If during a background index build in a collection on a secondary node this collection gets dropped on the primary, the secondary may crash with a segmentation fault once it replicates the collection drop.
Only background index builds are affected. Secondary failures affect replica set quorum and can potentially lead to unavailability of the replica set.
Use foreground index builds, or ensure that the secondaries have finished building the index before dropping the specific collection in which the index is being built.
Production release versions from 2.6.0 to 2.6.3 are affected by this bug.
The fix is included in the 2.6.4 production release.
The patch now ensures that IndexBuildBlock::fail() can never throw an exception.