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Currently we have documented for mongoc_client_set_read_concern:
It is a programming error to call this function on a client from a mongoc_client_pool_t. For pooled clients, set the read concern with the MongoDB URI instead.
I believe the reason it is considered a programming error is that it persists on the mongoc_client_t even after being pushed back onto the pool. For example:
pool = mongoc_client_pool_new (uri);
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client = mongoc_client_pool_pop (pool);
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rc = mongoc_read_concern_new ();
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mongoc_read_concern_set_level (rc, MONGOC_READ_CONCERN_LEVEL_AVAILABLE);
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mongoc_client_set_read_concern (client, rc);
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mongoc_client_pool_push (pool, client);
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client = mongoc_client_pool_pop (pool);
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level = mongoc_read_concern_get_level (mongoc_client_get_read_concern (client));
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printf ("rc=%s\n", level); /* prints "rc=available" */
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mongoc_read_concern_destroy (rc);
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This behavior seems surprising and error prone. I think the same applies to write concern and read preference. I think it would be simple enough to have mongoc_client_pool_push reset its state (clear the read/write concern and read preference) before being pushed back onto a pool.
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