elixir-client: Fix race condition (#2157)

Fixes
https://electricsql-04.sentry.io/issues/14068898/?project=4508410462404688&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&stream_index=2

.changeset/lucky-boats-lie.md

@@ -0,0 +1,5 @@
+---
+"@core/elixir-client": patch
+---
+
+Fix race condition in elixir client when multiple simultaneous clients are streaming the same shape

packages/elixir-client/lib/electric/client/fetch/monitor.ex

@@ -79,6 +79,7 @@ defmodule Electric.Client.Fetch.Monitor do
   @impl true
   def handle_continue({:start_request, request_id, request, client}, state) do
     {:ok, _pid} = Fetch.Request.start_link({request_id, request, client, self()})
+
     {:noreply, state}
   end
 
@@ -127,7 +128,7 @@ defmodule Electric.Client.Fetch.Monitor do
       send(pid, {:response, ref, response})
     end
 
-    {:stop, :normal, :ok, state}
+    {:stop, {:shutdown, :normal}, :ok, state}
   end
 
   @impl true
@@ -157,6 +158,6 @@ defmodule Electric.Client.Fetch.Monitor do
       send(pid, {:response, ref, {:error, reason}})
     end
 
-    {:stop, :normal, state}
+    {:stop, {:shutdown, :normal}, state}
   end
 end

packages/elixir-client/lib/electric/client/fetch/pool.ex

@@ -18,8 +18,11 @@ defmodule Electric.Client.Fetch.Pool do
   def request(%Client{} = client, %Fetch.Request{} = request, opts) do
     request_id = request_id(client, request)
 
-    # register this pid before making the request to avoid race conditions for
-    # very fast responses
+    # The monitor process is unique to the request and launches the actual
+    # request as a linked process.
+    #
+    # This coalesces requests, so no matter how many simultaneous
+    # clients we have, we only ever make one request to the backend.
     {:ok, monitor_pid} = start_monitor(request_id, request, client)
 
     try do
@@ -48,13 +51,12 @@ defmodule Electric.Client.Fetch.Pool do
   defp return_existing({:error, {:already_started, pid}}), do: {:ok, pid}
   defp return_existing(error), do: error
 
-  defp request_id(%Client{fetch: {fetch_impl, _}}, %Fetch.Request{shape_handle: nil} = request) do
-    %{endpoint: endpoint, shape: shape_definition} = request
-    {fetch_impl, URI.to_string(endpoint), shape_definition}
-  end
-
   defp request_id(%Client{fetch: {fetch_impl, _}}, %Fetch.Request{} = request) do
-    %{endpoint: endpoint, offset: offset, live: live, shape_handle: shape_handle} = request
-    {fetch_impl, URI.to_string(endpoint), shape_handle, Client.Offset.to_tuple(offset), live}
+    {
+      fetch_impl,
+      URI.to_string(request.endpoint),
+      request.headers,
+      Fetch.Request.params(request)
+    }
   end
 end

packages/elixir-client/lib/electric/client/fetch/request.ex

@@ -117,7 +117,7 @@ defmodule Electric.Client.Fetch.Request do
     |> Util.map_put_if("replica", to_string(replica), replica != :default)
     |> Util.map_put_if("handle", shape_handle, is_binary(shape_handle))
     |> Util.map_put_if("live", "true", live?)
-    |> Util.map_put_if("cursor", cursor, !is_nil(cursor))
+    |> Util.map_put_if("cursor", to_string(cursor), !is_nil(cursor))
     |> Util.map_put_if("database_id", database_id, !is_nil(database_id))
   end
 
@@ -133,9 +133,7 @@ defmodule Electric.Client.Fetch.Request do
 
   @doc false
   def start_link({request_id, request, client, monitor_pid}) do
-    GenServer.start_link(__MODULE__, {request_id, request, client, monitor_pid},
-      name: name(request_id)
-    )
+    GenServer.start_link(__MODULE__, {request_id, request, client, monitor_pid})
   end
 
   @impl true
@@ -161,16 +159,21 @@ defmodule Electric.Client.Fetch.Request do
 
     authenticated_request = Client.authenticate_request(client, request)
 
-    case fetcher.fetch(authenticated_request, fetcher_opts) do
-      {:ok, %Fetch.Response{status: status} = response} when status in 200..299 ->
-        reply(response, state)
+    try do
+      case fetcher.fetch(authenticated_request, fetcher_opts) do
+        {:ok, %Fetch.Response{status: status} = response} when status in 200..299 ->
+          reply(response, state)
 
-      {:ok, %Fetch.Response{} = response} ->
-        # Turn HTTP errors into errors
-        reply({:error, response}, state)
+        {:ok, %Fetch.Response{} = response} ->
+          # Turn HTTP errors into errors
+          reply({:error, response}, state)
 
+        error ->
+          reply(error, state)
+      end
+    rescue
       error ->
-        reply(error, state)
+        reply({:error, error}, state)
     end
 
     {:stop, :normal, state}