Add a `relaxed-idna` feature to allow some uses of `_` in hostnames.

This is added to aid migration from Codeberg Pages v2. Forgejo allows
both `_` and `-` in usernames, and it is necessary to be able to accept
host names like `user_name.codeberg.page` under a wildcard domain.
(It is not possible to get a TLS certificate for a host name like this,
so only a wildcard certificate will be able to cover it.)

.forgejo/workflows/ci.yaml

···
       - name: Build service
         run: |
           go build
+      - name: Run tests
+        run: |
+          go test ./src
       - name: Run static analysis
         run: |
           go vet

src/auth.go

···
 	// this also rejects invalid characters and labels
 	host, err = idnaProfile.ToASCII(host)
 	if err != nil {
-		return "", AuthError{http.StatusBadRequest,
-			fmt.Sprintf("malformed host name %q", host)}
+		if config.Feature("relaxed-idna") {
+			// unfortunately, the go IDNA library has some significant issues around its
+			// Unicode TR46 implementation: https://github.com/golang/go/issues/76804
+			// we would like to allow *just* the _ here, but adding `idna.StrictDomainName(false)`
+			// would also accept domains like `*.foo.bar` which should clearly be disallowed.
+			// as a workaround, accept a domain name if it is valid with all `_` characters
+			// replaced with an alphanumeric character (we use `a`); this allows e.g. `foo_bar.xxx`
+			// and `foo__bar.xxx`, as well as `_foo.xxx` and `foo_.xxx`. labels starting with
+			// an underscore are explicitly rejected below.
+			_, err = idnaProfile.ToASCII(strings.ReplaceAll(host, "_", "a"))
+		}
+		if err != nil {
+			return "", AuthError{http.StatusBadRequest,
+				fmt.Sprintf("malformed host name %q", host)}
+		}
 	}
-	if strings.HasPrefix(host, ".") {
+	if strings.HasPrefix(host, ".") || strings.HasPrefix(host, "_") {
 		return "", AuthError{http.StatusBadRequest,
-			fmt.Sprintf("host name %q is reserved", host)}
+			fmt.Sprintf("reserved host name %q", host)}
 	}
 	host = strings.TrimSuffix(host, ".")
 	return host, nil

src/pages_test.go

···
+package git_pages
+
+import (
+	"net/http"
+	"strings"
+	"testing"
+)
+
+func checkHost(t *testing.T, host string, expectOk string, expectErr string) {
+	host, err := GetHost(&http.Request{Host: host})
+	if expectErr != "" {
+		if err == nil || !strings.HasPrefix(err.Error(), expectErr) {
+			t.Errorf("%s: expect err %s, got err %s", host, expectErr, err)
+		}
+	}
+	if expectOk != "" {
+		if err != nil {
+			t.Errorf("%s: expect ok %s, got err %s", host, expectOk, err)
+		} else if host != expectOk {
+			t.Errorf("%s: expect ok %s, got ok %s", host, expectOk, host)
+		}
+	}
+}
+
+func TestHelloName(t *testing.T) {
+	config = &Config{Features: []string{}}
+
+	checkHost(t, "foo.bar", "foo.bar", "")
+	checkHost(t, "foo-baz.bar", "foo-baz.bar", "")
+	checkHost(t, "foo--baz.bar", "foo--baz.bar", "")
+	checkHost(t, "foo.bar.", "foo.bar", "")
+	checkHost(t, ".foo.bar", "", "reserved host name")
+	checkHost(t, "..foo.bar", "", "reserved host name")
+
+	checkHost(t, "ß.bar", "xn--zca.bar", "")
+	checkHost(t, "xn--zca.bar", "xn--zca.bar", "")
+
+	checkHost(t, "foo-.bar", "", "malformed host name")
+	checkHost(t, "-foo.bar", "", "malformed host name")
+	checkHost(t, "foo_.bar", "", "malformed host name")
+	checkHost(t, "_foo.bar", "", "malformed host name")
+	checkHost(t, "foo_baz.bar", "", "malformed host name")
+	checkHost(t, "foo__baz.bar", "", "malformed host name")
+	checkHost(t, "*.foo.bar", "", "malformed host name")
+
+	config = &Config{Features: []string{"relaxed-idna"}}
+
+	checkHost(t, "foo-.bar", "", "malformed host name")
+	checkHost(t, "-foo.bar", "", "malformed host name")
+	checkHost(t, "foo_.bar", "foo_.bar", "")
+	checkHost(t, "_foo.bar", "", "reserved host name")
+	checkHost(t, "foo_baz.bar", "foo_baz.bar", "")
+	checkHost(t, "foo__baz.bar", "foo__baz.bar", "")
+	checkHost(t, "*.foo.bar", "", "malformed host name")
+}