decouple email content from sender to ensure thread safety

If two emails fired off at once, they would previously replace each
other's content and possibly send the wrong email to the wrong person.
construct* methods now return the email content, which is sent
separately.

api.go

@@ -92,10 +92,11 @@ func (app *appContext) checkInvites() {
 				for address, settings := range notify {
 					if settings["notify-expiry"] {
 						go func() {
-							if err := app.email.constructExpiry(code, data, app); err != nil {
+							msg, err := app.email.constructExpiry(code, data, app)
+							if err != nil {
 								app.err.Printf("%s: Failed to construct expiry notification", code)
 								app.debug.Printf("Error: %s", err)
-							} else if err := app.email.send(address); err != nil {
+							} else if err := app.email.send(address, msg); err != nil {
 								app.err.Printf("%s: Failed to send expiry notification", code)
 								app.debug.Printf("Error: %s", err)
 							} else {
@@ -128,10 +129,11 @@ func (app *appContext) checkInvite(code string, used bool, username string) bool
 				for address, settings := range notify {
 					if settings["notify-expiry"] {
 						go func() {
-							if err := app.email.constructExpiry(code, inv, app); err != nil {
+							msg, err := app.email.constructExpiry(code, inv, app)
+							if err != nil {
 								app.err.Printf("%s: Failed to construct expiry notification", code)
 								app.debug.Printf("Error: %s", err)
-							} else if err := app.email.send(address); err != nil {
+							} else if err := app.email.send(address, msg); err != nil {
 								app.err.Printf("%s: Failed to send expiry notification", code)
 								app.debug.Printf("Error: %s", err)
 							} else {
@@ -220,10 +222,11 @@ func (app *appContext) NewUser(gc *gin.Context) {
 		for address, settings := range invite.Notify {
 			if settings["notify-creation"] {
 				go func() {
-					if err := app.email.constructCreated(req.Code, req.Username, req.Email, invite, app); err != nil {
+					msg, err := app.email.constructCreated(req.Code, req.Username, req.Email, invite, app)
+					if err != nil {
 						app.err.Printf("%s: Failed to construct user creation notification", req.Code)
 						app.debug.Printf("%s: Error: %s", req.Code, err)
-					} else if err := app.email.send(address); err != nil {
+					} else if err := app.email.send(address, msg); err != nil {
 						app.err.Printf("%s: Failed to send user creation notification", req.Code)
 						app.debug.Printf("%s: Error: %s", req.Code, err)
 					} else {
@@ -304,11 +307,12 @@ func (app *appContext) GenerateInvite(gc *gin.Context) {
 	if req.Email != "" && app.config.Section("invite_emails").Key("enabled").MustBool(false) {
 		app.debug.Printf("%s: Sending invite email", invite_code)
 		invite.Email = req.Email
-		if err := app.email.constructInvite(invite_code, invite, app); err != nil {
+		msg, err := app.email.constructInvite(invite_code, invite, app)
+		if err != nil {
 			invite.Email = fmt.Sprintf("Failed to send to %s", req.Email)
 			app.err.Printf("%s: Failed to construct invite email", invite_code)
 			app.debug.Printf("%s: Error: %s", invite_code, err)
-		} else if err := app.email.send(req.Email); err != nil {
+		} else if err := app.email.send(req.Email, msg); err != nil {
 			invite.Email = fmt.Sprintf("Failed to send to %s", req.Email)
 			app.err.Printf("%s: %s", invite_code, invite.Email)
 			app.debug.Printf("%s: Error: %s", invite_code, err)

email.go

@@ -17,14 +17,14 @@ import (
 
 // implements email sending, right now via smtp or mailgun.
 type emailClient interface {
-	send(address, fromName, fromAddr string, email email) error
+	send(address, fromName, fromAddr string, email *Email) error
 }
 
 type Mailgun struct {
 	client *mailgun.MailgunImpl
 }
 
-func (mg *Mailgun) send(address, fromName, fromAddr string, email email) error {
+func (mg *Mailgun) send(address, fromName, fromAddr string, email *Email) error {
 	message := mg.client.NewMessage(
 		fmt.Sprintf("%s <%s>", fromName, fromAddr),
 		email.subject,
@@ -45,7 +45,7 @@ type Smtp struct {
 	auth         smtp.Auth
 }
 
