fix sstring.find(): make find("") compatible with std::string

- Fixes #1001
- add compatibility test for find()

Closes #1002

include/seastar/core/sstring.hh

@@ -245,6 +245,13 @@ public:
  const char_type* c_str = s.str();
  const char_type* c_str_end = s.str() + s.size();
 
+ if (c_str == c_str_end) {
+ /* see https://en.cppreference.com/w/cpp/string/basic_string/find
+ * - an empty substring is found at pos if and only if pos <= size()
+ */
+ return (pos <= size()) ? pos : npos;
+ }
+
  while (it < end) {
  auto i = it;
  auto j = c_str;

tests/unit/sstring_test.cc

@@ -54,6 +54,29 @@ BOOST_AUTO_TEST_CASE(test_add_literal_to_sstring) {
 BOOST_AUTO_TEST_CASE(test_find_sstring) {
  BOOST_REQUIRE_EQUAL(sstring("abcde").find('b'), 1u);
  BOOST_REQUIRE_EQUAL(sstring("babcde").find('b',1), 2u);
+ BOOST_REQUIRE_EQUAL(sstring("").find("", 0), 0u);
+ BOOST_REQUIRE_EQUAL(sstring("").find("", 1), sstring::npos);
+}
+
+BOOST_AUTO_TEST_CASE(test_find_sstring_compatible) {
+ auto check_find = [](const char* s1, const char* s2, size_t pos) {
+ const auto xpos_ss = sstring(s1).find(s2, pos);
+ const auto xpos_std = std::string(s1).find(s2, pos);
+
+ // verify that std::string really has the same behavior as we just tested for sstring
+ if (xpos_ss == sstring::npos) { // sstring::npos may not equal std::string::npos ?
+ BOOST_REQUIRE_EQUAL(xpos_std, std::string::npos);
+ } else {
+ BOOST_REQUIRE_EQUAL(xpos_ss, xpos_std);
+ }
+ };
+
+ check_find("", "", 0);
+ check_find("", "", 1);
+ check_find("abcde", "", 0);
+ check_find("abcde", "", 1);
+ check_find("abcde", "", 5);
+ check_find("abcde", "", 6);
 }
 
 BOOST_AUTO_TEST_CASE(test_not_find_sstring) {