
+	}
+
+	return nil
+}