From 03a35880ed28904bbe35f401871d2ca0e01e417f Mon Sep 17 00:00:00 2001
From: Pau Ruiz Safont <pau.ruizsafont@cloud.com>
Date: Thu, 16 Nov 2023 14:21:41 +0000
Subject: [PATCH] lib: remove Failure "hd" exceptions

Instead raise the semantically correct No_RRA_available.

There's one last List.hd in the code, but it's correctly guarded so it
cannot ever be used with an empty list.

Signed-off-by: Pau Ruiz Safont <pau.ruizsafont@cloud.com>
---
 ChangeLog              |  3 +++
 lib/rrd.ml             |  6 +++++-
 lib/rrd_updates.ml     | 20 +++++++++++---------
 lib_test/unit_tests.ml |  2 +-
 4 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 5e3685c..2f23638 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,6 @@
+## v1.11.0 (16-Nov-2023)
+* lib: remove Failure "hd" exceptions
+
 ## v1.10.0 (02-Aug-2023)
 * rrd_updates: use yojson instead of ad-hoc json serialization
 
diff --git a/lib/rrd.ml b/lib/rrd.ml
index 4ffce3f..642a875 100644
--- a/lib/rrd.ml
+++ b/lib/rrd.ml
@@ -633,7 +633,11 @@ let query_named_ds rrd now ds_name cf =
     raise (Invalid_data_source ds_name)
   else
     let rras = find_best_rras rrd 0 (Some cf) (Int64.of_float now) in
-    Fring.peek (List.hd rras).rra_data.(n) 0
+    match rras with
+    | [] ->
+        raise No_RRA_Available
+    | rra :: _ ->
+        Fring.peek rra.rra_data.(n) 0
 
 (******************************************************************************)
 (* Marshalling/Unmarshalling functions                                        *)
diff --git a/lib/rrd_updates.ml b/lib/rrd_updates.ml
index daae0c5..e1e3a98 100644
--- a/lib/rrd_updates.ml
+++ b/lib/rrd_updates.ml
@@ -209,15 +209,19 @@ let json_of_t t =
     'xport' format. *)
 
 let create_multi prefixandrrds start interval cfopt =
-  let first_rrd = snd (List.hd prefixandrrds) in
+  let timestep, last_updated =
+    match prefixandrrds with
+    | (_, r) :: _ ->
+        (r.timestep, r.last_updated)
+    | [] ->
+        raise No_RRA_Available
+  in
 
-  let pdp_interval = Int64.to_int (Int64.div interval first_rrd.timestep) in
+  let pdp_interval = Int64.to_int (Int64.div interval timestep) in
 
   (* Sanity - make sure the RRDs are homogeneous *)
   let prefixandrrds =
-    List.filter
-      (fun (_prefix, rrd) -> rrd.timestep = first_rrd.timestep)
-      prefixandrrds
+    List.filter (fun (_prefix, rrd) -> rrd.timestep = timestep) prefixandrrds
   in
 
   (* Treat -ve start values as relative to the latest update. *)
@@ -282,12 +286,10 @@ let create_multi prefixandrrds start interval cfopt =
   let rras = List.flatten rras in
 
   (* The following timestep is that of the archive *)
-  let rra_timestep =
-    Int64.mul first_rrd.timestep (Int64.of_int first_rra.rra_pdp_cnt)
-  in
+  let rra_timestep = Int64.mul timestep (Int64.of_int first_rra.rra_pdp_cnt) in
 
   (* Get the last and first times of the CDPs to be returned *)
-  let last_cdp_time, _age = get_times first_rrd.last_updated rra_timestep in
+  let last_cdp_time, _age = get_times last_updated rra_timestep in
   let first_cdp_time_minus_one, _age =
     get_times (Int64.to_float start) rra_timestep
   in
diff --git a/lib_test/unit_tests.ml b/lib_test/unit_tests.ml
index 1ad4f5f..aa15b94 100644
--- a/lib_test/unit_tests.ml
+++ b/lib_test/unit_tests.ml
@@ -313,7 +313,7 @@ let suite_create_multi =
        )
   in
   let test_no_rrds () =
-    Alcotest.check_raises "should raise error" (Failure "hd") (fun () ->
+    Alcotest.check_raises "should raise error" (No_RRA_Available) (fun () ->
         let _ = RU.create_multi [] 0L 1L None in
         ()
     )