Normalize local usernames using PRECIS

This patch implements local username normalization using PRECIS
(https://tools.ietf.org/html/rfc7564,
https://tools.ietf.org/html/rfc7613)

It makes chasquid accept local email and authentication regardless of
the case. It covers both userdb and aliases.

Note that non-local usernames remain untouched.

internal/normalize/normalize.go

@@ -0,0 +1,31 @@
+// Package normalize contains functions to normalize usernames and addresses.
+package normalize
+
+import (
+	"blitiri.com.ar/go/chasquid/internal/envelope"
+	"golang.org/x/text/secure/precis"
+)
+
+// User normalices an username using PRECIS.
+// On error, it will also return the original username to simplify callers.
+func User(user string) (string, error) {
+	norm, err := precis.UsernameCaseMapped.String(user)
+	if err != nil {
+		return user, err
+	}
+
+	return norm, nil
+}
+
+// Name normalices an email address using PRECIS.
+// On error, it will also return the original address to simplify callers.
+func Addr(addr string) (string, error) {
+	user, domain := envelope.Split(addr)
+
+	user, err := User(user)
+	if err != nil {
+		return addr, err
+	}
+
+	return user + "@" + domain, nil
+}

internal/normalize/normalize_test.go

@@ -0,0 +1,64 @@
+package normalize
+
+import "testing"
+
+func TestUser(t *testing.T) {
+	valid := []struct{ user, norm string }{
+		{"ÑAndÚ", "ñandú"},
+		{"Pingüino", "pingüino"},
+	}
+	for _, c := range valid {
+		nu, err := User(c.user)
+		if nu != c.norm {
+			t.Errorf("%q normalized to %q, expected %q", c.user, nu, c.norm)
+		}
+		if err != nil {
+			t.Errorf("%q error: %v", c.user, err)
+		}
+
+	}
+
+	invalid := []string{
+		"á é", "a\te", "x ", "x\xa0y", "x\x85y", "x\vy", "x\fy", "x\ry",
+		"henry\u2163", "\u265a", "\u00b9",
+	}
+	for _, u := range invalid {
+		nu, err := User(u)
+		if err == nil {
+			t.Errorf("expected User(%+q) to fail, but did not", u)
+		}
+		if nu != u {
+			t.Errorf("%+q failed norm, but returned %+q", u, nu)
+		}
+	}
+}
+
+func TestAddr(t *testing.T) {
+	valid := []struct{ user, norm string }{
+		{"ÑAndÚ@pampa", "ñandú@pampa"},
+		{"Pingüino@patagonia", "pingüino@patagonia"},
+	}
+	for _, c := range valid {
+		nu, err := Addr(c.user)
+		if nu != c.norm {
+			t.Errorf("%q normalized to %q, expected %q", c.user, nu, c.norm)
+		}
+		if err != nil {
+			t.Errorf("%q error: %v", c.user, err)
+		}
+
+	}
+
+	invalid := []string{
+		"á é@i", "henry\u2163@throne",
+	}
+	for _, u := range invalid {
+		nu, err := Addr(u)
+		if err == nil {
+			t.Errorf("expected Addr(%+q) to fail, but did not", u)
+		}
+		if nu != u {
+			t.Errorf("%+q failed norm, but returned %+q", u, nu)
+		}
+	}
+}