[CDRIVER-5605] Mitigate a race on setting APM callbacks (#1641)

Author: vector-of-bool

src/libmongoc/src/mongoc/mongoc-client-pool.c

@@ -568,25 +568,30 @@ mongoc_client_pool_set_apm_callbacks (mongoc_client_pool_t *pool, mongoc_apm_cal
    BSON_ASSERT_PARAM (pool);
 
    mongoc_topology_t *const topology = BSON_ASSERT_PTR_INLINE (pool)->topology;
-   mc_tpld_modification tdmod;
+   mc_tpld_modification tdmod = mc_tpld_modify_begin (topology);
 
+   // Prevent setting callbacks more than once
    if (pool->apm_callbacks_set) {
+      mc_tpld_modify_drop (tdmod);
       MONGOC_ERROR ("Can only set callbacks once");
       return false;
    }
 
-   tdmod = mc_tpld_modify_begin (topology);
-
+   // Update callbacks on the pool
    if (callbacks) {
-      memcpy (&tdmod.new_td->apm_callbacks, callbacks, sizeof (mongoc_apm_callbacks_t));
-      memcpy (&pool->apm_callbacks, callbacks, sizeof (mongoc_apm_callbacks_t));
+      pool->apm_callbacks = *callbacks;
+   } else {
+      pool->apm_callbacks = (mongoc_apm_callbacks_t){0};
    }
+   pool->apm_context = context;
 
+   // Update callbacks on the topology
    mongoc_topology_set_apm_callbacks (topology, tdmod.new_td, callbacks, context);
-   tdmod.new_td->apm_context = context;
-   pool->apm_context = context;
+
+   // Signal that we have already set the callbacks
    pool->apm_callbacks_set = true;
 
+   // Save our updated topology
    mc_tpld_modify_commit (tdmod);
 
    return true;

src/libmongoc/src/mongoc/mongoc-topology-private.h

@@ -233,7 +233,7 @@ mongoc_topology_new (const mongoc_uri_t *uri, bool single_threaded);
 void
 mongoc_topology_set_apm_callbacks (mongoc_topology_t *topology,
                                    mongoc_topology_description_t *td,
-                                   mongoc_apm_callbacks_t *callbacks,
+                                   mongoc_apm_callbacks_t const *callbacks,
                                    void *context);
 
 void

src/libmongoc/src/mongoc/mongoc-topology.c

@@ -640,15 +640,15 @@ mongoc_topology_new (const mongoc_uri_t *uri, bool single_threaded)
 void
 mongoc_topology_set_apm_callbacks (mongoc_topology_t *topology,
                                    mongoc_topology_description_t *td,
-                                   mongoc_apm_callbacks_t *callbacks,
+                                   mongoc_apm_callbacks_t const *callbacks,
                                    void *context)
 {
    if (callbacks) {
-      memcpy (&td->apm_callbacks, callbacks, sizeof (mongoc_apm_callbacks_t));
-      memcpy (&topology->scanner->apm_callbacks, callbacks, sizeof (mongoc_apm_callbacks_t));
+      td->apm_callbacks = *callbacks;
+      topology->scanner->apm_callbacks = *callbacks;
    } else {
-      memset (&td->apm_callbacks, 0, sizeof (mongoc_apm_callbacks_t));
-      memset (&topology->scanner->apm_callbacks, 0, sizeof (mongoc_apm_callbacks_t));
+      td->apm_callbacks = (mongoc_apm_callbacks_t){0};
+      topology->scanner->apm_callbacks = (mongoc_apm_callbacks_t){0};
    }
 
    td->apm_context = context;