resubscribe when error in polling

ethers/providers/jsonrpc/subscriptions.nim

@@ -152,47 +152,62 @@ proc new*(_: type JsonRpcSubscriptions,
 
   let subscriptions = PollingSubscriptions(client: client)
 
-  proc getChanges(originalId: JsonNode): Future[JsonNode] {.async.} =
+  proc resubscribe(id: JsonNode) {.async: (raises: [CancelledError]).} =
     try:
-      let mappedId = subscriptions.subscriptionMapping[originalId]
-      let changes = await subscriptions.client.eth_getFilterChanges(mappedId)
-      if changes.kind == JNull:
-        return newJArray()
-      elif changes.kind != JArray:
-        raise newException(SubscriptionError,
-          "HTTP polling: unexpected value returned from eth_getFilterChanges." &
-          " Expected: JArray, got: " & $changes.kind)
-      return changes
-    except CancelledError as e:
-      raise e
-    except CatchableError as e:
-      if "filter not found" in e.msg:
       var newId: JsonNode
       # Log filters are stored in logFilters, block filters are not persisted
       # there is they do not need any specific data for their recreation.
       # We use this to determine if the filter was log or block filter here.
-        if subscriptions.logFilters.hasKey(originalId):
-          let filter = subscriptions.logFilters[originalId]
+      if subscriptions.logFilters.hasKey(id):
+        let filter = subscriptions.logFilters[id]
         newId = await subscriptions.client.eth_newFilter(filter)
       else:
         newId = await subscriptions.client.eth_newBlockFilter()
-        subscriptions.subscriptionMapping[originalId] = newId
-        return await getChanges(originalId)
-      else:
-        raise e
+      subscriptions.subscriptionMapping[id] = newId
+    except CancelledError as error:
+      raise error
+    except CatchableError:
+      # there's nothing further we can do here
+      discard
 
-  proc poll(id: JsonNode) {.async.} =
+  proc getChanges(id: JsonNode): Future[JsonNode] {.async: (raises: [CancelledError]).} =
+    try:
+      let mappedId = subscriptions.subscriptionMapping[id]
+      let changes = await subscriptions.client.eth_getFilterChanges(mappedId)
+      if changes.kind == JArray:
+        return changes
+    except KeyError as error:
+      raiseAssert "subscription mapping invalid: " & error.msg
+    except JsonRpcError:
+      await resubscribe(id)
+      # TODO: we could still miss some events between losing the subscription
+      # and resubscribing. We should probably adopt a strategy like ethers.js,
+      # whereby we keep track of the latest block number that we've seen
+      # filter changes for:
+      # https://github.com/ethers-io/ethers.js/blob/f97b92bbb1bde22fcc44100af78d7f31602863ab/packages/providers/src.ts/base-provider.ts#L977
+    except CancelledError as error:
+      raise error
+    except CatchableError:
+      # there's nothing we can do here
+      discard
+    return newJArray()
+
+  proc poll(id: JsonNode) {.async: (raises: [CancelledError]).} =
     for change in await getChanges(id):
       if callback =? subscriptions.getCallback(id):
         callback(id, change)
 
-  proc poll {.async.} =
-    untilCancelled:
+  proc poll {.async: (raises: []).} =
+    try:
+      while true:
         for id in toSeq subscriptions.callbacks.keys:
           await poll(id)
         await sleepAsync(pollingInterval)
+    except CancelledError:
+      discard
 
   subscriptions.polling = poll()
+  asyncSpawn subscriptions.polling
   subscriptions
 
 method close*(subscriptions: PollingSubscriptions) {.async.} =

testmodule/providers/jsonrpc/testJsonRpcSubscriptions.nim

@@ -1,4 +1,3 @@
-import pkg/serde
 import std/os
 import std/sequtils
 import std/importutils
@@ -106,13 +105,18 @@ suite "HTTP polling subscriptions - filter not found":
 
   privateAccess(PollingSubscriptions)
 
-  setup:
+  proc startServer() {.async.} =
     mockServer = MockRpcHttpServer.new()
     mockServer.start()
-
-    client = newRpcHttpClient()
     await client.connect("http://" & $mockServer.localAddress()[0])
 
+  proc stopServer() {.async.} =
+    await mockServer.stop()
+
+  setup:
+    client = newRpcHttpClient()
+    await startServer()
+
     subscriptions = PollingSubscriptions(
                       JsonRpcSubscriptions.new(
                         client,
@@ -174,3 +178,16 @@ suite "HTTP polling subscriptions - filter not found":
     await subscriptions.unsubscribe(id)
 
     check not subscriptions.subscriptionMapping.hasKey id
+
+  test "polling continues with new filter after temporary error":
+    let filter = EventFilter(address: Address.example, topics: @[array[32, byte].example])
+    let emptyHandler = proc(log: Log) = discard
+
+    let id = await subscriptions.subscribeLogs(filter, emptyHandler)
+
+    await stopServer()
+    mockServer.invalidateFilter(id)
+    await sleepAsync(50.milliseconds)
+    await startServer()
+
+    check eventually subscriptions.subscriptionMapping[id] != id