wait: implement waitid()

waitid() has a number of additional features that waitpid() is missing:

- WNOWAIT is only accepted for waitid() on Linux (and possibly other
  platforms)
- Support for waiting on PID file descriptors on Linux

For now support is added for all platforms with waitid() that have proper
siginfo_t support in libc. NetBSD support is currently a work in progress
[1].

Tests for the signal/exit code are currently skipped on MIPS platforms due
to multiple bugs in qemu-user in the translation of siginfo_t (one fixed
in January [2], one currently under review [3]).

[1] rust-lang/libc#2476
[2] https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04810.html
[3] https://lists.nongnu.org/archive/html/qemu-devel/2021-10/msg05433.html

src/sys/wait.rs

@@ -6,6 +6,8 @@ use crate::Result;
 use cfg_if::cfg_if;
 use libc::{self, c_int};
 use std::convert::TryFrom;
+#[cfg(any(target_os = "android", target_os = "linux"))]
+use std::os::unix::io::RawFd;
 
 libc_bitflags!(
     /// Controls the behavior of [`waitpid`].
@@ -225,6 +227,61 @@ impl WaitStatus {
             WaitStatus::Continued(pid)
         })
     }
+
+    /// Convert a `siginfo_t` as returned by `waitid` to a `WaitStatus`
+    ///
+    /// # Errors
+    ///
+    /// Returns an `Error` corresponding to `EINVAL` for invalid values.
+    ///
+    /// # Unsafe
+    ///
+    /// siginfo_t is actually a union, not all fields may be initialized.
+    /// The functions si_pid() and si_status() must be valid to call on
+    /// the passed siginfo_t.
+    #[cfg(any(
+        target_os = "android",
+        target_os = "freebsd",
+        target_os = "haiku",
+        target_os = "linux",
+    ))]
+    pub unsafe fn from_siginfo(siginfo: &libc::siginfo_t) -> Result<WaitStatus> {
+        assert!(siginfo.si_signo == libc::SIGCHLD);
+
+        let si_pid = siginfo.si_pid();
+        if si_pid == 0 {
+            return Ok(WaitStatus::StillAlive);
+        }
+
+        let pid = Pid::from_raw(si_pid);
+        let si_status = siginfo.si_status();
+
+        let status = match siginfo.si_code {
+            libc::CLD_EXITED => WaitStatus::Exited(pid, si_status),
+            libc::CLD_KILLED | libc::CLD_DUMPED => WaitStatus::Signaled(
+                pid,
+                Signal::try_from(si_status)?,
+                siginfo.si_code == libc::CLD_DUMPED,
+            ),
+            libc::CLD_STOPPED => WaitStatus::Stopped(pid, Signal::try_from(si_status)?),
+            libc::CLD_CONTINUED => WaitStatus::Continued(pid),
+            #[cfg(any(target_os = "android", target_os = "linux"))]
+            libc::CLD_TRAPPED => {
+                if si_status == libc::SIGTRAP | 0x80 {
+                    WaitStatus::PtraceSyscall(pid)
+                } else {
+                    WaitStatus::PtraceEvent(
+                        pid,
+                        Signal::try_from(si_status & 0xff)?,
+                        (si_status >> 8) as c_int,
+                    )
+                }
+            }
+            _ => return Err(Errno::EINVAL),
+        };
+
+        Ok(status)
+    }
 }
 
 /// Wait for a process to change status
@@ -260,3 +317,57 @@ pub fn waitpid<P: Into<Option<Pid>>>(pid: P, options: Option<WaitPidFlag>) -> Re
 pub fn wait() -> Result<WaitStatus> {
     waitpid(None, None)
 }
+
+/// The ID argument for `waitid`
+#[cfg(any(
+    target_os = "android",
+    target_os = "freebsd",
+    target_os = "haiku",
+    target_os = "linux",
+))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Id {
+    /// Wait for any child
+    All,
+    /// Wait for the child whose process ID matches the given PID
+    Pid(Pid),
+    /// Wait for the child whose process group ID matches the given PID
+    ///
+    /// If no PID is given, the caller's process group is used since Linux 5.4.
+    PGid(Option<Pid>),
+    /// Wait for the child referred to by the given PID file descriptor
+    #[cfg(any(target_os = "android", target_os = "linux"))]
+    PIDFd(RawFd),
+}
+
+/// Wait for a process to change status
+///
+/// See also [waitid(2)](https://pubs.opengroup.org/onlinepubs/9699919799/functions/waitid.html)
+#[cfg(any(
+    target_os = "android",
+    target_os = "freebsd",
+    target_os = "haiku",
+    target_os = "linux",
+))]
+pub fn waitid(id: Id, flags: WaitPidFlag) -> Result<WaitStatus> {
+    let (idtype, idval) = match id {
+        Id::All => (libc::P_ALL, 0),
+        Id::Pid(pid) => (libc::P_PID, pid.as_raw() as libc::id_t),
+        Id::PGid(pid) => (
+            libc::P_PGID,
+            pid.map(Pid::as_raw).unwrap_or(0) as libc::id_t,
+        ),
+        #[cfg(any(target_os = "android", target_os = "linux"))]
+        Id::PIDFd(fd) => (libc::P_PIDFD, fd as libc::id_t),
+    };
+
+    let siginfo = unsafe {
+        // Memory is zeroed rather than uninitialized, as not all platforms
+        // initialize the memory in the StillAlive case
+        let mut siginfo: libc::siginfo_t = std::mem::zeroed();
+        Errno::result(libc::waitid(idtype, idval, &mut siginfo, flags.bits()))?;
+        siginfo
+    };
+
+    unsafe { WaitStatus::from_siginfo(&siginfo) }
+}

