feat: implement dynamic request variable replacement in replacer

Replace the no-op reverseProxyReplacer.Replace with strings.NewReplacer
supporting {method}, {host}, {path}, {query}, {scheme}, {uri}, {proto}

reverseproxy_headers_replace_test.go

@@ -426,3 +426,105 @@ func BenchmarkReplacementApplyRegexp(b *testing.B) {
 		_ = r.apply(s)
 	}
 }
+
+func TestReverseProxyReplacerDynamicVars(t *testing.T) {
+	req, _ := http.NewRequest(http.MethodGet, "http://example.com/api/v1/users?sort=name&limit=10", nil)
+	req.Host = "example.com"
+	repl := newReverseProxyReplacer(req)
+
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{"method", "{method}", "GET"},
+		{"host", "{host}", "example.com"},
+		{"path", "{path}", "/api/v1/users"},
+		{"query", "{query}", "sort=name&limit=10"},
+		{"scheme", "{scheme}", "http"},
+		{"proto", "{proto}", "HTTP/1.1"},
+		{"combined", "X-{method}-{path}", "X-GET-/api/v1/users"},
+		{"no vars", "static-value", "static-value"},
+		{"empty", "", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := repl.Replace(tt.input); got != tt.want {
+				t.Errorf("Replace(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestReverseProxyReplacerNilRequest(t *testing.T) {
+	repl := newReverseProxyReplacer(nil)
+	if got := repl.Replace("{method}"); got != "{method}" {
+		t.Errorf("expected unchanged string with nil request, got %q", got)
+	}
+}
+
+func TestReverseProxyReplacerNilReplacer(t *testing.T) {
+	var repl *reverseProxyReplacer
+	if got := repl.Replace("{method}"); got != "{method}" {
+		t.Errorf("expected unchanged string with nil replacer, got %q", got)
+	}
+}
+
+func TestReverseProxyReplacerFromHeader(t *testing.T) {
+	hdr := make(http.Header)
+	repl := newReverseProxyReplacerFromHeader(hdr)
+	if got := repl.Replace("{method}"); got != "{method}" {
+		t.Errorf("expected unchanged string from header replacer, got %q", got)
+	}
+}
+
+func TestReverseProxyHeaderOpsWithDynamicVars(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if got := r.Header.Get("X-Forwarded-Path"); got != "/dynamic/path" {
+			t.Errorf("expected X-Forwarded-Path=/dynamic/path, got %q", got)
+		}
+		if got := r.Header.Get("X-Forwarded-Method"); got != "GET" {
+			t.Errorf("expected X-Forwarded-Method=GET, got %q", got)
+		}
+		if got := r.Header.Get("X-Forwarded-Host"); got != "client.example" {
+			t.Errorf("expected X-Forwarded-Host=client.example, got %q", got)
+		}
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("ok"))
+	}))
+	defer backend.Close()
+
+	target, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatalf("parse target: %v", err)
+	}
+
+	engine := New()
+	engine.GET("/dynamic/path", ReverseProxy(ReverseProxyConfig{
+		Target: target,
+		RequestHeaders: &HeaderOps{
+			Add: map[string][]string{
+				"X-Forwarded-Path":   {"{path}"},
+				"X-Forwarded-Method": {"{method}"},
+				"X-Forwarded-Host":   {"{host}"},
+			},
+		},
+	}))
+
+	proxy := httptest.NewServer(engine)
+	defer proxy.Close()
+
+	req, _ := http.NewRequest(http.MethodGet, proxy.URL+"/dynamic/path", nil)
+	req.Host = "client.example"
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	defer resp.Body.Close()
+	_, _ = io.ReadAll(resp.Body)
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("expected status 200, got %d", resp.StatusCode)
+	}
+}