CXX-2705: Throw an exception upon unsuccessful pool creation

[joshbsiegel]

Summary

This PR will ensure that a helpful exception is thrown to the user when a client pool cannot be constructed with the given URI.

Background

Prior to this PR, when the user provided an incorrect URI or the driver was unable to establish a connection to the server, an unhelpful error would display and the program would crash. This is because when libmongoc::client_pool_new is called with a URI that is unable to connect to a server, the function will return null. This value would then be made into a unique pointer, be assigned to _impl, and eventually would get caught down the line by any function that asserts that pool != nullptr.
This is the reason for the unhelpful messaging that existed before, the empty pool object was not caught at the source.

What's New

• construct_client_pool, which will attempt to create a mongoc_client_pool_t using client_pool_new_with_error and throw an exception upon error
• Changes in the pool initializer list to utilize this new function
• A simple test case for an invalid URI
• MOCK_POOL changes to ensure that existing tests can pass now that we are using client_pool_new_with_error instead of client_pool_new

[joshbsiegel]

I was unable to get mongoc_client_pool_t to be the return type of this function.

/home/ubuntu/code/mongo-cxx-driver/src/mongocxx/../mongocxx/pool.hpp:112:5: error: 'mongoc_client_pool_t' does not name a type
  112 |     mongoc_client_pool_t* initialize_impl(const uri& uri);

It might be something small that I forgot to include, but that's the reason why this function returns a void * and then casts the return value in the initializer list above.

[kevinAlbs]

I was unable to get mongoc_client_pool_t to be the return type of this function.

The C driver API is explicitly hidden in public C++ driver headers. The C driver is intended to be an implementation detail in the driver. The private .hh headers and .cpp may use the C driver API.

Rather than make a private method on pool.hpp, I suggest making a function in pool.cpp:

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 (!pool) {
        // If constructing a client pool failed, throw an exception from the bson_error_t.
        throw_exception<operation_exception>(error);
    }

    return pool;
}

[joshbsiegel]

const members of a class must be set in the initializer list of the constructor. Because we want to be able to call a function, check the response, and possibly throw an exception before setting _impl (a const member), this all must be done in a separate function to ensure that we can still set _impl in the initializer list upon successful creation of a client pool.

[joshbsiegel]

I'm checking by error code here because the interpose function for client_pool_new_with_error in the a pool is created with the correct MongoDB URI test returns nullptr to the pool. Because of this, writing the if statement as if (!pool) won't work. I'm unsure on if returning an empty mongoc_client_pool_t * from the interpose is the correct solution here but I understand why checking error.code isn't intuitive.

[kevinAlbs]

Checking if (error.code) may work, but I expect this may require bson_error_t error = {0}; to ensure error is initialized when no error occurs. mongoc_client_pool_new_with_error may not zero error if no error occurs.

I suggest checking if (!pool) and updating the test mocks to not use interpose to replace the C driver function call. visit can be used to call a function in addition to the C driver function.

Here is a suggested commit with a description: be69c67