From 82e5518961f41a6aad1d651f05869319426d39bd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20L=C3=A9tal?= <letal.vojtech@gmail.com>
Date: Thu, 30 Jul 2026 14:52:56 +0200
Subject: [PATCH] Log the server's error when registration is refused
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A refused registration is currently silent. RegisterResponse carries the
reason in Error — "invalid key: API key does not exist" for a bad auth
key — but do_register() only reads Node, which a successful response
omits too, so nothing tells the two apart.

The first visible symptom is therefore "node not found" from the
MapRequest that follows, reported by a different function and pointing at
peer fetching rather than at the key:

  E ml_coord: RegisterResponse error: invalid key: API key does not exist   (new)
  E ml_coord: MapResponse JSON parse failed near: node not found            (was all)

Tested against controlplane.tailscale.com on an ESP32-S3, with an unknown
machine key and an invalid auth key.

(cherry picked from commit 4cc3ddeddff3d65505fe4cc9cfa597c9bc51aa1b)
Upstream-PR: CamM2325/microlink#23
---
 components/microlink/src/ml_coord.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/components/microlink/src/ml_coord.c b/components/microlink/src/ml_coord.c
index 8da8d4d..7a02812 100644
--- a/components/microlink/src/ml_coord.c
+++ b/components/microlink/src/ml_coord.c
@@ -944,6 +944,14 @@ static int do_register(microlink_t *ml, ml_noise_state_t *noise) {
         free(resp_buf);
         return 0;  /* Not fatal - we'll get peers in MapResponse */
     }
+    /* The server reports a refused registration only in Error — a rejected auth
+     * key gives "invalid key: unable to validate API key". Node is absent either
+     * way, so without this the first symptom is MapResponse "node not found". */
+    const cJSON *reg_error = cJSON_GetObjectItem(resp_json, "Error");
+    if (cJSON_IsString(reg_error) && reg_error->valuestring[0] != '\0') {
+        ESP_LOGE(TAG, "RegisterResponse error: %s", reg_error->valuestring);
+    }
+
     parse_start[parse_len] = saved;
     free(resp_buf);