fix(quickproxy): ensure request body is preserved on fallback

pkg/server/quickproxy/quickproxy.go

@@ -5,8 +5,10 @@
 package quickproxy
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/http/httputil"
@@ -191,6 +193,15 @@ func (s *Service) Handler(fallback http.Handler) http.Handler {
 
 	for i := range s.rules {
 		s.rules[i].proxy.ErrorHandler = func(w http.ResponseWriter, r *http.Request, _ error) {
+			// ReverseProxy consumes and closes r.Body while attempting the
+			// upstream request, even when that attempt fails (per the
+			// http.RoundTripper contract). Restore a fresh copy from
+			// r.GetBody, set below, before handing the request to fallback.
+			if r.GetBody != nil {
+				if body, err := r.GetBody(); err == nil {
+					r.Body = body
+				}
+			}
 			fallback.ServeHTTP(w, r)
 		}
 	}
@@ -201,6 +212,22 @@ func (s *Service) Handler(fallback http.Handler) http.Handler {
 			fallback.ServeHTTP(w, r)
 			return
 		}
+
+		// Buffer the body so it can be replayed to fallback if the upstream
+		// attempt fails; see ErrorHandler above.
+		if r.Body != nil && r.Body != http.NoBody {
+			bodyBytes, err := io.ReadAll(r.Body)
+			r.Body.Close()
+			if err != nil {
+				http.Error(w, "failed to read request body", http.StatusInternalServerError)
+				return
+			}
+			r.Body = io.NopCloser(bytes.NewReader(bodyBytes))
+			r.GetBody = func() (io.ReadCloser, error) {
+				return io.NopCloser(bytes.NewReader(bodyBytes)), nil
+			}
+		}
+
 		rule.proxy.ServeHTTP(w, r)
 	})
 }

pkg/server/quickproxy/quickproxy_test.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 )
@@ -130,6 +131,75 @@ func TestHandler_UnreachableUpstreamFallsBack(t *testing.T) {
 	}
 }
 
+func TestHandler_UnreachableUpstreamFallsBackWithBody(t *testing.T) {
+	// A closed listener address: nothing is listening, so dialing fails and
+	// ReverseProxy invokes ErrorHandler. The fallback handler must still see
+	// the original request body, even though ReverseProxy consumed and
+	// closed it while attempting (and failing) the upstream request.
+	unreachable := "http://127.0.0.1:1"
+
+	svc, err := NewService([]Rule{{URLPrefix: "/", Target: unreachable}}, WithTimeout(500*time.Millisecond))
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	echoBody := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("fallback reading body: %v", err)
+		}
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write(body)
+	})
+
+	handler := svc.Handler(echoBody)
+
+	req := httptest.NewRequest(http.MethodPost, "/submit", strings.NewReader("payload=1"))
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "payload=1" {
+		t.Fatalf("body = %q, want payload=1", got)
+	}
+}
+
+func TestHandler_404FallsBackWithBody(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer upstream.Close()
+
+	svc, err := NewService([]Rule{{URLPrefix: "/", Target: upstream.URL}})
+	if err != nil {
+		t.Fatalf("NewService: %v", err)
+	}
+
+	echoBody := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("fallback reading body: %v", err)
+		}
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write(body)
+	})
+
+	handler := svc.Handler(echoBody)
+
+	req := httptest.NewRequest(http.MethodPut, "/missing", strings.NewReader("payload=2"))
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if got := rr.Body.String(); got != "payload=2" {
+		t.Fatalf("body = %q, want payload=2", got)
+	}
+}
+
 func TestHandler_NonNotFoundErrorsPassThrough(t *testing.T) {
 	codes := []int{http.StatusOK, http.StatusForbidden, http.StatusBadRequest, http.StatusInternalServerError}