rpc/client/http: Do not drop events even if the `out` channel is full (#6163)

```
// unbuffered
out, err := httpClient.Subscribe(ctx, "event.type=NewTx and account.name=Jack", 0)

// buffered
out, err := httpClient.Subscribe(ctx, "event.type=NewTx AND account.name=Jack", 20)
```

Before: when the `out` channel is buffered and becomes full, we drop an event (+ log the error)
After: when the `out` channel is buffered and becomes full, we block

**Before it was not apparent to the app when an event was dropped (looking at the logs is manual task). After this PR, if the user does not read from `out` on 1 subscription, all other subscriptions will be stuck too.**

Closes #6161

CHANGELOG_PENDING.md

@@ -65,6 +65,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 - [consensus] \#5987 Remove `time_iota_ms` from consensus params. Merge `tmproto.ConsensusParams` and `abci.ConsensusParams`. (@marbar3778)
 - [types] \#5994 Reduce the use of protobuf types in core logic. (@marbar3778)
   - `ConsensusParams`, `BlockParams`, `ValidatorParams`, `EvidenceParams`, `VersionParams`, `sm.Version` and `version.Consensus` have become native types. They still utilize protobuf when being sent over the wire or written to disk.
+- [rpc/client/http] \#6163 Do not drop events even if the `out` channel is full (@melekes)
 - [node] \#6059 Validate and complete genesis doc before saving to state store (@silasdavis)
 - [state] \#6067 Batch save state data (@githubsands & @cmwaters)
 

rpc/client/http/http.go

@@ -582,10 +582,15 @@ func (w *WSEvents) OnStop() {
 }
 
 // Subscribe implements EventsClient by using WSClient to subscribe given
-// subscriber to query. By default, returns a channel with cap=1. Error is
+// subscriber to query. By default, it returns a channel with cap=1. Error is
 // returned if it fails to subscribe.
 //
-// Channel is never closed to prevent clients from seeing an erroneous event.
+// When reading from the channel, keep in mind there's a single events loop, so
+// if you don't read events for this subscription fast enough, other
+// subscriptions will slow down in effect.
+//
+// The channel is never closed to prevent clients from seeing an erroneous
+// event.
 //
 // It returns an error if WSEvents is not running.
 func (w *WSEvents) Subscribe(ctx context.Context, subscriber, query string,
@@ -662,12 +667,15 @@ func (w *WSEvents) UnsubscribeAll(ctx context.Context, subscriber string) error
 func (w *WSEvents) redoSubscriptionsAfter(d time.Duration) {
 	time.Sleep(d)
 
-	w.mtx.RLock()
-	defer w.mtx.RUnlock()
+	ctx := context.Background()
+
+	w.mtx.Lock()
+	defer w.mtx.Unlock()
 	for q := range w.subscriptions {
-		err := w.ws.Subscribe(context.Background(), q)
+		err := w.ws.Subscribe(ctx, q)
 		if err != nil {
-			w.Logger.Error("Failed to resubscribe", "err", err)
+			w.Logger.Error("failed to resubscribe", "query", q, "err", err)
+			delete(w.subscriptions, q)
 		}
 	}
 }
@@ -706,18 +714,15 @@ func (w *WSEvents) eventListener() {
 			}
 
 			w.mtx.RLock()
-			if out, ok := w.subscriptions[result.Query]; ok {
-				if cap(out) == 0 {
-					out <- *result
-				} else {
-					select {
-					case out <- *result:
-					default:
-						w.Logger.Error("wanted to publish ResultEvent, but out channel is full", "result", result, "query", result.Query)
-					}
+			out, ok := w.subscriptions[result.Query]
+			w.mtx.RUnlock()
+			if ok {
+				select {
+				case out <- *result:
+				case <-w.Quit():
+					return
 				}
 			}
-			w.mtx.RUnlock()
 		case <-w.Quit():
 			return
 		}