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

[longqimin]

• seastar::sstring.find() is not compatible with std::string.find() #1001

[nyh]

Looks good. I left a suggestion on how to improve the test by making it more convincing that find's behavior is the same as std::string's, and not just our incorrect interpretation.

[nyh]

Since the goal of this test (or at least the newly added lines) is to check compatiblity with std::string, this implementation of the test is not convincing enough: you check that find("", 5) should be 5, but who said it should be 5?

What I would have done is to do something like

auto check_find = [] (const char* s1, const char* s2, int n1, unsigned int n2) {
    BOOST_REQUIRE_EQUAL(sstring(s1).find(s2, n), n2);
    // verify that std::string really has the same behavior as we just tested for sstring
    BOOST_REQUIRE_EQUAL(std::string(s1).find(s2, n), n2);
}
check_find("", "", 0, 0);
check_find("", "", 1, sstring::npos);
...

[longqimin]

test updated. but i found the npos of sstring/std::string may not be equal

[nyh]

Looks good.