xl: Fix rare freeze after many disk/network errors (#4438)

xl.storageDisks is sometimes passed to some low-level XL functions. Some disks in
xl.storageDisks are set to nil when they encounter some errors. This means all
elements in xl.storageDisks will be nil after some time which lead to an unusable XL.

cmd/xl-v1-utils.go

@@ -357,6 +357,24 @@ func shuffleDisks(disks []StorageAPI, distribution []int) (shuffledDisks []Stora
 	return shuffledDisks
 }
 
+// evalDisks - returns a new slice of disks where nil is set if
+// the correspond error in errs slice is not nil
+func evalDisks(disks []StorageAPI, errs []error) []StorageAPI {
+	if len(errs) != len(disks) {
+		errorIf(errors.New("unexpected disks/errors slice length"), "unable to evaluate internal disks")
+		return nil
+	}
+	newDisks := make([]StorageAPI, len(disks))
+	for index := range errs {
+		if errs[index] == nil {
+			newDisks[index] = disks[index]
+		} else {
+			newDisks[index] = nil
+		}
+	}
+	return newDisks
+}
+
 // Errors specifically generated by getPartSizeFromIdx function.
 var (
 	errPartSizeZero  = errors.New("Part size cannot be zero")