Log the error when Postgres closes an idle pool connection

AGENTS.md

@@ -175,8 +175,8 @@ templates/
   fullstack/     web/ (Vite + React + Tailwind + shadcn/ui), api/ (Hono + pg)
 scripts/         migrate, doctor, demo, support
 tests/           agents, blueprint, contracts, gateway, git, github-auth,
-                 host, images, policy, render, shell, teardown, templates,
-                 tools, workflow
+                 host, images, policy, render, shell, store, teardown,
+                 templates, tools, workflow
 ```
 
 There is no `tasks.ts`, `scaffold.ts`, `shell.ts`, `github.ts`, or `format.ts`:

app/store.ts

@@ -111,6 +111,13 @@ export function db(): pg.Pool {
 			allowExitOnIdle: true,
 			options: "-c statement_timeout=15000 -c lock_timeout=5000",
 		});
+		// Postgres can close an idle connection of the pool, for example in a
+		// restart or a failover. The pool then removes the client and emits
+		// "error". If no listener gets the event, Node stops the process. The
+		// next query gets a new connection, so the listener only logs the error.
+		pool.on("error", (error) => {
+			console.error("Lost an idle Postgres connection:", error);
+		});
 	}
 	return pool;
 }

tests/store.test.ts

@@ -0,0 +1,29 @@
+/**
+ * Postgres can close an idle connection of the pool, for example in a
+ * restart. The pool then emits "error", and an "error" event with no listener
+ * stops the gateway and the workflows host. No test connects to Postgres: the
+ * pool opens a connection only for a query.
+ */
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { db } from "../app/store.js";
+
+describe("db", () => {
+	afterEach(() => {
+		vi.restoreAllMocks();
+		vi.unstubAllEnvs();
+	});
+
+	it("logs the error of an idle connection and does not throw it", () => {
+		vi.stubEnv("DATABASE_URL", "postgres://factory@127.0.0.1:5432/factory");
+		const logged = vi.spyOn(console, "error").mockImplementation(() => {});
+		const error = new Error(
+			"terminating connection due to administrator command",
+		);
+
+		expect(() => db().emit("error", error)).not.toThrow();
+		expect(logged).toHaveBeenCalledWith(
+			"Lost an idle Postgres connection:",
+			error,
+		);
+	});
+});