perf(cmpp): isolate inbound transport capacity

gateway/internal/inbound/http_client.go

@@ -0,0 +1,26 @@
+package inbound
+
+import (
+	"net"
+	"net/http"
+	"time"
+)
+
+// NewAPIHTTPClient keeps enough loopback connections warm for the bounded CMPP
+// Submit window. Go's default of two idle connections per host otherwise causes
+// connection churn exactly when SubmitResp latency matters most.
+func NewAPIHTTPClient(maxConcurrency int) *http.Client {
+	if maxConcurrency < 1 {
+		maxConcurrency = 64
+	}
+	transport := http.DefaultTransport.(*http.Transport).Clone()
+	transport.MaxIdleConns = maxConcurrency + 16
+	transport.MaxIdleConnsPerHost = maxConcurrency
+	transport.MaxConnsPerHost = maxConcurrency
+	transport.IdleConnTimeout = 90 * time.Second
+	transport.DialContext = (&net.Dialer{
+		Timeout:   3 * time.Second,
+		KeepAlive: 30 * time.Second,
+	}).DialContext
+	return &http.Client{Transport: transport, Timeout: defaultHTTPTimeout}
+}