From 61e30f7cfa15cf5d9a175c1ca74ab967eb662363 Mon Sep 17 00:00:00 2001
From: Parker Selbert <parker@sorentwo.com>
Date: Thu, 4 Jan 2024 11:11:45 +0000
Subject: [PATCH] Handle rollback during postgres peer election
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Infrequently, the postgres peer election transaction returns `{:error,
:rollback}`. Now that return value is handled to prevent a match error.

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.

Closes #1007
---
 lib/oban/peers/postgres.ex | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/lib/oban/peers/postgres.ex b/lib/oban/peers/postgres.ex
index b0c2829d..67f6855d 100644
--- a/lib/oban/peers/postgres.ex
+++ b/lib/oban/peers/postgres.ex
@@ -90,14 +90,23 @@ defmodule Oban.Peers.Postgres do
 
     state =
       :telemetry.span([:oban, :peer, :election], meta, fn ->
-        {:ok, state} =
-          Repo.transaction(state.conf, fn ->
-            state
-            |> delete_expired_peers()
-            |> upsert_peer()
-          end)
-
-        {state, %{meta | leader: state.leader?}}
+        fun = fn ->
+          state
+          |> delete_expired_peers()
+          |> upsert_peer()
+        end
+
+        case Repo.transaction(state.conf, fun) do
+          {:ok, state} ->
+            {state, %{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}
+        end
       end)
 
     {:noreply, schedule_election(state)}