-func (sm *Smtp) send(address, fromName, fromAddr string, email email) error {
+func (sm *Smtp) send(address, fromName, fromAddr string, email *Email) error {
 	e := jEmail.NewEmail()
 	e.Subject = email.subject
 	e.From = fmt.Sprintf("%s <%s>", fromName, fromAddr)
@@ -68,12 +68,12 @@ func (sm *Smtp) send(address, fromName, fromAddr string, email email) error {
 
 // Emailer contains the email sender, email content, and methods to construct message content.
 type Emailer struct {
-	content            email
 	fromAddr, fromName string
 	sender             emailClient
 }
 
-type email struct {
+// Email stores content.
+type Email struct {
 	subject    string
 	html, text string
 }
@@ -140,10 +140,12 @@ func (emailer *Emailer) NewSMTP(server string, port int, password, host string,
 	}
 }
 
-func (email *Emailer) constructInvite(code string, invite Invite, app *appContext) error {
+func (emailer *Emailer) constructInvite(code string, invite Invite, app *appContext) (*Email, error) {
-	email.content.subject = app.config.Section("invite_emails").Key("subject").String()
+	email := &Email{
+		subject: app.config.Section("invite_emails").Key("subject").String(),
+	}
 	expiry := invite.ValidTill
-	d, t, expires_in := email.formatExpiry(expiry, false, app.datePattern, app.timePattern)
+	d, t, expires_in := emailer.formatExpiry(expiry, false, app.datePattern, app.timePattern)
 	message := app.config.Section("email").Key("message").String()
 	invite_link := app.config.Section("invite_emails").Key("url_base").String()
 	invite_link = fmt.Sprintf("%s/%s", invite_link, code)
@@ -152,7 +154,7 @@ func (email *Emailer) constructInvite(code string, invite Invite, app *appContex
 		fpath := app.config.Section("invite_emails").Key("email_" + key).String()
 		tpl, err := template.ParseFiles(fpath)
 		if err != nil {
-			return err
+			return nil, err
 		}
 		var tplData bytes.Buffer
 		err = tpl.Execute(&tplData, map[string]string{
@@ -163,25 +165,27 @@ func (email *Emailer) constructInvite(code string, invite Invite, app *appContex
 			"message":     message,
 		})
 		if err != nil {
-			return err
+			return nil, err
 		}
 		if key == "html" {
-			email.content.html = tplData.String()
+			email.html = tplData.String()
 		} else {
-			email.content.text = tplData.String()
+			email.text = tplData.String()
 		}
 	}
-	return nil
+	return email, nil
 }
 
-func (email *Emailer) constructExpiry(code string, invite Invite, app *appContext) error {
+func (emailer *Emailer) constructExpiry(code string, invite Invite, app *appContext) (*Email, error) {
-	email.content.subject = "Notice: Invite expired"
+	email := &Email{
+		subject: "Notice: Invite expired",
+	}
 	expiry := app.formatDatetime(invite.ValidTill)
 	for _, key := range []string{"html", "text"} {
 		fpath := app.config.Section("notifications").Key("expiry_" + key).String()
 		tpl, err := template.ParseFiles(fpath)
 		if err != nil {
-			return err
+			return nil, err
 		}
 		var tplData bytes.Buffer
 		err = tpl.Execute(&tplData, map[string]string{
@@ -189,19 +193,21 @@ func (email *Emailer) constructExpiry(code string, invite Invite, app *appContex
 			"expiry": expiry,
 		})
 		if err != nil {
-			return err
+			return nil, err
 		}
 		if key == "html" {
-			email.content.html = tplData.String()
+			email.html = tplData.String()
 		} else {
-			email.content.text = tplData.String()
+			email.text = tplData.String()
 		}
 	}
-	return nil
+	return email, nil
 }
 
