CXX-3100 Check return when acquiring client from pool (#1205)

* Throw exception in `pool::acquire()` on failure to acquire client.

* Define new error code: `k_pool_wait_queue_timeout`.

* Add a regression test to `src/mongocxx/test/pool.cpp`

---------

Co-authored-by: Kevin Albertson <[email protected]>
Co-authored-by: hansb <[email protected]>
Co-authored-by: Ezra Chung <[email protected]>

Author: 4 people

src/mongocxx/include/mongocxx/v_noabi/mongocxx/exception/error_code.hpp

@@ -108,6 +108,9 @@ enum class error_code : std::int32_t {
     // used.
     k_invalid_search_index_view,
 
+    // Timed out while waiting for a client to be returned to the pool
+    k_pool_wait_queue_timeout,
+
     // Add new constant string message to error_code.cpp as well!
 };
 

src/mongocxx/lib/mongocxx/v_noabi/mongocxx/exception/error_code.cpp

@@ -92,6 +92,8 @@ class error_category final : public std::error_category {
             case error_code::k_invalid_search_index_view:
                 return "invalid use of default constructed or moved-from "
                        "mongocxx::search_index_view object";
+            case error_code::k_pool_wait_queue_timeout:
+                return "timed out while waiting for a client to be returned to the pool";
             default:
                 return "unknown mongocxx error";
         }

src/mongocxx/lib/mongocxx/v_noabi/mongocxx/pool.cpp

@@ -131,8 +131,14 @@ pool::entry::operator bool() const noexcept {
 pool::entry::entry(pool::entry::unique_client p) : _client(std::move(p)) {}
 
 pool::entry pool::acquire() {
-    return entry(entry::unique_client(new client(libmongoc::client_pool_pop(_impl->client_pool_t)),
-                                      [this](client* client) { _release(client); }));
+    auto cli = libmongoc::client_pool_pop(_impl->client_pool_t);
+    if (!cli)
+        throw exception{
+            error_code::k_pool_wait_queue_timeout,
+            "failed to acquire client, possibly due to parameter 'waitQueueTimeoutMS' limits."};
+
+    return entry(
+        entry::unique_client(new client(cli), [this](client* client) { _release(client); }));
 }
 
 stdx::optional<pool::entry> pool::try_acquire() {

src/mongocxx/test/pool.cpp

@@ -26,6 +26,7 @@
 
 #include <bsoncxx/test/catch.hh>
 #include <mongocxx/test/catch_helpers.hh>
+#include <mongocxx/test/client_helpers.hh>
 
 namespace {
 using namespace mongocxx;
@@ -183,4 +184,18 @@ TEST_CASE("a pool is created with an invalid connection string", "[pool]") {
 
     REQUIRE_THROWS_AS(pool{mongocxx::uri(uristr)}, operation_exception);
 }
+
+TEST_CASE("acquiring a client throws if waitQueueTimeoutMS expires", "[pool]") {
+    instance::current();
+    mongocxx::pool pool{
+        mongocxx::uri{"mongodb://localhost:27017/?waitQueueTimeoutMS=1&maxPoolSize=1"},
+        options::pool(test_util::add_test_server_api())};
+    // Acquire only available client:
+    auto client = pool.acquire();
+    CHECK(client);
+    // Try to acquire again. Expect timeout:
+    REQUIRE_THROWS_WITH(pool.acquire(),
+                        Catch::Matchers::ContainsSubstring("failed to acquire client"));
+}
+
 }  // namespace