Rollup merge of #117957 - the8472:pidfd-wait, r=Mark-Simulacrum

if available use a Child's pidfd for kill/wait

This should get us closer to stabilization of pidfds since they now do something useful. And they're `CLOEXEC` now.

```
$ strace -ffe clone,sendmsg,recvmsg,execve,kill,pidfd_open,pidfd_send_signal,waitpid,waitid ./x test std --no-doc -- pidfd

[...]
running 1 tests
strace: Process 816007 attached
[pid 816007] pidfd_open(816006, 0)      = 3
[pid 816007] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f0c6b787990) = 816008
strace: Process 816008 attached
[pid 816007] recvmsg(3,  <unfinished ...>
[pid 816008] pidfd_open(816008, 0)      = 3
[pid 816008] sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, 0) = 0
[pid 816007] <... recvmsg resumed>{msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[4]}], msg_controllen=24, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 0
[pid 816008] execve("/usr/bin/false", ["false"], 0x7ffcf2100048 /* 105 vars */) = 0
[pid 816007] waitid(P_PIDFD, 4,  <unfinished ...>
[pid 816008] +++ exited with 1 +++
[pid 816007] <... waitid resumed>{si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=816008, si_uid=1001, si_status=1, si_utime=0, si_stime=0}, WEXITED, NULL) = 0
[pid 816007] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=816008, si_uid=1001, si_status=1, si_utime=0, si_stime=0} ---
[pid 816007] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLDstrace: Process 816009 attached
, child_tidptr=0x7f0c6b787990) = 816009
[pid 816007] recvmsg(3,  <unfinished ...>
[pid 816009] pidfd_open(816009, 0)      = 3
[pid 816009] sendmsg(5, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, 0) = 0
[pid 816007] <... recvmsg resumed>{msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[5]}], msg_controllen=24, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 0
[pid 816009] execve("/usr/bin/sleep", ["sleep", "1000"], 0x7ffcf2100048 /* 105 vars */) = 0
[pid 816007] waitid(P_PIDFD, 5, {}, WNOHANG|WEXITED, NULL) = 0
[pid 816007] pidfd_send_signal(5, SIGKILL, NULL, 0) = 0
[pid 816007] waitid(P_PIDFD, 5,  <unfinished ...>
[pid 816009] +++ killed by SIGKILL +++
[pid 816007] <... waitid resumed>{si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=816009, si_uid=1001, si_status=SIGKILL, si_utime=0, si_stime=0}, WEXITED, NULL) = 0
[pid 816007] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=816009, si_uid=1001, si_status=SIGKILL, si_utime=0, si_stime=0} ---
[pid 816007] +++ exited with 0 +++
```

1 files changed, 6 insertions, 0 deletions

diff --git a/library/std/src/os/linux/process.rs b/library/std/src/os/linux/process.rs
index 2b3ff76d7a4..51af432d056 100644
--- a/library/std/src/os/linux/process.rs
+++ b/library/std/src/os/linux/process.rs
@@ -152,6 +152,12 @@ pub trait CommandExt: Sealed {
     /// in a guaranteed race-free manner (e.g. if the `clone3` system call
     /// is supported). Otherwise, [`pidfd`] will return an error.
     ///
+    /// If a pidfd has been successfully created and not been taken from the `Child`
+    /// then calls to `kill()`, `wait()` and `try_wait()` will use the pidfd
+    /// instead of the pid. This can prevent pid recycling races, e.g.
+    /// those  caused by rogue libraries in the same process prematurely reaping
+    /// zombie children via `waitpid(-1, ...)` calls.
+    ///
     /// [`Command`]: process::Command
     /// [`Child`]: process::Child
     /// [`pidfd`]: fn@ChildExt::pidfd