feat: add .well-known/webfinger

config/config.go

@@ -1,5 +1,10 @@
 package config
 
+import (
+	"net/url"
+	"strings"
+)
+
 type Config struct {
 	Core     CoreConfig     `toml:"core"`
 	Web      WebConfig      `toml:"web"`
@@ -9,11 +14,34 @@ type Config struct {
 type WebConfig struct {
 	// Domain should be the instance's full domain, including https:// but without the trailing slash.
 	Domain string `toml:"domain"`
+	// WebFingerDomain should be the instance's WebFinger domain, used for usernames.
+	// .well-known/webfinger *must* listen on this domain.
+	// It should only be the bare domain, i.e. `mercury.localhost`
+	WebFingerDomain string `toml:"webfinger_domain"`
 
 	// Port is the port the server should listen on.
 	Port int `toml:"port"`
 }
 
+// WebFingerDomains returns the domains valid for WebFinger requests.
+// The first one is always the canonical domain.
+// This function is guaranteed to return at least one domain.
+func (c WebConfig) WebFingerDomains() []string {
+	domains := make([]string, 0, 2)
+
+	if c.WebFingerDomain != "" {
+		domains = append(domains, c.WebFingerDomain)
+	}
+
+	u, err := url.Parse(c.Domain)
+	if err != nil {
+		return append(domains, strings.TrimPrefix(
+			strings.TrimPrefix(c.Domain, "http://"), "https://"))
+	}
+
+	return append(domains, u.Host)
+}
+
 type CoreConfig struct {
 	Postgres  string `toml:"postgres"`
 	Dev       bool   `toml:"dev"`