Fix bad conversion of array-of-strings to string. (#817)

Fixes: #816.

The generic conversion to an SQL array computed its budget just slightly
too tightly, which only became obvious when converting an array of at
least 4 empty strings.

NEWS

@@ -1,4 +1,5 @@
 7.9.1
+ - Fix bad conversion of array of empty strings to string. (#816)
  - Move `[[likely]]` feature check back to compile time, to speed up configure.
  - Support `[[assume(...)]]`.
 7.9.0

include/pqxx/internal/conversions.hxx

@@ -1025,6 +1025,7 @@ public:
 
   static char *into_buf(char *begin, char *end, Container const &value)
   {
+    assert(begin <= end);
     std::size_t const budget{size_buffer(value)};
     if (internal::cmp_less(end - begin, budget))
       throw conversion_overrun{
@@ -1059,8 +1060,12 @@ public:
         // Use the tail end of the destination buffer as an intermediate
         // buffer.
         auto const elt_budget{pqxx::size_buffer(elt)};
+	assert(elt_budget < static_cast<std::size_t>(end - here));
         for (char const c : elt_traits::to_buf(end - elt_budget, end, elt))
         {
+	  // We copy the intermediate buffer into the final buffer, char by
+	  // char, with escaping where necessary.
+	  // TODO: This will not work for all encodings.  UTF8 & ASCII are OK.
           if (c == '\\' or c == '"')
             *here++ = '\\';
           *here++ = c;
@@ -1101,7 +1106,7 @@ public:
                      // but don't count the trailing zeroes.
                      std::size_t const elt_size{
                        pqxx::is_null(elt) ? std::size(s_null) :
-                                            elt_traits::size_buffer(elt) - 1};
+                                            elt_traits::size_buffer(elt)};
                      return acc + 2 * elt_size + 2;
                    });
   }

test/unit/test_array.cxx

@@ -420,6 +420,25 @@ void test_generate_escaped_strings()
 }
 
 
+void test_array_generate_empty_strings()
+{
+  // Reproduce #816: Under-budgeted conversion of empty strings in arrays.
+  PQXX_CHECK_EQUAL(
+    pqxx::to_string(std::vector<std::string>({""})),
+    "{\"\"}",
+    "Array of one empty string came out wrong.");
+  PQXX_CHECK_EQUAL(
+    pqxx::to_string(std::vector<std::string>({"", "", "", ""})),
+    "{\"\",\"\",\"\",\"\"}",
+    "Array of 4 empty strings came out wrong.");
+  PQXX_CHECK_EQUAL(
+    pqxx::to_string(std::vector<std::string>(
+        {"", "", "", "", "", "", "", "", "", "", "", ""})),
+    "{\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\"}",
+    "Array of 12 empty strings came out wrong.");
+}
+
+
 void test_array_generate()
 {
   test_generate_empty_array();
@@ -696,4 +715,5 @@ PQXX_REGISTER_TEST(test_array_parses_quoted_strings);
 PQXX_REGISTER_TEST(test_array_parses_multidim_arrays);
 PQXX_REGISTER_TEST(test_array_at_checks_bounds);
 PQXX_REGISTER_TEST(test_array_iterates_in_row_major_order);
+PQXX_REGISTER_TEST(test_array_generate_empty_strings);
 } // namespace

test/unit/test_stream_to.cxx

@@ -544,6 +544,21 @@ void test_stream_to_moves_into_optional()
 }
 
 
+void test_stream_to_empty_strings()
+{
+  // Reproduce #816: Streaming an array of 4 or more empty strings to a table
+  // using stream_to crashes.
+  pqxx::connection cx;
+  pqxx::transaction tx{cx};
+  tx.exec0("CREATE TEMP TABLE strs (list text[])");
+  std::vector<std::string> empties{"", "", "", ""};
+  auto stream{pqxx::stream_to::table(tx, {"strs"})};
+  stream.write_values(std::variant<std::vector<std::string>>{empties});
+  stream.complete();
+  tx.commit();
+}
+
+
 PQXX_REGISTER_TEST(test_stream_to);
 PQXX_REGISTER_TEST(test_container_stream_to);
 PQXX_REGISTER_TEST(test_stream_to_does_nonnull_optional);
@@ -553,4 +568,5 @@ PQXX_REGISTER_TEST(test_stream_to_quotes_arguments);
 PQXX_REGISTER_TEST(test_stream_to_optionals);
 PQXX_REGISTER_TEST(test_stream_to_escaping);
 PQXX_REGISTER_TEST(test_stream_to_moves_into_optional);
+PQXX_REGISTER_TEST(test_stream_to_empty_strings);
 } // namespace