fix: do not panic on cleaning up failed iterators (influxdata#21666)

We have seen occasional panics in Iterators.Close()
when cleaning up after failed iterator creation.
This commit checks for nil on any iterator to be
closed, and now returns any errors generated by
that Close().

Closes influxdata#19579
Closes influxdata#19476

CHANGELOG.md

@@ -4,6 +4,7 @@ v1.8.11 [unreleased]
 -	[#18429](https://github.com/influxdata/influxdb/pull/18429): Add option to authenticate prometheus remote read
 -	[#21592](https://github.com/influxdata/influxdb/pull/21592): fix: avoid rewriting fields.idx unnecessarily
 -	[#21659](https://github.com/influxdata/influxdb/pull/21659): fix: do not close connection twice in DigestWithOptions
+-	[#21666](https://github.com/influxdata/influxdb/pull/21666): fix: do not panic on cleaning up failed iterators
 
 v1.8.10 [2021-10-11]
 -------------------

query/iterator.go

@@ -43,11 +43,22 @@ func (a Iterators) Stats() IteratorStats {
 }
 
 // Close closes all iterators.
-func (a Iterators) Close() error {
+// We are seeing an occasional panic in this function
+// which looks like a nil reference from one
+// itr.Close() call, thus we check for nil elements
+// in the slice a.  This is often called as error
+// clean-up, so the state of the iterators may be
+// unhappy.
+func (a Iterators) Close() (err error) {
+	err = nil
 	for _, itr := range a {
-		itr.Close()
+		if itr != nil {
+			if e := itr.Close(); e != nil && err == nil {
+				err = e
+			}
+		}
 	}
-	return nil
+	return err
 }
 
 // filterNonNil returns a slice of iterators that removes all nil iterators.