fix(gateway): clamp the derived BEAM scheduler count (#2243)

deploy/self-hosting/.env.example

@@ -106,3 +106,10 @@ FLUXER_DISCOVERY_ENABLED=true
 # durable state elsewhere.
 #FLUXER_VALKEY_MAXMEMORY=192mb
 #FLUXER_VALKEY_MAXMEMORY_POLICY=noeviction
+
+# The gateway derives its BEAM scheduler count from the container CPU quota,
+# clamped to this range. The floor matters: a single scheduler lets one blocking
+# operation stall every websocket on the node. The ceiling stops a large host
+# from starting far more schedulers than the container can actually use.
+#FLUXER_ERLANG_SCHEDULERS_MIN=2
+#FLUXER_ERLANG_SCHEDULERS_MAX=16

fluxer_gateway/scripts/docker_entrypoint.sh

@@ -37,12 +37,27 @@ available_cpu_count() {
 	fi
 }
 
+clamp_int() {
+	value="$1"
+	min="$2"
+	max="$3"
+	if [ "$value" -lt "$min" ]; then
+		value="$min"
+	fi
+	if [ "$value" -gt "$max" ]; then
+		value="$max"
+	fi
+	echo "$value"
+}
+
+: "${FLUXER_ERLANG_SCHEDULERS_MIN:=2}"
+: "${FLUXER_ERLANG_SCHEDULERS_MAX:=16}"
 : "${FLUXER_ERLANG_NODE_NAME:[email protected]}"
 : "${FLUXER_ERLANG_COOKIE:=fluxer_gateway_dev_cookie}"
 : "${FLUXER_ERLANG_DIST_PORT:=8081}"
 
 if ! is_positive_int "${FLUXER_ERLANG_SCHEDULERS:-}"; then
-	FLUXER_ERLANG_SCHEDULERS="$(available_cpu_count)"
+	FLUXER_ERLANG_SCHEDULERS="$(clamp_int "$(available_cpu_count)" "$FLUXER_ERLANG_SCHEDULERS_MIN" "$FLUXER_ERLANG_SCHEDULERS_MAX")"
 fi
 
 if ! is_positive_int "${FLUXER_ERLANG_DIRTY_CPU_SCHEDULERS:-}"; then