little update.

libmxclient/mxapi/discover.go

@@ -0,0 +1,32 @@
+package mxapi
+
+import (
+	"context"
+	"encoding/json"
+
+	"maunium.net/go/mautrix"
+	"maunium.net/go/mautrix/id"
+)
+
+func Discover(mxid string) (string, error) {
+
+	localpart, hs, err := id.UserID(mxid).ParseAndValidateRelaxed()
+	if err != nil {
+		return "", err
+	}
+
+	wk, err := mautrix.DiscoverClientAPI(context.Background(), hs)
+	if err != nil {
+		return "", err
+	}
+	if wk != nil {
+		hs = wk.Homeserver.BaseURL
+	}
+
+	out, err := json.Marshal(map[string]string{"mxid": mxid, "homeserver": hs, "loginname": localpart})
+	if err != nil {
+		return "", err
+	}
+
+	return string(out), nil
+}