Older driver versions allowed invalid index fields. When one of these invalid index fields exists, it gets dumped by mongodump and then upon mongorestore, the process encounters a "The field 'XXX' is not valid for an index specification" error.  For example:

2019-09-27T04:03:10.421-0700	Failed: testdb.food: error creating indexes for testdb.food: createIndex error: (InvalidIndexSpecificationOption) The field 'safe' is not valid for an index specification. Specification: { key: { a: 1.0 }, name: "a_1", ns: "testdb.food", safe: true }

Possible workarounds:

• Delete and re-create the index. Not a great option since dropping the index to re-create it could significantly affect performance.
• Perform the mongorestore with the "–noIndexRestore" option

Reproduction steps:

1. On a deployment running 3.2 or earlier, create an index with an invalid option using the mongo shell. A common invalid option previously introduced by one of the drivers was "safe".
2. Upgrade it to 3.4
3. Take a mongodump (any version, including 4.2.0)
4. Run mongorestore (any version, including 4.2.0)

Example of creating an index with an invalid option (on deployment running 3.2.23):

> db.foo.insert({a:1})
WriteResult({ "nInserted" : 1 })
> db.foo.drop()
true
> db.foo.insert({"a":1})
WriteResult({ "nInserted" : 1 })
> db.food.createIndex({"a":1},{"safe": true})
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}