Auto merge of #81825 - voidc:pidfd, r=joshtriplett

Add Linux-specific pidfd process extensions (take 2)

Continuation of #77168.
I addressed the following concerns from the original PR:

- make `CommandExt` and `ChildExt` sealed traits
- wrap file descriptors in `PidFd` struct representing ownership over the fd
- add `take_pidfd` to take the fd out of `Child`
- close fd when dropped

Tracking Issue: #82971

src/test/ui/command/command-create-pidfd.rs

@@ -0,0 +1,45 @@
+// run-pass
+// only-linux - pidfds are a linux-specific concept
+
+#![feature(linux_pidfd)]
+#![feature(rustc_private)]
+
+extern crate libc;
+
+use std::io::Error;
+use std::os::linux::process::{ChildExt, CommandExt};
+use std::process::Command;
+
+fn has_clone3() -> bool {
+    let res = unsafe { libc::syscall(libc::SYS_clone3, 0, 0) };
+    let err = (res == -1)
+        .then(|| Error::last_os_error())
+        .expect("probe syscall should not succeed");
+    err.raw_os_error() != Some(libc::ENOSYS)
+}
+
+fn main() {
+    // pidfds require the clone3 syscall
+    if !has_clone3() {
+        return;
+    }
+
+    // We don't assert the precise value, since the standard library
+    // might have opened other file descriptors before our code runs.
+    let _ = Command::new("echo")
+        .create_pidfd(true)
+        .spawn()
+        .unwrap()
+        .pidfd().expect("failed to obtain pidfd");
+
+    let _ = Command::new("echo")
+        .create_pidfd(false)
+        .spawn()
+        .unwrap()
+        .pidfd().expect_err("pidfd should not have been created when create_pid(false) is set");
+
+    let _ = Command::new("echo")
+        .spawn()
+        .unwrap()
+        .pidfd().expect_err("pidfd should not have been created");
+}

src/test/ui/command/command-pre-exec.rs

@@ -8,8 +8,6 @@
 // ignore-sgx no processes
 #![feature(process_exec, rustc_private)]
 
-extern crate libc;
-
 use std::env;
 use std::io::Error;
 use std::os::unix::process::CommandExt;
@@ -17,6 +15,23 @@ use std::process::Command;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 
+#[cfg(not(target_os = "linux"))]
+fn getpid() -> u32 {
+    use std::process;
+    process::id()
+}
+
+/// We need to directly use the getpid syscall instead of using `process::id()`
+/// because the libc wrapper might return incorrect values after a process was
+/// forked.
+#[cfg(target_os = "linux")]
+fn getpid() -> u32 {
+    extern crate libc;
+    unsafe {
+        libc::syscall(libc::SYS_getpid) as _
+    }
+}
+
 fn main() {
     if let Some(arg) = env::args().nth(1) {
         match &arg[..] {
@@ -68,14 +83,12 @@ fn main() {
     };
     assert_eq!(output.raw_os_error(), Some(102));
 
-    let pid = unsafe { libc::getpid() };
-    assert!(pid >= 0);
+    let pid = getpid();
     let output = unsafe {
         Command::new(&me)
             .arg("empty")
             .pre_exec(move || {
-                let child = libc::getpid();
-                assert!(child >= 0);
+                let child = getpid();
                 assert!(pid != child);
                 Ok(())
             })

src/test/ui/process/process-panic-after-fork.rs

@@ -23,6 +23,21 @@ use std::sync::atomic::{AtomicU32, Ordering};
 
 use libc::c_int;
 
+#[cfg(not(target_os = "linux"))]
+fn getpid() -> u32 {
+    process::id()
+}
+
+/// We need to directly use the getpid syscall instead of using `process::id()`
+/// because the libc wrapper might return incorrect values after a process was
+/// forked.
+#[cfg(target_os = "linux")]
+fn getpid() -> u32 {
+    unsafe {
+        libc::syscall(libc::SYS_getpid) as _
+    }
+}
+
 /// This stunt allocator allows us to spot heap allocations in the child.
 struct PidChecking<A> {
     parent: A,
@@ -44,7 +59,7 @@ impl<A> PidChecking<A> {
     fn check(&self) {
         let require_pid = self.require_pid.load(Ordering::Acquire);
         if require_pid != 0 {
-            let actual_pid = process::id();
+            let actual_pid = getpid();
             if require_pid != actual_pid {
                 unsafe {
                     libc::raise(libc::SIGUSR1);