CA-408126 follow-up: Fix negative ds_min and RRD values in historical archives (#6359)

When reading RRD archives from XML, make sure ds_min is (almost never)
negative, this will convert negative values to NaNs as well.

Author: last-genius

ocaml/libs/xapi-rrd/lib/rrd.ml

@@ -744,7 +744,11 @@ let from_xml input =
           let name = get_el "name" i in
           let type_ = get_el "type" i in
           let min_hb = get_el "minimal_heartbeat" i in
-          let min = get_el "min" i in
+          (* CA-408126 - work around negative data in historical RRDs
+             where ds_min could have been incorrectly set to neg_infinity.
+             Setting ds_min to 0. means Fring.make below will turn negative
+             historical values to NaNs.*)
+          let min = max (float_of_string (get_el "min" i)) 0. in
           let max = get_el "max" i in
           ignore (get_el "last_ds" i) ;
           let value = get_el "value" i in
@@ -767,7 +771,7 @@ let from_xml input =
                   failwith "Bad format"
               )
           ; ds_mrhb= float_of_string min_hb
-          ; ds_min= float_of_string min
+          ; ds_min= min
           ; ds_max= float_of_string max
           ; ds_last= VT_Unknown
           ; (* float_of_string "last_ds"; *)