-func (email *Emailer) constructCreated(code, username, address string, invite Invite, app *appContext) error {
+func (emailer *Emailer) constructCreated(code, username, address string, invite Invite, app *appContext) (*Email, error) {
-	email.content.subject = "Notice: User created"
+	email := &Email{
+		subject: "Notice: User created",
+	}
 	created := app.formatDatetime(invite.Created)
 	var tplAddress string
 	if app.config.Section("email").Key("no_username").MustBool(false) {
@@ -213,7 +219,7 @@ func (email *Emailer) constructCreated(code, username, address string, invite In
 		fpath := app.config.Section("notifications").Key("created_" + key).String()
 		tpl, err := template.ParseFiles(fpath)
 		if err != nil {
-			return err
+			return nil, err
 		}
 		var tplData bytes.Buffer
 		err = tpl.Execute(&tplData, map[string]string{
@@ -223,26 +229,28 @@ func (email *Emailer) constructCreated(code, username, address string, invite In
 			"time":     created,
 		})
 		if err != nil {
-			return err
+			return nil, err
 		}
 		if key == "html" {
-			email.content.html = tplData.String()
+			email.html = tplData.String()
 		} else {
-			email.content.text = tplData.String()
+			email.text = tplData.String()
 		}
 	}
-	return nil
+	return email, nil
 }
 
-func (email *Emailer) constructReset(pwr Pwr, app *appContext) error {
+func (emailer *Emailer) constructReset(pwr Pwr, app *appContext) (*Email, error) {
-	email.content.subject = app.config.Section("password_resets").Key("subject").MustString("Password reset - Jellyfin")
+	email := &Email{
-	d, t, expires_in := email.formatExpiry(pwr.Expiry, true, app.datePattern, app.timePattern)
+		subject: app.config.Section("password_resets").Key("subject").MustString("Password reset - Jellyfin"),
+	}
+	d, t, expires_in := emailer.formatExpiry(pwr.Expiry, true, app.datePattern, app.timePattern)
 	message := app.config.Section("email").Key("message").String()
 	for _, key := range []string{"html", "text"} {
 		fpath := app.config.Section("password_resets").Key("email_" + key).String()
 		tpl, err := template.ParseFiles(fpath)
 		if err != nil {
-			return err
+			return nil, err
 		}
 		var tplData bytes.Buffer
 		err = tpl.Execute(&tplData, map[string]string{
@@ -254,17 +262,17 @@ func (email *Emailer) constructReset(pwr Pwr, app *appContext) error {
 			"message":     message,
 		})
 		if err != nil {
-			return err
+			return nil, err
 		}
 		if key == "html" {
-			email.content.html = tplData.String()
+			email.html = tplData.String()
 		} else {
-			email.content.text = tplData.String()
+			email.text = tplData.String()
 		}
 	}
-	return nil
+	return email, nil
 }
 
-func (emailer *Emailer) send(address string) error {
+func (emailer *Emailer) send(address string, email *Email) error {
-	return emailer.sender.send(address, emailer.fromName, emailer.fromAddr, emailer.content)
+	return emailer.sender.send(address, emailer.fromName, emailer.fromAddr, email)
 }

pwreset.go

@@ -71,10 +71,11 @@ func pwrMonitor(app *appContext, watcher *fsnotify.Watcher) {
 						app.err.Printf("Couldn't find email for user \"%s\". Make sure it's set", pwr.Username)
 						return
 					}
-					if err := app.email.constructReset(pwr, app); err != nil {
+					msg, err := app.email.constructReset(pwr, app)
+					if err != nil {
 						app.err.Printf("Failed to construct password reset email for %s", pwr.Username)
 						app.debug.Printf("%s: Error: %s", pwr.Username, err)
-					} else if err := app.email.send(address); err != nil {
+					} else if err := app.email.send(address, msg); err != nil {
 						app.err.Printf("Failed to send password reset email to \"%s\"", address)
 						app.debug.Printf("%s: Error: %s", pwr.Username, err)
 					} else {