aliases: Skip resolution logic for non-local addresses

This patch skips the resolution logic if the address is not local.
Today, the resolution logic handles that case transparently, and returns
the original email address, so this should be a no-op.

However, having an explicit early check makes the resolution logic more
robust, and will simplify future patches.

Note this also means that the `alias-resolve` hook is no longer run for
non-local aliases, which should also help simplify their implementation.

internal/aliases/aliases.go

@@ -145,7 +145,7 @@ func (v *Resolver) Resolve(addr string) ([]Recipient, error) {
 // Exists check that the address exists in the database.
 // It returns the cleaned address, and a boolean indicating the result.
 // The clean address can be used to look it up in other databases, even if it
-// doesn't exist.
+// doesn't exist. It must only be called for local addresses.
 func (v *Resolver) Exists(addr string) (string, bool) {
 	addr = v.cleanIfLocal(addr)
 
@@ -171,6 +171,13 @@ func (v *Resolver) resolve(rcount int, addr string) ([]Recipient, error) {
 		return nil, ErrRecursionLimitExceeded
 	}
 
+	// If the address is not local, we return it as-is, so delivery is
+	// attempted against it.
+	// Example: an alias that resolves to a non-local address.
+	if _, ok := v.domains[envelope.DomainOf(addr)]; !ok {
+		return []Recipient{{addr, EMAIL}}, nil
+	}
+
 	// Drop suffixes and chars to get the "clean" address before resolving.
 	// This also means that we will return the clean version if there's no
 	// match, which our callers can rely upon.

internal/aliases/aliases_test.go

@@ -13,6 +13,7 @@ type Cases []struct {
 }
 
 func (cases Cases) check(t *testing.T, r *Resolver) {
+	t.Helper()
 	for _, c := range cases {
 		got, err := r.Resolve(c.addr)
 		if err != nil {
@@ -26,6 +27,7 @@ func (cases Cases) check(t *testing.T, r *Resolver) {
 }
 
 func mustExist(t *testing.T, r *Resolver, addrs ...string) {
+	t.Helper()
 	for _, addr := range addrs {
 		if _, ok := r.Exists(addr); !ok {
 			t.Errorf("address %q does not exist, it should", addr)
@@ -34,6 +36,7 @@ func mustExist(t *testing.T, r *Resolver, addrs ...string) {
 }
 
 func mustNotExist(t *testing.T, r *Resolver, addrs ...string) {
+	t.Helper()
 	for _, addr := range addrs {
 		if _, ok := r.Exists(addr); ok {
 			t.Errorf("address %q exists, it should not", addr)
@@ -43,20 +46,22 @@ func mustNotExist(t *testing.T, r *Resolver, addrs ...string) {
 
 func TestBasic(t *testing.T) {
 	resolver := NewResolver()
+	resolver.AddDomain("localA")
+	resolver.AddDomain("localB")
 	resolver.aliases = map[string][]Recipient{
-		"a@b": {{"c@d", EMAIL}, {"e@f", EMAIL}},
-		"e@f": {{"cmd", PIPE}},
-		"cmd": {{"x@y", EMAIL}}, // it's a trap!
+		"a@localA": {{"c@d", EMAIL}, {"e@localB", EMAIL}},
+		"e@localB": {{"cmd", PIPE}},
+		"cmd":      {{"x@y", EMAIL}}, // it's a trap!
 	}
 
 	cases := Cases{
-		{"a@b", []Recipient{{"c@d", EMAIL}, {"cmd", PIPE}}},
-		{"e@f", []Recipient{{"cmd", PIPE}}},
+		{"a@localA", []Recipient{{"c@d", EMAIL}, {"cmd", PIPE}}},
+		{"e@localB", []Recipient{{"cmd", PIPE}}},
 		{"x@y", []Recipient{{"x@y", EMAIL}}},
 	}
 	cases.check(t, resolver)
 
-	mustExist(t, resolver, "a@b", "e@f", "cmd")
+	mustExist(t, resolver, "a@localA", "e@localB", "cmd")
 	mustNotExist(t, resolver, "x@y")
 }
 
@@ -68,6 +73,7 @@ func TestAddrRewrite(t *testing.T) {
 		"abc@def":  {{"x@y", EMAIL}},
 		"ñoño@def": {{"x@y", EMAIL}},
 		"recu@def": {{"ab+cd@p-q.com", EMAIL}},
+		"remo@def": {{"x-@y-z.com", EMAIL}},
 	}
 	resolver.DropChars = ".~"
 	resolver.SuffixSep = "-+"
@@ -96,6 +102,7 @@ func TestAddrRewrite(t *testing.T) {
 		{"x.y@z.com", []Recipient{{"x.y@z.com", EMAIL}}},
 		{"x-@y-z.com", []Recipient{{"x-@y-z.com", EMAIL}}},
 		{"x+blah@y", []Recipient{{"x+blah@y", EMAIL}}},
+		{"remo@def", []Recipient{{"x-@y-z.com", EMAIL}}},
 	}
 	cases.check(t, resolver)
 }
@@ -143,7 +150,9 @@ func TestExistsRewrite(t *testing.T) {
 }
 
 func TestTooMuchRecursion(t *testing.T) {
-	resolver := Resolver{}
+	resolver := NewResolver()
+	resolver.AddDomain("b")
+	resolver.AddDomain("d")
 	resolver.aliases = map[string][]Recipient{
 		"a@b": {{"c@d", EMAIL}},
 		"c@d": {{"a@b", EMAIL}},