synchronizer: ensure fairness between wallets

Scenario (prior to change):
User opens wallet1 with 10k addresses, and then immediately opens wallet2
with 100 addresses.
wallet1 will synchronise first, fully, and only then will wallet2 start syncing.

Now, wallet1 and wallet2 will sync concurrently (and wallet2 will finish much
sooner as expected).

electrum/synchronizer.py

@@ -61,6 +61,10 @@ class SynchronizerBase(NetworkJobOnDefaultServer):
     def __init__(self, network: 'Network'):
         self.asyncio_loop = network.asyncio_loop
         self._reset_request_counters()
+        # Ensure fairness between Synchronizers. e.g. if multiple wallets
+        # are open, a large wallet should not starve the small wallets:
+        self._network_request_semaphore = asyncio.Semaphore(100)
+
         NetworkJobOnDefaultServer.__init__(self, network)
 
     def _reset(self):
@@ -106,7 +110,8 @@ class SynchronizerBase(NetworkJobOnDefaultServer):
             self.scripthash_to_address[h] = addr
             self._requests_sent += 1
             try:
-                await self.session.subscribe('blockchain.scripthash.subscribe', [h], self.status_queue)
+                async with self._network_request_semaphore:
+                    await self.session.subscribe('blockchain.scripthash.subscribe', [h], self.status_queue)
             except RPCError as e:
                 if e.message == 'history too large':  # no unique error code
                     raise GracefulDisconnect(e, log_level=logging.ERROR) from e
@@ -167,7 +172,8 @@ class Synchronizer(SynchronizerBase):
         self.requested_histories.add((addr, status))
         h = address_to_scripthash(addr)
         self._requests_sent += 1
-        result = await self.interface.get_history_for_scripthash(h)
+        async with self._network_request_semaphore:
+            result = await self.interface.get_history_for_scripthash(h)
         self._requests_answered += 1
         self.logger.info(f"receiving history {addr} {len(result)}")
         hashes = set(map(lambda item: item['tx_hash'], result))
@@ -210,7 +216,8 @@ class Synchronizer(SynchronizerBase):
     async def _get_transaction(self, tx_hash, *, allow_server_not_finding_tx=False):
         self._requests_sent += 1
         try:
-            raw_tx = await self.interface.get_transaction(tx_hash)
+            async with self._network_request_semaphore:
+                raw_tx = await self.interface.get_transaction(tx_hash)
         except RPCError as e:
             # most likely, "No such mempool or blockchain transaction"
             if allow_server_not_finding_tx: