Summary
mongoc_collection_create_indexes_with_opts uses mongoc_client_command_with_opts rather than mongoc_collection_write_command_with_opts. As a result, write command behavior is not applied:

• Write concern is not inherited from the parent mongoc_collection_t
• A server reply with a "writeConcernError" does not result in a failing return.

Background & Motivation

mongoc_collection_create_indexes_with_opts documents:

If no write concern is provided in opts, the collection’s write concern is used.

That appears untrue. When updating the create_index test function in test-mongoc-opts.c to switch from the deprecated mongoc_collection_create_index_with_opts to mongoc_collection_create_indexes_with_opts, the test failed with:

$ ./cmake-build/src/libmongoc/test-libmongoc -d --no-fork --match "/inheritance/create_index*"

[...]

Expected: { "createIndexes" : "collection", "writeConcern" : { "w" : "collection" } }
, Got: { "createIndexes" : "collection", "indexes" : [ { "key" : { "foo" : "bar" }, "name" : "foo_bar" } ], "$db" : "database", "lsid" : { "id" : { "$binary" : { "base64" : "rZ2NhtrGTSKAmjJJzi9Ang==", "subType" : "04" } } }, "$clusterTime" : { "clusterTime" : { "$timestamp" : { "t" : 1, "i" : 1 } }, "signature" : { "hash" : { "$binary" : { "base64" : "", "subType" : "00" } }, "keyId" : { "$numberLong" : "6446735049323708417" } }, "operationTime" : { "$timestamp" : { "t" : 1, "i" : 1 } } } }
, writeConcern: not found

Discovered when removing deprecated mongoc_collection_create_index_with_opts for C driver 2.0.