fix(daemon): prefer stderr over stale stdout in gateway restart diagnostics

Refs #93001.

src/daemon/diagnostics.test.ts

@@ -36,4 +36,21 @@ describe("readLastGatewayErrorLine", () => {
       "gateway stdout current",
     );
   });
+
+  it("prefers the current stderr error over a stale stdout match on linux", async () => {
+    const stateDir = makeTempStateDir();
+    const homeDir = makeTempStateDir();
+    const env = { HOME: homeDir, OPENCLAW_STATE_DIR: stateDir };
+    const stateLogs = resolveGatewayLogPaths(env);
+    fs.mkdirSync(stateLogs.logDir, { recursive: true });
+    // stderr carries the real, current failure; stdout carries an older matching
+    // line. On non-darwin platforms stderr is the strongest failure signal, so
+    // it must win instead of the stale stdout match.
+    fs.writeFileSync(stateLogs.stderrPath, "failed to bind gateway socket EADDRINUSE\n", "utf8");
+    fs.writeFileSync(stateLogs.stdoutPath, "gateway start blocked: stale prior reason\n", "utf8");
+
+    await expect(readLastGatewayErrorLine(env, { platform: "linux" })).resolves.toBe(
+      "failed to bind gateway socket EADDRINUSE",
+    );
+  });
 });

src/daemon/diagnostics.ts

@@ -40,7 +40,10 @@ export async function readLastGatewayErrorLine(
       : resolveGatewayLogPaths(env);
   const stderrRaw = readStderr ? await fs.readFile(stderrPath, "utf8").catch(() => "") : "";
   const stdoutRaw = await fs.readFile(stdoutPath, "utf8").catch(() => "");
-  const lines = [...stderrRaw.split(/\r?\n/), ...stdoutRaw.split(/\r?\n/)].map((line) =>
+  // stderr is the strongest failure signal on non-darwin platforms, so place it
+  // last and scan from the end: the most recent stderr error line then wins over
+  // any (possibly stale) stdout match, matching the stderr-first fallback below.
+  const lines = [...stdoutRaw.split(/\r?\n/), ...stderrRaw.split(/\r?\n/)].map((line) =>
     line.trim(),
   );
   for (let i = lines.length - 1; i >= 0; i -= 1) {