linux::process: adding getfd() public api.

library/std/src/os/linux/process.rs

@@ -86,6 +86,15 @@ impl PidFd {
     pub fn try_wait(&self) -> Result<Option<ExitStatus>> {
         Ok(self.inner.try_wait()?.map(FromInner::from_inner))
     }
+
+    /// Obtains a duplicate of the file descriptor `targetfd`
+    ///
+    /// So we can get file descriptors from another process
+    /// without relying on that process to explicitly send them via
+    /// `SCM_RIGHTS` or similar mechanisms
+    pub fn getfd(&self, targetfd: RawFd) -> Result<OwnedFd> {
+        self.inner.getfd(targetfd)
+    }
 }
 
 impl AsInner<InnerPidFd> for PidFd {

library/std/src/sys/pal/unix/linux/pidfd.rs

@@ -1,5 +1,5 @@
 use crate::io;
-use crate::os::fd::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
+use crate::os::fd::{AsRawFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::sys::fd::FileDesc;
 use crate::sys::process::ExitStatus;
 use crate::sys::{AsInner, FromInner, IntoInner, cvt};
@@ -95,6 +95,13 @@ impl PidFd {
         .map(drop)
     }
 
+    #[cfg(any(test, target_env = "gnu", target_env = "musl"))]
+    pub fn getfd(&self, targetfd: RawFd) -> io::Result<OwnedFd> {
+        let fd =
+            cvt(unsafe { libc::syscall(libc::SYS_pidfd_getfd, self.0.as_raw_fd(), targetfd, 0) })?;
+        Ok(unsafe { OwnedFd::from_raw_fd(fd as RawFd) })
+    }
+
     pub fn wait(&self) -> io::Result<ExitStatus> {
         let r = self.waitid(libc::WEXITED)?;
         match r {