From 6cc44c03e24fa24aa65fddd9d6c87ee550f42bcb Mon Sep 17 00:00:00 2001
From: Sean Lang <sean.lang@thescore.com>
Date: Thu, 29 Aug 2024 16:37:18 -0400
Subject: [PATCH] add was_leader metadata to oban.peer.election event

---
 lib/oban/peers/global.ex   | 2 +-
 lib/oban/peers/postgres.ex | 4 ++--
 lib/oban/telemetry.ex      | 3 ++-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/lib/oban/peers/global.ex b/lib/oban/peers/global.ex
index 9799f4a3..24e0a44c 100644
--- a/lib/oban/peers/global.ex
+++ b/lib/oban/peers/global.ex
@@ -96,7 +96,7 @@ defmodule Oban.Peers.Global do
       :telemetry.span([:oban, :peer, :election], meta, fn ->
         locked? = :global.set_lock(key(state), nodes(), 0)
 
-        {locked?, %{meta | leader: locked?}}
+        {locked?, %{was_leader: meta.leader}, %{meta | leader: locked?}}
       end)
 
     if locked?, do: notify_lock(state.conf)
diff --git a/lib/oban/peers/postgres.ex b/lib/oban/peers/postgres.ex
index b94f9f77..238a9c49 100644
--- a/lib/oban/peers/postgres.ex
+++ b/lib/oban/peers/postgres.ex
@@ -106,14 +106,14 @@ defmodule Oban.Peers.Postgres do
 
         case Repo.transaction(state.conf, fun, retry: 1) do
           {:ok, state} ->
-            {state, %{meta | leader: state.leader?}}
+            {state, %{was_leader: meta.leader}, %{meta | leader: state.leader?}}
 
           {:error, :rollback} ->
             # The peer maintains its current `leader?` status on rollback—this may cause
             # inconsistency if the leader encounters an error and multiple rollbacks happen in
             # sequence. That tradeoff is acceptable because the situation is unlikely and less of
             # an issue than crashing the peer.
-            {state, meta}
+            {state, %{was_leader: meta.leader}, meta}
         end
       end)
 
diff --git a/lib/oban/telemetry.ex b/lib/oban/telemetry.ex
index a3937900..4fe6bc4d 100644
--- a/lib/oban/telemetry.ex
+++ b/lib/oban/telemetry.ex
@@ -177,13 +177,14 @@ defmodule Oban.Telemetry do
   | event        | measures       | metadata                                                       |
   | ------------ | -------------- | -------------------------------------------------------------- |
   | `:start`     | `:system_time` | `:conf`, `:leader`, `:peer`,                                   |
-  | `:stop`      | `:duration`    | `:conf`, `:leader`, `:peer`,                                   |
+  | `:stop`      | `:duration`    | `:conf`, `:leader`, `:peer`, `:was_leader`,                    |
   | `:exception` | `:duration`    | `:conf`, `:leader`, `:peer`, `:kind`, `:reason`, `:stacktrace` |
 
   #### Metadata
 
   * `:conf`, `:kind`, `:reason`, `:stacktrace` — see the explanation in notifier metadata above
   * `:leader` — whether the peer is the current leader
+  * `:was_leader` — whether the peer was the leader before the election occurred
   * `:peer` — the module used for peering
 
   ## Queue Shutdown Events