CDRIVER-5834 Do not compare uninitialized bytes in _mongoc_server_description_equal

[apolukhin]

In mongoc_cluster_check_interval function the bson_error_t error; is constructed on the stack, its error message is not zero initialised

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-cluster.c

Line 2771 in db54ccf

bson_error_t error;

. In case of an error in mongoc_cluster_run_command_parts function, a short error message is written to error, leaving some of the bytes uninitialized. That error is then passed to mongoc_topology_description_invalidate_server

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-cluster.c

Line 2840 in db54ccf

mongoc_topology_description_invalidate_server (tdmod.new_td, server_id, &error);

, which in turn copies the whole error message buffer into sd in mongoc_server_description_handle_hello

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-topology-description.c

Line 2168 in db54ccf

mongoc_server_description_handle_hello (sd, hello_response, rtt_msec, error);

, including uninitialized bytes

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-server-description.c

Line 741 in db54ccf

_mongoc_server_description_set_error (sd, error);

After that, attempt to call _mongoc_server_description_equal

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-topology-description.c

Line 2204 in db54ccf

sd_changed = !_mongoc_server_description_equal (prev_sd, sd);

results in comparison of uninitilized bytes of std->error at

mongo-c-driver/src/libmongoc/src/mongoc/mongoc-server-description.c

Line 1154 in db54ccf

if (0 != memcmp (&sd1->error, &sd2->error, sizeof (bson_error_t))) {

Resulting in sanitizer error:

Uninitialized bytes in MemcmpInterceptorCommon at offset 109 inside [0x71d0000200f8, 512)
==366158==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x9ac794 in bcmp /-S/contrib/libs/clang18-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc:859:10
    #1 0x1dc3136 in _mongoc_server_description_equal /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-server-description.c:1154:13
    #2 0x1de1ea0 in mongoc_topology_description_handle_hello /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-topology-description.c:2204:21
    #3 0x1d867d7 in _mongoc_topology_update_no_lock /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-topology.c:101:4
    #4 0x1d867d7 in _mongoc_topology_scanner_cb /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-topology.c:188:7
    #5 0x1d9349d in _async_error_or_timeout /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-topology-scanner.c:742:7
    #6 0x1d99b7d in mongoc_async_run /-S/contrib/libs/mongo-c-driver/libmongoc/src/mongoc/mongoc-async.c

[eramongodb]

Thank you very much for the contribution and the thorough PR description.

[kevinAlbs]

Thank you for the fix!