fix: stale order map on pooled headerSorter breaks orderless header sorting

header.go

@@ -223,6 +223,10 @@ func (h Header) SortedKeyValues(exclude map[string]bool) (kvs []HeaderKeyValues,
 		mutex.RUnlock()
 	}
 	hs.kvs = kvs
+	// Reset any order left on the sorter by a previous SortedKeyValuesBy
+	// call, otherwise a pooled sorter sorts by the stale order instead of
+	// lexicographically.
+	hs.order = nil
 	sort.Sort(hs)
 	return kvs, hs
 }

header_test.go

@@ -359,3 +359,28 @@ func TestHTTP1HeaderOrder(t *testing.T) {
 		t.Fatalf("got:\n%swant:\n%s", buf.String(), expected)
 	}
 }
+
+func TestHeaderSorterPoolReuse(t *testing.T) {
+	// A sorter used by SortedKeyValuesBy keeps its order map. When it is
+	// pulled from the pool again by SortedKeyValues (no order), the stale
+	// order must not be used.
+	ordered := Header{
+		"Zebra": {"1"},
+		"Apple": {"2"},
+	}
+	// Deliberately the reverse of lexicographic order.
+	_, hs := ordered.SortedKeyValuesBy(map[string]int{"zebra": 0, "apple": 1}, nil)
+	headerSorterPool.Put(hs)
+
+	plain := Header{
+		"Apple":  {"1"},
+		"Banana": {"2"},
+		"Zebra":  {"3"},
+	}
+	kvs, _ := plain.SortedKeyValues(nil)
+	for i := 1; i < len(kvs); i++ {
+		if kvs[i-1].Key > kvs[i].Key {
+			t.Fatalf("keys not sorted lexicographically: %q before %q", kvs[i-1].Key, kvs[i].Key)
+		}
+	}
+}