Major - P3 There is a test failure logging this
[ReplicaSetFixture:job2:initsync] | 2020-05-08T02:18:47.571+00:00 E INITSYNC 21200 [ReplCoordExtern-1] "Initial sync attempt failed","attr":{"attemptsLeft":9,"error":"OplogOperationUnsupported: error applying batch :: caused by :: Applying command to feature compatibility version collection not supported in initial sync: { op: \"c\", ns: \"admin.$cmd\", ui: UUID(\"530a46b7-9cb5-43d1-852b-14304575fb7f\"), o: { startIndexBuild: \"system.version\", indexBuildUUID: UUID(\"0097eb7f-6ade-4581-ae37-a3170059a242\"), indexes: [ { v: 2, key: { a: 1.0 }, name: \"a_1\" } ] }, ts: Timestamp(1588904327, 35), t: 1, wall: new Date(1588904327267), v: 2 }"}
saying that "Initial sync attempt failed" and "Applying command to feature compatibility version collection not supported in initial sync" where the oplog entry is
{startIndexBuild: "system.version",....}Looking at the relevant code, we appear to have a whitelist of oplog cmds that are allowed on the admin.system.version collection during initial sync. The comment explains that changing the FCV value during initial sync is not allowed. Index builds, however, are not changing the FCV value.
The index build oplog entries - start, commit, abort – should all be added to the whitelist.