sstring: implement push_back()

to be more compatible with basic_string<>
this is also what we expect from a sequence-alike STL containers.

Fixes scylladb#2104
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

include/seastar/core/sstring.hh

@@ -346,6 +346,29 @@ public:
         return *this;
     }
 
+    /**
+     * Append the given character to the end of the string
+     * @param ch  The character to append
+     * @note unlike @c std::basic_string, sstring does not preallocate, so
+     *       this call always lead to reallocation if external storage is
+     *       used.
+     */
+    constexpr void push_back(char_type ch) {
+        if (is_internal()) {
+            if (static_cast<Size>(u.internal.size) < max_size) {
+                u.internal.str[u.internal.size++] = ch;
+                if (NulTerminate) {
+                    u.internal.str[u.internal.size] = '\0';
+                }
+                return;
+            }
+        }
+        basic_sstring new_str(initialized_later(), u.external.size + 1);
+        std::copy(begin(), end(), new_str.begin());
+        new_str.u.external.str[new_str.u.external.size - 1] = ch;
+        *this = std::move(new_str);
+    }
+
     /**
      *  Resize string and use the specified @c op to modify the content and the length
      *  @param n  new size

tests/unit/sstring_test.cc

@@ -199,6 +199,27 @@ BOOST_AUTO_TEST_CASE(test_append) {
     BOOST_REQUIRE_EQUAL(sstring("aba").append("1234", 0), "aba");
 }
 
+BOOST_AUTO_TEST_CASE(test_push_back) {
+    {
+        // append in internal storage
+        basic_sstring<char, uint32_t, 15> s("0123456789");
+        s.push_back('a');
+        BOOST_REQUIRE_EQUAL(s, "0123456789a");
+    }
+    {
+        // append causing spilling to external storage
+        basic_sstring<char, uint32_t, 15> s("0123456789abcde");
+        s.push_back('f');
+        BOOST_REQUIRE_EQUAL(s, "0123456789abcdef");
+    }
+    {
+        // append with external storage
+        basic_sstring<char, uint32_t, 15> s("0123456789abcdef");
+        s.push_back('g');
+        BOOST_REQUIRE_EQUAL(s, "0123456789abcdefg");
+    }
+}
+
 BOOST_AUTO_TEST_CASE(test_replace) {
     BOOST_REQUIRE_EQUAL(sstring("abc").replace(1,1, "xyz", 1), "axc");
     BOOST_REQUIRE_EQUAL(sstring("abc").replace(3,2, "xyz", 2), "abcxy");