處理 Google Cloud Messaging for Android (GCM) token 重複問題 (canonical_ids)

事情是這樣的，拖了很久才開始處理這段 XD 拖到連 GCM 都要下線了：

As of April 10, 2018, Google has deprecated GCM. The GCM server and client APIs are deprecated and will be removed as soon as April 11, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure, plus many new features. See the migration guide to learn more.

為何要處理？主因是一隻 android mobile device 很有可能因為隱私狀態改變、重新安裝軟體後，導致 GCM token 變更，如果當年沒有設計 global uuid 來辨識移動裝置的話（像是綁個人帳號、用 wifi mac_address 等），那 token 肯定會收到一卡車多，這時要發送訊息時，就可能出現多個 token 對應到同一位使用者，造成用戶被 Push  轟炸 (這些在 Firebase 設計理念上都有改善了，例如可以單純對全部 android user 發訊息等等），特別是 QA 常常安裝反安裝程式...

原先我們依賴著 wifi mac_address ，但在某版 Android 發現他是可變的 Orz （忘了是隱私提升還是某OEM廠的bug） 對於維護 GCM token ，就只剩下一條路：試著對他發送訊息，檢查回應是不是有變化。

連續動作：

$check_failed_db_ids = array();
$tokens = array(
array( 'id' => 'db record id', 'token' => 'GCM token'),
// ...
);

$payload = array(
'registration_ids' => array(),
'dry_run' => true,
'data' => array(
'body' => 'test'
),
);

while(count($tokens) > 0) {
//
// Step 1: init
//
$payload['registration_ids'] = array();
$current_task = array_splice($tokens, 0, 900);
foreach($current_task as $items) {
array_push($payload['registration_ids'], $items['token']);
}

//
// Step 2: call gcm api
//
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://android.googleapis.com/gcm/send");
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Authorization: key=".$api_key,
'Content-Type: application/json'
));
curl_setopt($ch, CURLOPT_POST , true);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($payload));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
//curl_setopt($ch, CURLOPT_VERBOSE, true);
$ret = curl_exec($ch);
curl_close($ch);

$ret_json = @json_decode($ret, true);
if (isset($ret_json['results'])) {
$echo_cnt = count($ret_json['results']);
for ($i=0;$i<$echo_cnt; ++$i) {
// 通常有 error 都可以考慮去掉了；當有 registration_id 則是代表已轉換 token
// 完整範例跟解釋：https://developers.google.com/cloud-messaging/http#example-responses
if (isset($ret_json['results'][$i]['error']) || isset($ret_json['results'][$i]['registration_id'])) {
array_push($check_failed_db_ids, $i);

}
}
}

//
// Step 3: update db record & reset $check_failed_db_ids
//
// ...

}