BUG/MINOR: fix HTTP header add filter to append to existing header value

The Gateway API "add" semantics require that if the header is already
present, the new value is appended to the existing one with a comma
separator.  The previous implementation emitted a plain add-header rule
which adds a second header line; the echo backend does not merge them, so
the conformance test received only the newly added value.

Two rules are now emitted per add entry: a conditional set-header that
constructs "<existing>,<new>" when the header is already present, and a
fallback add-header for when it is absent.  The same fix is applied to
responseHeaderModifierRules using res.hdr.

Author: ivanmatmati

k8s/gate/haproxy/filters/http_filters.go

@@ -429,11 +429,26 @@ func requestHeaderModifierRules(f *gatewayv1.HTTPHeaderFilter) models.HTTPReques
 		})
 	}
 	for _, h := range f.Add {
-		rules = append(rules, &models.HTTPRequestRule{
-			Type:      "add-header",
-			HdrName:   string(h.Name),
-			HdrFormat: h.Value,
-		})
+		name := string(h.Name)
+		// Gateway API "add" semantics: append to the existing header value with a
+		// comma separator.  Two rules are needed: one that combines when the header
+		// is already present, and one that sets it when it is absent.
+		rules = append(rules,
+			&models.HTTPRequestRule{
+				Type:      "set-header",
+				HdrName:   name,
+				HdrFormat: fmt.Sprintf("%%[req.hdr(%s)],%s", name, h.Value),
+				Cond:      "if",
+				CondTest:  fmt.Sprintf("{ hdr_cnt(%s) gt 0 }", name),
+			},
+			&models.HTTPRequestRule{
+				Type:      "add-header",
+				HdrName:   name,
+				HdrFormat: h.Value,
+				Cond:      "unless",
+				CondTest:  fmt.Sprintf("{ hdr_cnt(%s) gt 0 }", name),
+			},
+		)
 	}
 	for _, name := range f.Remove {
 		rules = append(rules, &models.HTTPRequestRule{
@@ -455,11 +470,23 @@ func responseHeaderModifierRules(f *gatewayv1.HTTPHeaderFilter) models.HTTPRespo
 		})
 	}
 	for _, h := range f.Add {
-		rules = append(rules, &models.HTTPResponseRule{
-			Type:      "add-header",
-			HdrName:   string(h.Name),
-			HdrFormat: h.Value,
-		})
+		name := string(h.Name)
+		rules = append(rules,
+			&models.HTTPResponseRule{
+				Type:      "set-header",
+				HdrName:   name,
+				HdrFormat: fmt.Sprintf("%%[res.hdr(%s)],%s", name, h.Value),
+				Cond:      "if",
+				CondTest:  fmt.Sprintf("{ hdr_cnt(%s) gt 0 }", name),
+			},
+			&models.HTTPResponseRule{
+				Type:      "add-header",
+				HdrName:   name,
+				HdrFormat: h.Value,
+				Cond:      "unless",
+				CondTest:  fmt.Sprintf("{ hdr_cnt(%s) gt 0 }", name),
+			},
+		)
 	}
 	for _, name := range f.Remove {
 		rules = append(rules, &models.HTTPResponseRule{

k8s/gate/haproxy/filters/http_filters_test.go

@@ -346,11 +346,35 @@ func TestToHAProxyRules(t *testing.T) {
 				},
 			},
 		}, "")
-		assert.Len(t, result.HTTPRequestRules, 3)
+		// 1 set-header + 2 rules per add (conditional set + fallback add) + 1 del-header
+		assert.Len(t, result.HTTPRequestRules, 4)
 		assert.Empty(t, result.HTTPResponseRules)
 		assert.False(t, result.IsRedirect)
 	})
 
+	t.Run("add header appends to existing value", func(t *testing.T) {
+		result := ToHAProxyRules([]gatewayv1.HTTPRouteFilter{
+			{
+				Type: gatewayv1.HTTPRouteFilterRequestHeaderModifier,
+				RequestHeaderModifier: &gatewayv1.HTTPHeaderFilter{
+					Add: []gatewayv1.HTTPHeader{{Name: "X-Append", Value: "new-val"}},
+				},
+			},
+		}, "")
+		require.Len(t, result.HTTPRequestRules, 2)
+		// Rule 0: conditional set-header combining existing + new value
+		r0 := result.HTTPRequestRules[0]
+		assert.Equal(t, "set-header", r0.Type)
+		assert.Equal(t, "X-Append", r0.HdrName)
+		assert.Equal(t, "%[req.hdr(X-Append)],new-val", r0.HdrFormat)
+		assert.Equal(t, "if", r0.Cond)
+		assert.Equal(t, "{ hdr_cnt(X-Append) gt 0 }", r0.CondTest)
+		// Rule 1: add-header when header is absent
+		r1 := result.HTTPRequestRules[1]
+		assert.Equal(t, "add-header", r1.Type)
+		assert.Equal(t, "unless", r1.Cond)
+	})
+
 	t.Run("response header modifier", func(t *testing.T) {
 		result := ToHAProxyRules([]gatewayv1.HTTPRouteFilter{
 			{