test/sys/test_wait.rs

@@ -23,6 +23,33 @@ fn test_wait_signal() {
     }
 }
 
+#[test]
+#[cfg(any(
+    target_os = "android",
+    target_os = "freebsd",
+    target_os = "haiku",
+    target_os = "linux",
+))]
+#[cfg(not(any(target_arch = "mips", target_arch = "mips64")))]
+fn test_waitid_signal() {
+    let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+    // Safe: The child only calls `pause` and/or `_exit`, which are async-signal-safe.
+    match unsafe{fork()}.expect("Error: Fork Failed") {
+      Child => {
+          pause();
+          unsafe { _exit(123) }
+      },
+      Parent { child } => {
+          kill(child, Some(SIGKILL)).expect("Error: Kill Failed");
+          assert_eq!(
+              waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+              Ok(WaitStatus::Signaled(child, SIGKILL, false)),
+          );
+      },
+    }
+}
+
 #[test]
 fn test_wait_exit() {
     let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
@@ -36,6 +63,29 @@ fn test_wait_exit() {
     }
 }
 
+#[test]
+#[cfg(any(
+    target_os = "android",
+    target_os = "freebsd",
+    target_os = "haiku",
+    target_os = "linux",
+))]
+#[cfg(not(any(target_arch = "mips", target_arch = "mips64")))]
+fn test_waitid_exit() {
+    let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+    // Safe: Child only calls `_exit`, which is async-signal-safe.
+    match unsafe{fork()}.expect("Error: Fork Failed") {
+      Child => unsafe { _exit(12); },
+      Parent { child } => {
+          assert_eq!(
+              waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+              Ok(WaitStatus::Exited(child, 12)),
+          );
+      }
+    }
+}
+
 #[test]
 fn test_waitstatus_from_raw() {
     let pid = Pid::from_raw(1);
@@ -57,6 +107,25 @@ fn test_waitstatus_pid() {
     }
 }
 
+#[test]
+#[cfg(any(
+    target_os = "android",
+    target_os = "freebsd",
+    target_os = "haiku",
+    target_os = "linux",
+))]
+fn test_waitid_pid() {
+    let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+    match unsafe { fork() }.unwrap() {
+        Child => unsafe { _exit(0) },
+        Parent { child } => {
+            let status = waitid(Id::Pid(child), WaitPidFlag::WEXITED).unwrap();
+            assert_eq!(status.pid(), Some(child));
+        }
+    }
+}
+
 #[cfg(any(target_os = "linux", target_os = "android"))]
 // FIXME: qemu-user doesn't implement ptrace on most arches
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
@@ -77,7 +146,7 @@ mod ptrace {
         unsafe { _exit(0) }
     }
 
-    fn ptrace_parent(child: Pid) {
+    fn ptrace_wait_parent(child: Pid) {
         // Wait for the raised SIGTRAP
         assert_eq!(waitpid(child, None), Ok(WaitStatus::Stopped(child, SIGTRAP)));
         // We want to test a syscall stop and a PTRACE_EVENT stop
@@ -94,14 +163,57 @@ mod ptrace {
         assert_eq!(waitpid(child, None), Ok(WaitStatus::Exited(child, 0)));
     }
 
+    fn ptrace_waitid_parent(child: Pid) {
+        // Wait for the raised SIGTRAP
+        //
+        // Unlike waitpid(), waitid() can distinguish trap events from regular
+        // stop events, so unlike ptrace_wait_parent(), we get a PtraceEvent here
+        assert_eq!(
+            waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+            Ok(WaitStatus::PtraceEvent(child, SIGTRAP, 0)),
+        );
+        // We want to test a syscall stop and a PTRACE_EVENT stop
+        assert!(ptrace::setoptions(child, Options::PTRACE_O_TRACESYSGOOD | Options::PTRACE_O_TRACEEXIT).is_ok());
+
+        // First, stop on the next system call, which will be exit()
+        assert!(ptrace::syscall(child, None).is_ok());
+        assert_eq!(
+            waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+            Ok(WaitStatus::PtraceSyscall(child)),
+        );
+        // Then get the ptrace event for the process exiting
+        assert!(ptrace::cont(child, None).is_ok());
+        assert_eq!(
+            waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+            Ok(WaitStatus::PtraceEvent(child, SIGTRAP, Event::PTRACE_EVENT_EXIT as i32)),
+        );
+        // Finally get the normal wait() result, now that the process has exited
+        assert!(ptrace::cont(child, None).is_ok());
+        assert_eq!(
+            waitid(Id::Pid(child), WaitPidFlag::WEXITED),
+            Ok(WaitStatus::Exited(child, 0)),
+        );
+    }
+
     #[test]
     fn test_wait_ptrace() {
         require_capability!("test_wait_ptrace", CAP_SYS_PTRACE);
         let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
 
         match unsafe{fork()}.expect("Error: Fork Failed") {
             Child => ptrace_child(),
-            Parent { child } => ptrace_parent(child),
+            Parent { child } => ptrace_wait_parent(child),
+        }
+    }
+
+    #[test]
+    fn test_waitid_ptrace() {
+        require_capability!("test_waitid_ptrace", CAP_SYS_PTRACE);
+        let _m = crate::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+        match unsafe{fork()}.expect("Error: Fork Failed") {
+            Child => ptrace_child(),
+            Parent { child } => ptrace_waitid_parent(child),
         }
     }
 }