From d88bdd70a807d6d092bf6fb2d00a9186dd2e92ac Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Fri, 24 Apr 2026 10:15:19 -0500
Subject: [PATCH] fix(map): harden :preload_forecast against slow NFS +
 LiveStash crash
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Two user-visible crashes in MapLive seen on prod pods today:

1. `:preload_forecast` handle_info timed out a Task.async_stream subtask
   after 10 s and the `Enum.map(fn {:ok, h} -> h end)` match cascaded
   the `:exit, :timeout` into the LiveView process, terminating the
   session. Adds `on_timeout: :kill_task` and flat-maps only :ok
   elements — a slow NFS forecast hour now drops from the preload
   payload instead of killing the LV.

2. `LiveStash.stash/1` crashed with `** (ArgumentError) errors were
   found at the given arguments: 2nd argument: not a valid match
   specification` inside `:ets.select_replace/2` on OTP 28. The dep
   (live_stash 0.2.0) has a latent bug against the tightened OTP 28
   match-spec validator. Both call sites (MapLive.select_band,
   SubmitLive.switch_tab) now route through a new
   `MicrowavepropWeb.LiveStashGuard.stash/1` wrapper that rescues
   ArgumentError, logs a warning, and returns the socket unchanged.
   Stash is a convenience for reconnect state — failing it does not
   need to kill the user's session.

Drop the guard module once the upstream match-spec fix lands.
---
 .../live/live_stash_guard.ex                  | 34 +++++++++++++++++++
 lib/microwaveprop_web/live/map_live.ex        | 14 ++++++--
 lib/microwaveprop_web/live/submit_live.ex     |  2 +-
 3 files changed, 46 insertions(+), 4 deletions(-)
 create mode 100644 lib/microwaveprop_web/live/live_stash_guard.ex

diff --git a/lib/microwaveprop_web/live/live_stash_guard.ex b/lib/microwaveprop_web/live/live_stash_guard.ex
new file mode 100644
index 00000000..e9c053a3
--- /dev/null
+++ b/lib/microwaveprop_web/live/live_stash_guard.ex
@@ -0,0 +1,34 @@
+defmodule MicrowavepropWeb.LiveStashGuard do
+  @moduledoc """
+  Tiny wrapper around `LiveStash.stash/1` that swallows the
+  `ArgumentError: not a valid match specification` crash LiveStash 0.2.0
+  raises under OTP 28.
+
+  The stash is a convenience for state persistence across reconnects;
+  a failure is not fatal to the user's session. We log and return the
+  socket unchanged instead of letting the LiveView process terminate
+  mid-`handle_event`.
+
+  Remove this module and inline `LiveStash.stash/1` calls once the
+  upstream match-spec fix lands in the `live_stash` hex package.
+  """
+
+  alias Phoenix.LiveView.Socket
+
+  require Logger
+
+  @doc """
+  Best-effort wrapper around `LiveStash.stash/1`.
+
+  Returns the (possibly unchanged) socket no matter what the adapter
+  raises.
+  """
+  @spec stash(Socket.t()) :: Socket.t()
+  def stash(socket) do
+    LiveStash.stash(socket)
+  rescue
+    e in ArgumentError ->
+      Logger.warning("LiveStashGuard: stash skipped (#{Exception.message(e)})")
+      socket
+  end
+end
diff --git a/lib/microwaveprop_web/live/map_live.ex b/lib/microwaveprop_web/live/map_live.ex
index 0d53860b..5ea59b00 100644
--- a/lib/microwaveprop_web/live/map_live.ex
+++ b/lib/microwaveprop_web/live/map_live.ex
@@ -232,7 +232,7 @@ defmodule MicrowavepropWeb.MapLive do
       socket
       |> assign(selected_band: band, valid_times: valid_times, selected_time: selected_time)
       |> assign(:tracking_now?, tracking_now?(selected_time, valid_times, DateTime.utc_now()))
-      |> LiveStash.stash()
+      |> MicrowavepropWeb.LiveStashGuard.stash()
       |> push_event("update_scores", %{scores: Propagation.pack_scores(scores)})
       |> push_event("update_band_info", %{band_info: band_info(band)})
       |> push_timeline()
@@ -412,6 +412,10 @@ defmodule MicrowavepropWeb.MapLive do
     # since the JS hook expects `hours` in valid_time order.
     forecast_times = Enum.reject(socket.assigns.valid_times, &(selected && DateTime.compare(&1, selected) == :eq))
 
+    # `on_timeout: :kill_task` + explicit filter on :ok keeps a slow NFS
+    # read from cascading the whole stream's :timeout exit up into the
+    # LiveView process. A timed-out forecast hour just drops from the
+    # preload payload; the client re-requests it on demand.
     hours =
       forecast_times
       |> Task.async_stream(
@@ -423,9 +427,13 @@ defmodule MicrowavepropWeb.MapLive do
         end,
         max_concurrency: 4,
         ordered: true,
-        timeout: 10_000
+        timeout: 10_000,
+        on_timeout: :kill_task
       )
-      |> Enum.map(fn {:ok, h} -> h end)
+      |> Enum.flat_map(fn
+        {:ok, h} -> [h]
+        {:exit, _} -> []
+      end)
 
     {:noreply,
      socket
diff --git a/lib/microwaveprop_web/live/submit_live.ex b/lib/microwaveprop_web/live/submit_live.ex
index 98edb3f2..66dfa1c1 100644
--- a/lib/microwaveprop_web/live/submit_live.ex
+++ b/lib/microwaveprop_web/live/submit_live.ex
@@ -57,7 +57,7 @@ defmodule MicrowavepropWeb.SubmitLive do
   @impl true
   def handle_event("switch_tab", %{"tab" => tab}, socket) do
     socket = assign(socket, active_tab: String.to_existing_atom(tab))
-    {:noreply, LiveStash.stash(socket)}
+    {:noreply, MicrowavepropWeb.LiveStashGuard.stash(socket)}
   end
 
   def handle_event("validate", %{"contact" => contact_params}, socket) do