CA-404597 - rrd: Fix incorrect processing of Gauge and Absolute data …

…sources (#6233)

Some recent changes related to RRDs likely exposed a long-standing
latent issue
where the RRD library would process the passed-in values for Gauge and
Absolute
data sources incorrectly leading to constant values changing from update
to
update, for example:
```
$ rrd2csv memory_total_kib
timestamp, AVERAGE:host:8b533333-91e1-4698-bd17-95b9732ffbb6:memory_total_kib
2025-01-15T08:41:40Z, 33351000
2025-01-15T08:41:45Z, 33350000
2025-01-15T08:41:50Z, 33346000
2025-01-15T08:41:55Z, 33352000
```

Instead of treating Gauge and Absolute data sources as a
variation on the rate-based Derive data source type, expecting
time-based
calculations to cancel each other out, do not undertake any calculations
on
non-rate data sources at all.

First commit adds a failing unit test, second makes it pass.

===

I've verified these changes through manual testing, they've also passed
the testcases that discovered this issue: SNMP memory testcases (JobIDs
4197305, 4196759, 4196744) and ShimMemory testcase (4197050). This
branch also passed Ring3 BST+BVT (210577)

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

@@ -341,9 +341,10 @@ let rra_update rrd proc_pdp_st elapsed_pdp_st pdps =
   Array.iter updatefn rrd.rrd_rras
 
 (* We assume that the data being given is of the form of a rate; that is,
-   it's dependent on the time interval between updates. To be able to
-   deal with gauge DSs, we multiply by the interval so that it cancels
-   the subsequent divide by interval later on *)
+   it's dependent on the time interval between updates.
+   Gauge and Absolute data sources are simply kept as is without any
+   time-based calculations, while Derive data sources will be changed according
+   to the time passed since the last measurement. (see CA-404597) *)
 let process_ds_value ds value interval new_rrd =
   if interval > ds.ds_mrhb then
     nan
@@ -360,10 +361,8 @@ let process_ds_value ds value interval new_rrd =
 
     let rate =
       match (ds.ds_ty, new_rrd) with
-      | Absolute, _ | Derive, true ->
+      | Absolute, _ | Derive, true | Gauge, _ ->
           value_raw
-      | Gauge, _ ->
-          value_raw *. interval
       | Derive, false -> (
         match (ds.ds_last, value) with
         | VT_Int64 x, VT_Int64 y ->
@@ -433,7 +432,14 @@ let ds_update rrd timestamp valuesandtransforms new_rrd =
       if Utils.isnan value then
         ds.ds_unknown_sec <- pre_int
       else
-        ds.ds_value <- ds.ds_value +. (pre_int *. value /. interval)
+        (* CA-404597 - Gauge and Absolute values should be passed as-is,
+           without being involved in time-based calculations at all.
+           This applies to calculations below as well *)
+        match ds.ds_ty with
+        | Gauge | Absolute ->
+            ds.ds_value <- value
+        | Derive ->
+            ds.ds_value <- ds.ds_value +. (pre_int *. value /. interval)
     )
     v2s ;
 
@@ -450,7 +456,13 @@ let ds_update rrd timestamp valuesandtransforms new_rrd =
             let raw =
               let proc_pdp_st = get_float_time last_updated rrd.timestep in
               let occu_pdp_st = get_float_time timestamp rrd.timestep in
-              ds.ds_value /. (occu_pdp_st -. proc_pdp_st -. ds.ds_unknown_sec)
+
+              match ds.ds_ty with
+              | Gauge | Absolute ->
+                  ds.ds_value
+              | Derive ->
+                  ds.ds_value
+                  /. (occu_pdp_st -. proc_pdp_st -. ds.ds_unknown_sec)
             in
             (* Apply the transform after the raw value has been calculated *)
             let raw = apply_transform_function transform raw in
@@ -473,8 +485,12 @@ let ds_update rrd timestamp valuesandtransforms new_rrd =
           ds.ds_value <- 0.0 ;
           ds.ds_unknown_sec <- post_int
         ) else (
-          ds.ds_value <- post_int *. value /. interval ;
-          ds.ds_unknown_sec <- 0.0
+          ds.ds_unknown_sec <- 0.0 ;
+          match ds.ds_ty with
+          | Gauge | Absolute ->
+              ds.ds_value <- value
+          | Derive ->
+              ds.ds_value <- post_int *. value /. interval
         )
       )
       v2s

ocaml/libs/xapi-rrd/lib_test/unit_tests.ml

@@ -107,6 +107,110 @@ let test_length_invariants rrd () =
   let check_length dss rra = check_length_of_fring dss rra.rra_data in
   Array.iter (check_length rrd.rrd_dss) rrd.rrd_rras
 
