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Operating System:
ALL
Steps To Reproduce:
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To reproduce the questionable behavior we'll need:

MongoDB replicaset with primary instance and at least one secondary instance.

Sufficiently large collection t for enough time to be available for observation.

Some index already built on that collection. E. g. db.t.createIndex({foo:1})

Steps to reproduce are:

Start building new index on the collection.
db.t.createIndex({foo:1,_id:1})

Wait until primary reports index build completion.

Drop unique index before secondaries catch up with building new index. db.t.dropIndex('foo_1').
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To reproduce the questionable behavior we'll need: MongoDB replicaset with primary instance and at least one secondary instance. Sufficiently large collection t for enough time to be available for observation. Some index already built on that collection. E. g. db.t.createIndex({foo:1}) Steps to reproduce are: Start building new index on the collection. db.t.createIndex({foo:1,_id:1}) Wait until primary reports index build completion. Drop unique index before secondaries catch up with building new index. db.t.dropIndex('foo_1') .

Observed behavior is:

Secondary mongod instances block on Global lock and effectively all operations except building index are waiting for Global lock.
Replication is effectively paused.
Due to replication pause all operations with writeConcern:majority on the primary instance are blocked. E. g. db.t.insertOne({foo:1}, {w:'majority'}) will hang until one of secondaries catch up with primary.
All read operations with readPreference set to secondaryPreferred also hang until indices are built on that secondary. E. g.
db.getMongo().setReadPref('secondaryPreferred');
db.t.find();
will hang.

Results of db.currentOp() from secondary instance with hanged db.t.find() are attached to the issue.

This behavior is very surprising and isn't documented, so I consider it a bug.

duplicates

SERVER-21307 Replicated DDL (catalog) operation during background index build blocks reads/writes