Introduce a fast proxy mode to improve HTTP/1.1 performances with backends

Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>

pkg/proxy/httputil/builder.go

@@ -0,0 +1,54 @@
+package httputil
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net/http"
+	"net/url"
+	"time"
+
+	"github.com/traefik/traefik/v3/pkg/config/dynamic"
+	"github.com/traefik/traefik/v3/pkg/metrics"
+)
+
+// TransportManager manages transport used for backend communications.
+type TransportManager interface {
+	Get(name string) (*dynamic.ServersTransport, error)
+	GetRoundTripper(name string) (http.RoundTripper, error)
+	GetTLSConfig(name string) (*tls.Config, error)
+}
+
+// ProxyBuilder handles the http.RoundTripper for httputil reverse proxies.
+type ProxyBuilder struct {
+	bufferPool             *bufferPool
+	transportManager       TransportManager
+	semConvMetricsRegistry *metrics.SemConvMetricsRegistry
+}
+
+// NewProxyBuilder creates a new ProxyBuilder.
+func NewProxyBuilder(transportManager TransportManager, semConvMetricsRegistry *metrics.SemConvMetricsRegistry) *ProxyBuilder {
+	return &ProxyBuilder{
+		bufferPool:             newBufferPool(),
+		transportManager:       transportManager,
+		semConvMetricsRegistry: semConvMetricsRegistry,
+	}
+}
+
+// Update does nothing.
+func (r *ProxyBuilder) Update(_ map[string]*dynamic.ServersTransport) {}
+
+// Build builds a new httputil.ReverseProxy with the given configuration.
+func (r *ProxyBuilder) Build(cfgName string, targetURL *url.URL, shouldObserve, passHostHeader bool, flushInterval time.Duration) (http.Handler, error) {
+	roundTripper, err := r.transportManager.GetRoundTripper(cfgName)
+	if err != nil {
+		return nil, fmt.Errorf("getting RoundTripper: %w", err)
+	}
+
+	if shouldObserve {
+		// Wrapping the roundTripper with the Tracing roundTripper,
+		// to handle the reverseProxy client span creation.
+		roundTripper = newObservabilityRoundTripper(r.semConvMetricsRegistry, roundTripper)
+	}
+
+	return buildSingleHostProxy(targetURL, passHostHeader, flushInterval, roundTripper, r.bufferPool), nil
+}