| 1 | //! Classic Unix double-fork daemonization. |
| 2 | //! |
| 3 | //! Only `libc` is used for all syscalls so that this module has zero |
| 4 | //! dependency on any async runtime and can safely be called before Tokio |
| 5 | //! is ever created. |
| 6 | |
| 7 | use std::io; |
| 8 | |
| 9 | /// Double-fork daemonization. |
| 10 | /// |
| 11 | /// ## Process tree after the call returns |
| 12 | /// |
| 13 | /// ```text |
| 14 | /// original caller |
| 15 | /// ├── [fork #1] first child |
| 16 | /// │ ├── setsid() — new session, no controlling terminal |
| 17 | /// │ └── [fork #2] grandchild ← the actual daemon |
| 18 | /// │ ├── dup2 stdin/stdout/stderr → /dev/null |
| 19 | /// │ └── daemon_fn() |
| 20 | /// └── waitpid(first child) → returns normally to caller |
| 21 | /// ``` |
| 22 | /// |
| 23 | /// * **Original process** — waits for the first child (so it does not |
| 24 | /// become a zombie) then **returns normally**. The `daemon_fn` closure |
| 25 | /// is dropped (not called) here. |
| 26 | /// |
| 27 | /// * **First child** — calls `setsid(2)` to detach from the controlling |
| 28 | /// terminal, forks a second time, then exits immediately via `_exit(0)` |
| 29 | /// so that the grandchild is re-parented to `init` / `systemd`. |
| 30 | /// |
| 31 | /// * **Grandchild** — the daemon proper: its standard file descriptors are |
| 32 | /// redirected to `/dev/null` so that stray reads/writes cannot interact |
| 33 | /// with a terminal, then `daemon_fn()` is invoked. |
| 34 | /// |
| 35 | /// # Panics |
| 36 | /// |
| 37 | /// Panics in the original process if `fork()` fails. |
| 38 | /// |
| 39 | /// # Safety |
| 40 | /// |
| 41 | /// This function **must** be called before any multi-threaded runtime |
| 42 | /// (Tokio, Rayon, …) is started. Forking a multi-threaded process is |
| 43 | /// undefined behaviour. |
| 44 | pub fn spawn_daemon<F: FnOnce()>(daemon_fn: F) { |
| 45 | // ── Fork #1 ─────────────────────────────────────────────────────────── |
| 46 | let pid1 = unsafe { libc::fork() }; |
| 47 | |
| 48 | if pid1 < 0 { |
| 49 | panic!("fork() #1 failed: {}", io::Error::last_os_error()); |
| 50 | } |
| 51 | |
| 52 | if pid1 > 0 { |
| 53 | // ── Original process (parent) ───────────────────────────────────── |
| 54 | // Reap the first child so it does not linger as a zombie. |
| 55 | unsafe { |
| 56 | let mut status: libc::c_int = 0; |
| 57 | libc::waitpid(pid1, &mut status, 0); |
| 58 | } |
| 59 | // `daemon_fn` is intentionally *dropped* here — it must only run in |
| 60 | // the grandchild. |
| 61 | return; |
| 62 | } |
| 63 | |
| 64 | // ── First child ─────────────────────────────────────────────────────── |
| 65 | // Become the leader of a brand-new session. This severs the connection |
| 66 | // to any controlling terminal inherited from the parent. |
| 67 | if unsafe { libc::setsid() } < 0 { |
| 68 | // Cannot recover; exit without running Rust destructors so we do not |
| 69 | // flush/corrupt stdio buffers shared with the parent. |
| 70 | unsafe { libc::_exit(1) }; |
| 71 | } |
| 72 | |
| 73 | // ── Fork #2 ─────────────────────────────────────────────────────────── |
| 74 | let pid2 = unsafe { libc::fork() }; |
| 75 | |
| 76 | if pid2 < 0 { |
| 77 | unsafe { libc::_exit(1) }; |
| 78 | } |
| 79 | |
| 80 | if pid2 > 0 { |
| 81 | // First child exits immediately. The grandchild is now an orphan |
| 82 | // and will be re-parented to PID 1 (init / systemd), ensuring it |
| 83 | // can never accidentally acquire a controlling terminal. |
| 84 | // `_exit` is used (rather than `exit`) to avoid double-flushing |
| 85 | // any stdio buffers inherited from the original process. |
| 86 | unsafe { libc::_exit(0) }; |
| 87 | } |
| 88 | |
| 89 | // ── Grandchild — the actual daemon ──────────────────────────────────── |
| 90 | |
| 91 | // Redirect the standard file descriptors to /dev/null so that any |
| 92 | // accidental reads or writes do not interact with a terminal or pipe. |
| 93 | unsafe { |
| 94 | let dev_null = libc::open(b"/dev/null\0".as_ptr() as *const libc::c_char, libc::O_RDWR); |
| 95 | |
| 96 | if dev_null >= 0 { |
| 97 | // It is fine if any of these dup2 calls fail — we are already |
| 98 | // detached, and the daemon will soon open its own log files. |
| 99 | libc::dup2(dev_null, libc::STDIN_FILENO); |
| 100 | libc::dup2(dev_null, libc::STDOUT_FILENO); |
| 101 | libc::dup2(dev_null, libc::STDERR_FILENO); |
| 102 | |
| 103 | // Close the temporary fd only if it is not one of the three |
| 104 | // standard slots (which would have been installed above). |
| 105 | if dev_null > libc::STDERR_FILENO { |
| 106 | libc::close(dev_null); |
| 107 | } |
| 108 | } |
| 109 | } |
| 110 | |
| 111 | // Hand control to the daemon entry point. |
| 112 | // |
| 113 | // In normal usage `daemon_fn` is `|| server::run()`, which is `-> !` |
| 114 | // and never returns. The `_exit` below is a safety net for any |
| 115 | // `daemon_fn` that does return. |
| 116 | daemon_fn(); |
| 117 | |
| 118 | unsafe { libc::_exit(0) }; |
| 119 | } |