auth: Implement an Authenticator type

This patch implements an Authenticator type, which connections use to
do authentication and user existence checks.

It simplifies the abstractions (the server doesn't need to know about
userdb, or keep track of domain-userdb maps), and lays the foundation
for other types of authentication backends which will come in later
patches.

internal/smtpsrv/conn.go

@@ -30,7 +30,6 @@ import (
 	"blitiri.com.ar/go/chasquid/internal/set"
 	"blitiri.com.ar/go/chasquid/internal/tlsconst"
 	"blitiri.com.ar/go/chasquid/internal/trace"
-	"blitiri.com.ar/go/chasquid/internal/userdb"
 	"blitiri.com.ar/go/spf"
 )
 
@@ -120,9 +119,9 @@ type Conn struct {
 	// Are we using TLS?
 	onTLS bool
 
-	// User databases, aliases and local domains, taken from the server at
+	// Authenticator, aliases and local domains, taken from the server at
 	// creation time.
-	userDBs      map[string]*userdb.DB
+	authr        *auth.Authenticator
 	localDomains *set.String
 	aliasesR     *aliases.Resolver
 	dinfo        *domaininfo.DB
@@ -897,7 +896,11 @@ func (c *Conn) AUTH(params string) (code int, msg string) {
 		return 535, fmt.Sprintf("error decoding AUTH response: %v", err)
 	}
 
-	if auth.Authenticate(c.userDBs[domain], user, passwd) {
+	authOk, err := c.authr.Authenticate(user, domain, passwd)
+	if err != nil {
+		c.tr.Errorf("error authenticating %q@%q: %v", user, domain, err)
+	}
+	if authOk {
 		c.authUser = user
 		c.authDomain = domain
 		c.completedAuth = true
@@ -929,11 +932,11 @@ func (c *Conn) userExists(addr string) bool {
 	// look up "user" in our databases if the domain is local, which is what
 	// we want.
 	user, domain := envelope.Split(addr)
-	udb := c.userDBs[domain]
-	if udb == nil {
-		return false
+	ok, err := c.authr.Exists(user, domain)
+	if err != nil {
+		c.tr.Errorf("error checking if user %q exists: %v", addr, err)
 	}
-	return udb.HasUser(user)
+	return ok
 }
 
 func (c *Conn) readCommand() (cmd, params string, err error) {