From 11e1cb97537476ce805262952a5c615000a6cf78 Mon Sep 17 00:00:00 2001
From: Siraaj Khandkar <siraaj@khandkar.net>
Date: Sun, 27 Sep 2015 17:07:30 -0400
Subject: [PATCH] Manage global reductions delta ourselves

We can get between-calls-delta directly from erlang:statistics(reductions),
but then if some other process also calls it - we'll get incorrect data on
the next call.
Managing deltas ourselves here, will at least reduce the possible callers to
only those with knowledge of our table ID.
---
 src/beam_stats.app.src                    |  2 +-
 src/beam_stats.erl                        |  3 +--
 src/beam_stats_delta.erl                  | 13 +++++++++++++
 test/beam_stats_consumer_statsd_SUITE.erl |  4 +++-
 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/beam_stats.app.src b/src/beam_stats.app.src
index 9c8d943..4e49f59 100644
--- a/src/beam_stats.app.src
+++ b/src/beam_stats.app.src
@@ -1,7 +1,7 @@
 {application, beam_stats,
  [
   {description, "Periodic VM stats production and consumption."},
-  {vsn, "0.14.2"},
+  {vsn, "0.14.3"},
   {registered, []},
   {applications,
     [ kernel
diff --git a/src/beam_stats.erl b/src/beam_stats.erl
index af5167c..b3e445e 100644
--- a/src/beam_stats.erl
+++ b/src/beam_stats.erl
@@ -18,7 +18,6 @@
 -spec collect(beam_stats_delta:t()) ->
     t().
 collect(DeltasServer) ->
-    {_, DeltaOfReductions} = beam_stats_source:erlang_statistics(reductions),
     { {io_bytes_in  , DeltaOfIOBytesIn}
     , {io_bytes_out , DeltaOfIOBytesOut}
     } = beam_stats_delta:of_io(DeltasServer),
@@ -29,7 +28,7 @@ collect(DeltasServer) ->
     , io_bytes_in      = DeltaOfIOBytesIn
     , io_bytes_out     = DeltaOfIOBytesOut
     , context_switches = beam_stats_delta:of_context_switches(DeltasServer)
-    , reductions       = DeltaOfReductions
+    , reductions       = beam_stats_delta:of_reductions(DeltasServer)
     , run_queue        = beam_stats_source:erlang_statistics(run_queue)
     , ets              = beam_stats_ets:collect()
     , processes        = beam_stats_processes:collect()
diff --git a/src/beam_stats_delta.erl b/src/beam_stats_delta.erl
index d563507..42f2399 100644
--- a/src/beam_stats_delta.erl
+++ b/src/beam_stats_delta.erl
@@ -9,6 +9,7 @@
     , stop/1
     , of_context_switches/1
     , of_io/1
+    , of_reductions/1
     ]).
 
 -record(?MODULE,
@@ -62,6 +63,18 @@ of_io(?T{erlang_statistics=Table}) ->
     , {io_bytes_out , DeltaOut}
     }.
 
+% We can get between-calls-delta directly from erlang:statistics(reductions),
+% but then if some other process also calls it - we'll get incorrect data on
+% the next call.
+% Managing deltas ourselves here, will at least reduce the possible callers to
+% only those with knowledge of our table ID.
+-spec of_reductions(t()) ->
+    non_neg_integer().
+of_reductions(?T{erlang_statistics=Table}) ->
+    Key = reductions,
+    {Current, _} = beam_stats_source:erlang_statistics(Key),
+    delta(Table, Key, Current).
+
 -spec delta(ets:tid(), atom(), non_neg_integer()) ->
     non_neg_integer().
 delta(Table, Key, CurrentTotal) ->
diff --git a/test/beam_stats_consumer_statsd_SUITE.erl b/test/beam_stats_consumer_statsd_SUITE.erl
index 88f82e7..a5b3779 100644
--- a/test/beam_stats_consumer_statsd_SUITE.erl
+++ b/test/beam_stats_consumer_statsd_SUITE.erl
@@ -60,6 +60,7 @@ t_full_cycle(_Cfg) ->
         [ {io_bytes_in      , 6}
         , {io_bytes_out     , 14}
         , {context_switches , 10}
+        , {reductions       , 18}
         ]
     ),
     ct:log("meck_expect_beam_stats ok~n"),
@@ -150,6 +151,7 @@ meck_expect_beam_stats(Overrides) ->
     IOBytesIn       = hope_kv_list:get(Overrides, io_bytes_in , 3),
     IOBytesOut      = hope_kv_list:get(Overrides, io_bytes_out, 7),
     ContextSwitches = hope_kv_list:get(Overrides, context_switches, 5),
+    Reductions      = hope_kv_list:get(Overrides, reductions, 9),
     Pid0 = list_to_pid("<0.0.0>"),
     Pid1 = list_to_pid("<0.1.0>"),
     Pid2 = list_to_pid("<0.2.0>"),
@@ -266,7 +268,7 @@ meck_expect_beam_stats(Overrides) ->
     meck:expect(beam_stats_source, erlang_statistics,
         fun (io              ) -> {{input, IOBytesIn}, {output, IOBytesOut}}
         ;   (context_switches) -> {ContextSwitches, 0}
-        ;   (reductions      ) -> {0, 9} % 1st element is unused
+        ;   (reductions      ) -> {Reductions, undefined} % 2nd element is unused
         ;   (run_queue       ) -> 17
         end
     ),
-- 
2.20.1