store completers similarly to subscriptions so we can await them

lib/electrumx_rpc/electrumx_chain_height_service.dart

@@ -7,4 +7,8 @@ abstract class ElectrumxChainHeightService {
   static Map<Coin, StreamSubscription<dynamic>?> subscriptions = {};
   // Used to hold chain height subscriptions for each coin as in:
   // ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin] = sub;
+
+  static Map<Coin, Completer<int>?> completers = {};
+  // Used to hold chain height completers for each coin as in:
+  // ElectrumxChainHeightService.completers[cryptoCurrency.coin] = completer;
 }

lib/wallets/wallet/wallet_mixin_interfaces/electrumx_interface.dart

@@ -805,25 +805,79 @@ mixin ElectrumXInterface<T extends Bip39HDCurrency> on Bip39HDWallet<T> {
   Future<int> fetchChainHeight() async {
     try {
       // Don't set a stream subscription if one already exists.
+      await _manageChainHeightSubscription();
+
+      if (_latestHeight == null) {
+        // Probably waiting on the subscription to receive the latest block
+        // height, fallback to cached value
+        return info.cachedChainHeight;
+      } else {
+        return _latestHeight!;
+      }
+    } catch (e, s) {
+      Logging.instance.log(
+          "Exception rethrown in fetchChainHeight\nError: $e\nStack trace: $s",
+          level: LogLevel.Error);
+      // completer.completeError(e, s);
+      // return Future.error(e, s);
+      rethrow;
+    }
+  }
+
+  Future<void> _manageChainHeightSubscription() async {
     if (ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin] ==
         null) {
-        return _manageChainHeightSubscription();
+      // No subscription exists for this coin yet, so create one.
+      //
+      // Set up to wait for the first response.
+      final Completer<int> completer = Completer<int>();
+      ElectrumxChainHeightService.completers[cryptoCurrency.coin] ??= completer;
+
+      // Make sure we only complete once.
+      final isFirstResponse = _latestHeight == null;
+
+      // Subscribe to block headers.
+      final subscription =
+          subscribableElectrumXClient.subscribeToBlockHeaders();
+
+      // Set stream subscription.
+      ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin] =
+          subscription.responseStream.asBroadcastStream().listen((event) {
+        final response = event;
+        if (response != null &&
+            response is Map &&
+            response.containsKey('height')) {
+          final int chainHeight = response['height'] as int;
+          // print("Current chain height: $chainHeight");
+
+          _latestHeight = chainHeight;
+
+          if (isFirstResponse) {
+            // If the completer is not completed, complete it.
+            if (!ElectrumxChainHeightService
+                .completers[cryptoCurrency.coin]!.isCompleted) {
+              // Complete the completer, returning the chain height.
+              ElectrumxChainHeightService.completers[cryptoCurrency.coin]!
+                  .complete(chainHeight);
             }
-      // Don't set a stream subscription if one already exists.
+          }
-      else {
+        } else {
-        //IF there's already a wallet for a coin the chain height might not be
+          Logging.instance.log(
-        // stored for current wallet
+              "blockchain.headers.subscribe returned malformed response\n"
-        // Check if the stream subscription is paused.
+              "Response: $response",
+              level: LogLevel.Error);
+        }
+      });
+    } else {
+      // A subscription already exists.
+      //
+      // Resume the stream subscription if it's paused.
       if (ElectrumxChainHeightService
           .subscriptions[cryptoCurrency.coin]!.isPaused) {
         // If it's paused, resume it.
         ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin]!
             .resume();
       }
-        if (_latestHeight == null) {
-          //Get the chain height
-          return  _manageChainHeightSubscription();
-        }
 
       // Causes synchronization to stall.
       // // Check if the stream subscription is active by pinging it.
@@ -838,58 +892,29 @@ mixin ElectrumXInterface<T extends Bip39HDCurrency> on Bip39HDWallet<T> {
       //   return completer.future;
       // }
 
-        if (_latestHeight != null) {
+      // If there's no completer set for this coin, something's gone wrong.
-          return _latestHeight!;
+      //
+      // The completer is always set before the subscription, so this should
+      // never happen.
+      if (ElectrumxChainHeightService.completers[cryptoCurrency.coin] == null) {
+        // Clear this coin's subscription.
+        await ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin]!
+            .cancel();
+        ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin] = null;
+
+        // Clear this coin's completer.
+        ElectrumxChainHeightService.completers[cryptoCurrency.coin] = null;
+
+        // Retry/recurse.
+        return await _manageChainHeightSubscription();
       }
     }
 
-      // Probably waiting on the subscription to receive the latest block height
+    // Wait for the first response.
-      // fallback to cached value
+    _latestHeight = await ElectrumxChainHeightService
-      return info.cachedChainHeight;
+        .completers[cryptoCurrency.coin]!.future;
-    } catch (e, s) {
-      Logging.instance.log(
-          "Exception rethrown in fetchChainHeight\nError: $e\nStack trace: $s",
-          level: LogLevel.Error);
-      // completer.completeError(e, s);
-      // return Future.error(e, s);
-      rethrow;
-    }
-  }
 
-  Future<int> _manageChainHeightSubscription() async {
+    return;
-    final Completer<int> completer = Completer<int>();
-    // Make sure we only complete once.
-    final isFirstResponse = _latestHeight == null;
-
-    // Subscribe to block headers.
-    final subscription =
-    subscribableElectrumXClient.subscribeToBlockHeaders();
-
-    // set stream subscription
-    ElectrumxChainHeightService.subscriptions[cryptoCurrency.coin] =
-        subscription.responseStream.asBroadcastStream().listen((event) {
-          final response = event;
-          if (response != null &&
-              response is Map &&
-              response.containsKey('height')) {
-            final int chainHeight = response['height'] as int;
-            // print("Current chain height: $chainHeight");
-
-            _latestHeight = chainHeight;
-
-            if (isFirstResponse && !completer.isCompleted) {
-              // Return the chain height.
-              completer.complete(chainHeight);
-            }
-          } else {
-            Logging.instance.log(
-                "blockchain.headers.subscribe returned malformed response\n"
-                    "Response: $response",
-                level: LogLevel.Error);
-          }
-        });
-
-    return _latestHeight ?? await completer.future;
   }
 
   Future<int> fetchTxCount({required String addressScriptHash}) async {