+let absolute_rrd =
+  let rra = rra_create CF_Average 100 1 0.5 in
+  let rra2 = rra_create CF_Average 100 10 0.5 in
+  let rra3 = rra_create CF_Average 100 100 0.5 in
+  let rra4 = rra_create CF_Average 100 1000 0.5 in
+  let ts = 1000000000.0 in
+  let ds = ds_create "foo" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds2 = ds_create "bar" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds3 = ds_create "baz" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds4 = ds_create "boo" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let rrd = rrd_create [|ds; ds2; ds3; ds4|] [|rra; rra2; rra3; rra4|] 1L ts in
+  let id = Identity in
+  for i = 1 to 100000 do
+    let t = 1000000.0 +. (0.7 *. float_of_int i) in
+    let v1 =
+      (0, {value= VT_Float (0.5 +. (0.5 *. sin (t /. 10.0))); transform= id})
+    in
+    let v2 =
+      (1, {value= VT_Float (1.5 +. (0.5 *. cos (t /. 80.0))); transform= id})
+    in
+    let v3 =
+      (2, {value= VT_Float (3.5 +. (0.5 *. sin (t /. 700.0))); transform= id})
+    in
+    let v4 =
+      (3, {value= VT_Float (6.5 +. (0.5 *. cos (t /. 5000.0))); transform= id})
+    in
+    ds_update rrd t [|v1; v2; v3; v4|] false
+  done ;
+  rrd
+
+let absolute_rrd_CA_404597 () =
+  let rra = rra_create CF_Average 100 1 0.5 in
+  let rra2 = rra_create CF_Average 100 10 0.5 in
+  let rra3 = rra_create CF_Average 100 100 0.5 in
+  let rra4 = rra_create CF_Average 100 1000 0.5 in
+  let ts = 1000000000.0 in
+  let ds = ds_create "foo" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds2 = ds_create "bar" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds3 = ds_create "baz" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let ds4 = ds_create "boo" Absolute ~mrhb:10.0 (VT_Float 0.0) in
+  let rrd = rrd_create [|ds; ds2; ds3; ds4|] [|rra; rra2; rra3; rra4|] 1L ts in
+  let id = Identity in
+  for i = 1 to 100000 do
+    let t = 1000000.0 +. (0.7 *. float_of_int i) in
+    let ((_, val1) as v1) =
+      (0, {value= VT_Float (0.5 +. (0.5 *. sin (t /. 10.0))); transform= id})
+    in
+    let ((_, val2) as v2) =
+      (1, {value= VT_Float (1.5 +. (0.5 *. cos (t /. 80.0))); transform= id})
+    in
+    let ((_, val3) as v3) =
+      (2, {value= VT_Float (3.5 +. (0.5 *. sin (t /. 700.0))); transform= id})
+    in
+    let ((_, val4) as v4) =
+      (3, {value= VT_Float (6.5 +. (0.5 *. cos (t /. 5000.0))); transform= id})
+    in
+    ds_update rrd t [|v1; v2; v3; v4|] false ;
+
+    Array.iter2
+      (fun ds value ->
+        compare_float __LOC__ ds.ds_value
+          (float_of_string (ds_value_to_string value.value))
+      )
+      rrd.rrd_dss [|val1; val2; val3; val4|]
+  done
+
+(** Verify that Gauge data soruce values are correctly handled by the RRD lib
+    and that timestamps do not cause absolute values to fluctuate *)
+let gauge_rrd_CA_404597 () =
+  let rra = rra_create CF_Average 100 1 0.5 in
+  let rra2 = rra_create CF_Average 100 10 0.5 in
+  let rra3 = rra_create CF_Average 100 100 0.5 in
+  let rra4 = rra_create CF_Average 100 1000 0.5 in
+  let ts = 1000000000.0 in
+  let ds = ds_create "foo" Gauge ~mrhb:10.0 (VT_Float 0.0) in
+  let ds2 = ds_create "bar" Gauge ~mrhb:10.0 (VT_Float 0.0) in
+  let ds3 = ds_create "baz" Gauge ~mrhb:10.0 (VT_Float 0.0) in
+  let ds4 = ds_create "boo" Gauge ~mrhb:10.0 (VT_Float 0.0) in
+  let rrd = rrd_create [|ds; ds2; ds3; ds4|] [|rra; rra2; rra3; rra4|] 1L ts in
+  let id = Identity in
+  for i = 1 to 100000 do
+    let t = 1000000.0 +. (0.7 *. float_of_int i) in
+    let ((_, val1) as v1) =
+      (0, {value= VT_Float (0.5 +. (0.5 *. sin (t /. 10.0))); transform= id})
+    in
+    let ((_, val2) as v2) =
+      (1, {value= VT_Float (1.5 +. (0.5 *. cos (t /. 80.0))); transform= id})
+    in
+    let ((_, val3) as v3) =
+      (2, {value= VT_Float (3.5 +. (0.5 *. sin (t /. 700.0))); transform= id})
+    in
+    let ((_, val4) as v4) =
+      (3, {value= VT_Float (6.5 +. (0.5 *. cos (t /. 5000.0))); transform= id})
+    in
+    ds_update rrd t [|v1; v2; v3; v4|] false ;
+
+    Array.iter2
+      (fun ds value ->
+        compare_float __LOC__ ds.ds_value
+          (float_of_string (ds_value_to_string value.value))
+      )
+      rrd.rrd_dss [|val1; val2; val3; val4|]
+  done
+
 let gauge_rrd =
   let rra = rra_create CF_Average 100 1 0.5 in
   let rra2 = rra_create CF_Average 100 10 0.5 in
@@ -328,12 +432,15 @@ let regression_suite =
   ; ("CA-329043 (1)", `Quick, test_ranges ca_329043_rrd_1)
   ; ("CA-329043 (2)", `Quick, test_ranges ca_329043_rrd_2)
   ; ("CA-329813", `Quick, test_ranges ca_329813_rrd)
+  ; ("CA-404597 (1)", `Quick, gauge_rrd_CA_404597)
+  ; ("CA-404597 (2)", `Quick, absolute_rrd_CA_404597)
   ]
 
 let () =
   Alcotest.run "Test RRD library"
     [
-      ("Gauge RRD", rrd_suite gauge_rrd)
+      ("Absolute RRD", rrd_suite absolute_rrd)
+    ; ("Gauge RRD", rrd_suite gauge_rrd)
     ; ("RRD for CA-322008", rrd_suite ca_322008_rrd)
     ; ("RRD for CA-329043", rrd_suite ca_329043_rrd_1)
     ; ("RRD for CA-329813", rrd_suite ca_329813_rrd)