Rollup merge of rust-lang#96725 - nico-abram:win_tid, r=ChrisDenton

Expose process windows_process_extensions_main_thread_handle on Windows

~~I did not find any tests in https://github.com/rust-lang/rust/blob/7d3e03666a93bd2b0f78b3933f9305832af771a5/library/std/src/sys/windows/process/tests.rs that actually launch processes, so I haven't added tests for this.~~ I ran the following locally, to check that it works as expected:
```rs
#![feature(windows_process_extensions_main_thread_handle)]

fn main() {
    use std::os::windows::process::{ChildExt, CommandExt};
    const CREATE_SUSPENDED: u32 = 0x00000004;

    let proc = std::process::Command::new("cmd")
        .args(["/C", "echo hello"])
        .creation_flags(CREATE_SUSPENDED)
        .spawn()
        .unwrap();

    extern "system" {
        fn ResumeThread(_: *mut std::ffi::c_void) -> u32;
    }
    unsafe {
        ResumeThread(proc.main_thread_handle());
    }

    let output = proc.wait_with_output().unwrap();
    let str_output = std::str::from_utf8(&output.stdout[..]).unwrap();
    println!("{}", str_output);
}

```

Without the feature attribute it wouldn't compile, and commenting the `ResumeThread` line makes it hang forever, showing that it works.

Trakcing issue rust-lang#96723

Author: JohnTitor

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

@@ -180,3 +180,17 @@ impl CommandExt for process::Command {
         self
     }
 }
+
+#[unstable(feature = "windows_process_extensions_main_thread_handle", issue = "96723")]
+pub trait ChildExt: Sealed {
+    /// Extracts the main thread raw handle, without taking ownership
+    #[unstable(feature = "windows_process_extensions_main_thread_handle", issue = "96723")]
+    fn main_thread_handle(&self) -> BorrowedHandle<'_>;
+}
+
+#[unstable(feature = "windows_process_extensions_main_thread_handle", issue = "96723")]
+impl ChildExt for process::Child {
+    fn main_thread_handle(&self) -> BorrowedHandle<'_> {
+        self.handle.main_thread_handle()
+    }
+}

library/std/src/sys/windows/process.rs

@@ -14,7 +14,7 @@ use crate::io::{self, Error, ErrorKind};
 use crate::mem;
 use crate::num::NonZeroI32;
 use crate::os::windows::ffi::{OsStrExt, OsStringExt};
-use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle};
+use crate::os::windows::io::{AsHandle, AsRawHandle, BorrowedHandle, FromRawHandle, IntoRawHandle};
 use crate::path::{Path, PathBuf};
 use crate::ptr;
 use crate::sys::args::{self, Arg};
@@ -334,13 +334,14 @@ impl Command {
             ))
         }?;
 
-        // We close the thread handle because we don't care about keeping
-        // the thread id valid, and we aren't keeping the thread handle
-        // around to be able to close it later.
         unsafe {
-            drop(Handle::from_raw_handle(pi.hThread));
-
-            Ok((Process { handle: Handle::from_raw_handle(pi.hProcess) }, pipes))
+            Ok((
+                Process {
+                    handle: Handle::from_raw_handle(pi.hProcess),
+                    main_thread_handle: Handle::from_raw_handle(pi.hThread),
+                },
+                pipes,
+            ))
         }
     }
 }
@@ -609,6 +610,7 @@ impl From<File> for Stdio {
 /// for the process to terminate.
 pub struct Process {
     handle: Handle,
+    main_thread_handle: Handle,
 }
 
 impl Process {
@@ -621,6 +623,10 @@ impl Process {
         unsafe { c::GetProcessId(self.handle.as_raw_handle()) as u32 }
     }
 
+    pub fn main_thread_handle(&self) -> BorrowedHandle<'_> {
+        self.main_thread_handle.as_handle()
+    }
+
     pub fn wait(&mut self) -> io::Result<ExitStatus> {
         unsafe {
             let res = c::WaitForSingleObject(self.handle.as_raw_handle(), c::INFINITE);

library/std/src/sys/windows/process/tests.rs

@@ -24,6 +24,31 @@ fn test_raw_args() {
     );
 }
 
+#[test]
+fn test_thread_handle() {
+    use crate::os::windows::io::BorrowedHandle;
+    use crate::os::windows::process::{ChildExt, CommandExt};
+    const CREATE_SUSPENDED: u32 = 0x00000004;
+
+    let p = Command::new("cmd").args(&["/C", "exit 0"]).creation_flags(CREATE_SUSPENDED).spawn();
+    assert!(p.is_ok());
+    let mut p = p.unwrap();
+
+    extern "system" {
+        fn ResumeThread(_: BorrowedHandle<'_>) -> u32;
+    }
+    unsafe {
+        ResumeThread(p.main_thread_handle());
+    }
+
+    crate::thread::sleep(crate::time::Duration::from_millis(100));
+
+    let res = p.try_wait();
+    assert!(res.is_ok());
+    assert!(res.unwrap().is_some());
+    assert!(p.try_wait().unwrap().unwrap().success());
+}
+
 #[test]
 fn test_make_command_line() {
     fn test_wrapper(prog: &str, args: &[&str], force_quotes: bool) -> String {