CXX-1187 Handle null return values from libmongoc for uri's

Closes #581

Author: aseering

src/mongocxx/test/uri.cpp

@@ -26,6 +26,34 @@ TEST_CASE("URI", "[uri]") {
         REQUIRE_NOTHROW(mongocxx::uri{mongocxx::uri::k_default_uri});
         REQUIRE(mongocxx::uri{}.to_string() ==
                 mongocxx::uri{mongocxx::uri::k_default_uri}.to_string());
+
+        mongocxx::uri u{};
+
+        // Values that should be empty with a blank URI.
+        REQUIRE(u.auth_mechanism() == "");
+        REQUIRE(u.auth_source() == "admin");
+        REQUIRE(u.database() == "");
+        REQUIRE(u.hosts().size() == 1);
+        REQUIRE(u.hosts()[0].name == "localhost");
+        // Don't check 'u.hosts()[0].family'.  Value is platform-dependent.
+        REQUIRE(u.hosts()[0].port == 27017);
+        REQUIRE(u.options().empty());
+        REQUIRE(u.password() == "");
+        REQUIRE(u.read_concern().acknowledge_level() == mongocxx::read_concern::level::k_server_default);
+        REQUIRE(u.read_concern().acknowledge_string().empty());
+        REQUIRE(u.read_preference().mode() == mongocxx::read_preference::read_mode::k_primary);
+        REQUIRE(!u.read_preference().tags());
+        REQUIRE(!u.read_preference().max_staleness());
+        REQUIRE(u.replica_set() == "");
+        REQUIRE(u.ssl() == false);
+        REQUIRE(u.to_string() == mongocxx::uri::k_default_uri);
+        REQUIRE(u.username() == "");
+        REQUIRE(u.write_concern().journal() == false);
+        REQUIRE(u.write_concern().majority() == false);
+        REQUIRE(!u.write_concern().nodes());
+        REQUIRE(u.write_concern().timeout() == std::chrono::milliseconds{0});
+        REQUIRE(u.write_concern().acknowledge_level());
+        REQUIRE(*u.write_concern().acknowledge_level() == mongocxx::write_concern::level::k_default);
     }
 
     SECTION("Valid URI") {

src/mongocxx/uri.cpp

@@ -28,6 +28,19 @@
 namespace mongocxx {
 MONGOCXX_INLINE_NAMESPACE_BEGIN
 
+namespace {
+
+// Some of the 'uri_get_*' string accessors may return nullptr.
+// Check for this case and convert to the empty string.
+std::string to_string_null_safe(const char* str) {
+    if (str == nullptr) {
+        return std::string{};
+    }
+    return str;
+}
+
+} // namespace
+
 const std::string uri::k_default_uri = "mongodb://localhost:27017";
 
 uri::uri(std::unique_ptr<impl>&& implementation) {
@@ -47,15 +60,15 @@ uri& uri::operator=(uri&&) noexcept = default;
 uri::~uri() = default;
 
 std::string uri::auth_mechanism() const {
-    return libmongoc::uri_get_auth_mechanism(_impl->uri_t);
+    return to_string_null_safe(libmongoc::uri_get_auth_mechanism(_impl->uri_t));
 }
 
 std::string uri::auth_source() const {
     return libmongoc::uri_get_auth_source(_impl->uri_t);
 }
 
 std::string uri::database() const {
-    return libmongoc::uri_get_database(_impl->uri_t);
+    return to_string_null_safe(libmongoc::uri_get_database(_impl->uri_t));
 }
 
 std::vector<uri::host> uri::hosts() const {
@@ -75,7 +88,7 @@ bsoncxx::document::view uri::options() const {
 }
 
 std::string uri::password() const {
-    return libmongoc::uri_get_password(_impl->uri_t);
+    return to_string_null_safe(libmongoc::uri_get_password(_impl->uri_t));
 }
 
 class read_concern uri::read_concern() const {
@@ -91,7 +104,7 @@ class read_preference uri::read_preference() const {
 }
 
 std::string uri::replica_set() const {
-    return libmongoc::uri_get_replica_set(_impl->uri_t);
+    return to_string_null_safe(libmongoc::uri_get_replica_set(_impl->uri_t));
 }
 
 std::string uri::to_string() const {
@@ -103,7 +116,7 @@ bool uri::ssl() const {
 }
 
 std::string uri::username() const {
-    return libmongoc::uri_get_username(_impl->uri_t);
+    return to_string_null_safe(libmongoc::uri_get_username(_impl->uri_t));
 }
 
 class write_concern uri::write_concern() const {