remove interpose in MOCK_POOL_NOSSL

Returning `nullptr` can result in exceptions. Removing the call to `interpose` will have the default behavior call the C driver function. The behavior can be overriden by the test.

Use `visit` in test functions that do not need to override the C driver function. `visit` is called in addition to the C driver function.

Author: kevinAlbs

src/mongocxx/pool.cpp

@@ -37,7 +37,7 @@ MONGOCXX_INLINE_NAMESPACE_BEGIN
 static mongoc_client_pool_t* construct_client_pool(mongoc_uri_t* uri) {
     bson_error_t error;
     auto pool = libmongoc::client_pool_new_with_error(uri, &error);
-    if (error.code) {
+    if (!pool) {
         // If constructing a client pool failed, throw an exception from the bson_error_t.
         throw_exception<operation_exception>(error);
     }

src/mongocxx/test/pool.cpp

@@ -35,15 +35,15 @@ TEST_CASE("a pool is created with the correct MongoDB URI", "[pool]") {
     instance::current();
 
     bool destroy_called = false;
-    client_pool_destroy->interpose([&](::mongoc_client_pool_t*) { destroy_called = true; });
+    client_pool_destroy->visit([&](::mongoc_client_pool_t*) { destroy_called = true; });
 
     std::string expected_uri("mongodb://mongodb.example.com:9999");
     uri mongodb_uri{expected_uri};
 
     std::string actual_uri{};
     bool new_called = false;
 
-    client_pool_new_with_error->interpose([&](const mongoc_uri_t* uri, bson_error_t* error) {
+    client_pool_new_with_error->visit([&](const mongoc_uri_t* uri, bson_error_t* error) {
         new_called = true;
         actual_uri = mongoc_uri_get_string(uri);
         error->code = 0;
@@ -89,7 +89,7 @@ TEST_CASE(
 
     ::mongoc_ssl_opt_t interposed = {};
 
-    client_pool_set_ssl_opts->interpose(
+    client_pool_set_ssl_opts->visit(
         [&](::mongoc_client_pool_t*, const ::mongoc_ssl_opt_t* opts) {
             set_tls_opts_called = true;
             interposed = *opts;
@@ -114,13 +114,13 @@ TEST_CASE("calling acquire on a pool returns an entry that manages its client",
     instance::current();
 
     bool pop_called = false;
-    client_pool_pop->interpose([&](::mongoc_client_pool_t*) {
+    client_pool_pop->visit([&](::mongoc_client_pool_t*) {
         pop_called = true;
         return nullptr;
     });
 
     bool push_called = false;
-    client_pool_push->interpose(
+    client_pool_push->visit(
         [&](::mongoc_client_pool_t*, ::mongoc_client_t*) { push_called = true; });
 
     SECTION("entry releases its client at end of scope") {

src/third_party/catch/include/helpers.hpp

@@ -57,17 +57,10 @@ MONGOCXX_INLINE_NAMESPACE_END
 
 #define MOCK_POOL_NOSSL                                                                        \
     auto client_pool_new_with_error = libmongoc::client_pool_new_with_error.create_instance(); \
-    client_pool_new_with_error                                                                 \
-        ->interpose([](const mongoc_uri_t*, bson_error_t*) { return nullptr; })                \
-        .forever();                                                                            \
     auto client_pool_destroy = libmongoc::client_pool_destroy.create_instance();               \
-    client_pool_destroy->interpose([&](::mongoc_client_pool_t*) {}).forever();                 \
     auto client_pool_pop = libmongoc::client_pool_pop.create_instance();                       \
-    client_pool_pop->interpose([](::mongoc_client_pool_t*) { return nullptr; }).forever();     \
     auto client_pool_push = libmongoc::client_pool_push.create_instance();                     \
-    client_pool_push->interpose([](::mongoc_client_pool_t*, ::mongoc_client_t*) {}).forever(); \
     auto client_pool_try_pop = libmongoc::client_pool_try_pop.create_instance();               \
-    client_pool_try_pop->interpose([](::mongoc_client_pool_t*) { return nullptr; }).forever();
 
 #if defined(MONGOCXX_ENABLE_SSL) && defined(MONGOC_ENABLE_SSL)
 #define MOCK_POOL                                                                          \