SSE implementation that sheds stuck clients (#14413)

* sse implementation that sheds stuck clients

* Radek and James feedback

* Refactor event streamer code for readability

* less-flaky test signaling

* test case where queue fills; fixes

* add changelog entry

* james and preston feedback

* swap our Subscription interface with an alias

* event.Data can be nil for the payload attr event

* deepsource

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>

api/headers.go

@@ -1,5 +1,7 @@
 package api
 
+import "net/http"
+
 const (
 	VersionHeader                 = "Eth-Consensus-Version"
 	ExecutionPayloadBlindedHeader = "Eth-Execution-Payload-Blinded"
@@ -10,3 +12,9 @@ const (
 	EventStreamMediaType          = "text/event-stream"
 	KeepAlive                     = "keep-alive"
 )
+
+// SetSSEHeaders sets the headers needed for a server-sent event response.
+func SetSSEHeaders(w http.ResponseWriter) {
+	w.Header().Set("Content-Type", EventStreamMediaType)
+	w.Header().Set("Connection", KeepAlive)
+}

api/server/structs/conversions.go

@@ -15,6 +15,7 @@ import (
 	"github.com/prysmaticlabs/prysm/v5/encoding/bytesutil"
 	"github.com/prysmaticlabs/prysm/v5/math"
 	enginev1 "github.com/prysmaticlabs/prysm/v5/proto/engine/v1"
+	ethv1 "github.com/prysmaticlabs/prysm/v5/proto/eth/v1"
 	eth "github.com/prysmaticlabs/prysm/v5/proto/prysm/v1alpha1"
 )
 
@@ -1508,3 +1509,37 @@ func PendingConsolidationsFromConsensus(cs []*eth.PendingConsolidation) []*Pendi
 	}
 	return consolidations
 }
+
+func HeadEventFromV1(event *ethv1.EventHead) *HeadEvent {
+	return &HeadEvent{
+		Slot:                      fmt.Sprintf("%d", event.Slot),
+		Block:                     hexutil.Encode(event.Block),
+		State:                     hexutil.Encode(event.State),
+		EpochTransition:           event.EpochTransition,
+		ExecutionOptimistic:       event.ExecutionOptimistic,
+		PreviousDutyDependentRoot: hexutil.Encode(event.PreviousDutyDependentRoot),
+		CurrentDutyDependentRoot:  hexutil.Encode(event.CurrentDutyDependentRoot),
+	}
+}
+
+func FinalizedCheckpointEventFromV1(event *ethv1.EventFinalizedCheckpoint) *FinalizedCheckpointEvent {
+	return &FinalizedCheckpointEvent{
+		Block:               hexutil.Encode(event.Block),
+		State:               hexutil.Encode(event.State),
+		Epoch:               fmt.Sprintf("%d", event.Epoch),
+		ExecutionOptimistic: event.ExecutionOptimistic,
+	}
+}
+
+func EventChainReorgFromV1(event *ethv1.EventChainReorg) *ChainReorgEvent {
+	return &ChainReorgEvent{
+		Slot:                fmt.Sprintf("%d", event.Slot),
+		Depth:               fmt.Sprintf("%d", event.Depth),
+		OldHeadBlock:        hexutil.Encode(event.OldHeadBlock),
+		NewHeadBlock:        hexutil.Encode(event.NewHeadBlock),
+		OldHeadState:        hexutil.Encode(event.OldHeadState),
+		NewHeadState:        hexutil.Encode(event.NewHeadState),
+		Epoch:               fmt.Sprintf("%d", event.Epoch),
+		ExecutionOptimistic: event.ExecutionOptimistic,
+	}
+}