Reset an inherited ignored SIGCHLD at startup on Linux

src/runtime/bin_entry/mod.rs

@@ -180,6 +180,10 @@ pub(crate) unsafe extern "C" fn main(argc: c_int, argv: *const *const c_char) ->
         libc::signal(libc::SIGXFSZ, libc::SIG_IGN);
     }
 
+    // macOS and FreeBSD keep children waitable after execve.
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    keep_children_waitable();
+
     // Windows-only startup. Must run before anything reads
     // `Bun.env`/`process.env` (env conversion).
     #[cfg(windows)]
@@ -212,6 +216,21 @@ pub(crate) unsafe extern "C" fn main(argc: c_int, argv: *const *const c_char) ->
     Global::exit(0)
 }
 
+/// An inherited SIG_IGN on SIGCHLD makes Linux reap our children itself: wait4() fails with ECHILD.
+#[cfg(any(target_os = "linux", target_os = "android"))]
+fn keep_children_waitable() {
+    // SAFETY: `old` is a valid out-pointer and a null new action only queries;
+    // `SIGCHLD` and `SIG_DFL` are a valid signal and disposition.
+    unsafe {
+        let mut old: libc::sigaction = bun_core::ffi::zeroed();
+        if libc::sigaction(libc::SIGCHLD, core::ptr::null(), &raw mut old) == 0
+            && old.sa_sigaction == libc::SIG_IGN
+        {
+            libc::signal(libc::SIGCHLD, libc::SIG_DFL);
+        }
+    }
+}
+
 /// Linux's `expand_fdtable()` waits for an RCU grace period (tens of ms on a
 /// many-core machine) each time the fd table doubles past 64 once a second
 /// thread exists. Single-threaded it is a memcpy and the table never shrinks,

test/js/bun/spawn/spawn-sigchld-ignored.test.ts

@@ -0,0 +1,132 @@
+import { describe, expect, test } from "bun:test";
+import { bunEnv, bunExe, isLinux, libcPathForDlopen, tempDir } from "harness";
+import { join } from "node:path";
+
+// An ignored SIGCHLD survives execve. While it is ignored, Linux reaps the
+// children of the process itself and wait4() fails with ECHILD, so the exit
+// status of every child is lost.
+
+// `trap "" CHLD` sets SIGCHLD to SIG_IGN in bash and `exec` hands that to the
+// next program. dash leaves SIGCHLD alone, so /bin/sh cannot be the launcher.
+function launcher(ignore: string) {
+  return ["bash", "-c", `trap "" ${ignore}; exec "$0" "$@"`];
+}
+
+// Bit n-1 of a signal mask in /proc/<pid>/status is set for signal n.
+// SigIgn has the ignored signals, SigCgt the ones with a handler.
+function signals(status: string, field: "SigIgn" | "SigCgt") {
+  const mask = parseInt(new RegExp(`^${field}:\\s*([0-9a-f]+)$`, "m").exec(status)?.[1].slice(-8) ?? "0", 16);
+  return { SIGHUP: (mask & 1) !== 0, SIGINT: (mask & 2) !== 0, SIGCHLD: (mask & (1 << 16)) !== 0 };
+}
+
+const printStatus = `console.log(require("node:fs").readFileSync("/proc/self/status", "utf8"))`;
+
+const launcherIgnoresSigchld =
+  isLinux &&
+  Bun.which("bash") != null &&
+  signals(Bun.spawnSync({ cmd: [...launcher("CHLD"), "cat", "/proc/self/status"] }).stdout.toString(), "SigIgn")
+    .SIGCHLD;
+
+const cc = Bun.which("cc") ?? Bun.which("clang") ?? Bun.which("gcc");
+
+describe.skipIf(!launcherIgnoresSigchld)("bun started with SIGCHLD ignored", () => {
+  async function run(ignore: string, args: string[], options: { cwd?: string; env?: typeof bunEnv } = {}) {
+    await using proc = Bun.spawn({
+      cmd: [...launcher(ignore), bunExe(), ...args],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+      // Without the reset, `bun run` with BUN_FEATURE_FLAG_NO_ORPHANS=1 does not stop on SIGTERM.
+      killSignal: "SIGKILL",
+      ...options,
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    return { stdout, stderr, exitCode };
+  }
+
+  test.concurrent.each([
+    ["Bun.spawnSync", `Bun.spawnSync(["sh", "-c", "exit 7"]).exitCode`, "7"],
+    ["Bun.spawn", `await Bun.spawn(["sh", "-c", "exit 7"]).exited`, "7"],
+    ["child_process.spawnSync", `require("node:child_process").spawnSync("sh", ["-c", "exit 7"]).status`, "7"],
+    ["Bun.$", `(await Bun.$\`sh -c "exit 5"\`.nothrow().quiet()).exitCode`, "5"],
+  ])("%s reports the exit code of the child", async (_, expression, expected) => {
+    const { stdout, stderr, exitCode } = await run("CHLD", ["-e", `console.log(${expression})`]);
+    expect(stderr).toBe("");
+    expect(stdout).toBe(expected + "\n");
+    expect(exitCode).toBe(0);
+  });
+
+  // system() forks and waits inside libc, not through bun's spawn. The wait
+  // status of a command that exits holds its exit code in bits 8 to 15.
+  test.concurrent("system() through bun:ffi reports the exit code of the command", async () => {
+    // A skipped describe still runs its callback, and libcPathForDlopen() throws on Windows.
+    const libc = JSON.stringify(libcPathForDlopen());
+    const { stdout, stderr, exitCode } = await run("CHLD", [
+      "-e",
+      `const { symbols } = require("bun:ffi").dlopen(${libc}, { system: { args: ["cstring"], returns: "int" } });
+       console.log(symbols.system(Buffer.from("exit 7\\0")) >> 8);`,
+    ]);
+    expect(stderr).toBe("");
+    expect(stdout).toBe("7\n");
+    expect(exitCode).toBe(0);
+  });
+
+  test.concurrent.each([
+    ["bun run", {}],
+    ["bun run with BUN_FEATURE_FLAG_NO_ORPHANS=1", { BUN_FEATURE_FLAG_NO_ORPHANS: "1" }],
+  ])("%s exits with the exit code of the script", async (_, env) => {
+    using dir = tempDir("sigchld-ignored", {
+      "package.json": JSON.stringify({ name: "sigchld-ignored", scripts: { fail: "exit 3" } }),
+    });
+    const { stderr, exitCode } = await run("CHLD", ["run", "fail"], { cwd: String(dir), env: { ...bunEnv, ...env } });
+    expect(stderr).toBe('$ exit 3\nerror: script "fail" exited with code 3\n');
+    expect(exitCode).toBe(3);
+  });
+
+  // Only SIGCHLD is put back to its default. `nohup bun` depends on the others.
+  test.concurrent("SIGHUP and SIGINT stay ignored", async () => {
+    const { stdout, stderr, exitCode } = await run("CHLD HUP INT", ["-e", printStatus]);
+    expect(stderr).toBe("");
+    expect(signals(stdout, "SigIgn")).toEqual({ SIGHUP: true, SIGINT: true, SIGCHLD: false });
+    expect(exitCode).toBe(0);
+  });
+
+  // Only an exact SIG_IGN is replaced. A preloaded library runs before main(),
+  // and the handler that it installs stays.
+  test.concurrent.skipIf(cc == null)("a SIGCHLD handler installed before main() stays", async () => {
+    using dir = tempDir("sigchld-handler", {
+      "handler.c": `
+        #include <signal.h>
+        static void on_sigchld(int sig) { (void)sig; }
+        __attribute__((constructor)) static void install(void) {
+          struct sigaction sa = {0};
+          sa.sa_handler = on_sigchld;
+          sigaction(SIGCHLD, &sa, 0);
+        }
+      `,
+    });
+    const library = join(String(dir), "handler.so");
+    await using compile = Bun.spawn({
+      cmd: [cc!, "-shared", "-fPIC", "-o", library, join(String(dir), "handler.c")],
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [compileStdout, compileStderr, compileExitCode] = await Promise.all([
+      compile.stdout.text(),
+      compile.stderr.text(),
+      compile.exited,
+    ]);
+    expect({ stdout: compileStdout, stderr: compileStderr, exitCode: compileExitCode }).toEqual({
+      stdout: "",
+      stderr: "",
+      exitCode: 0,
+    });
+
+    const { stdout, stderr, exitCode } = await run("CHLD", ["-e", printStatus], {
+      env: { ...bunEnv, LD_PRELOAD: library },
+    });
+    expect(stderr).toBe("");
+    expect(signals(stdout, "SigCgt").SIGCHLD).toBe(true);
+    expect(exitCode).toBe(0);
+  });
+});