Add std:🧵:add_spawn_hook.

library/std/src/thread/mod.rs

@@ -188,6 +188,11 @@ mod current;
 pub use current::current;
 pub(crate) use current::{current_id, drop_current, set_current, try_current};
 
+mod spawnhook;
+
+#[unstable(feature = "thread_spawn_hook", issue = "none")]
+pub use spawnhook::add_spawn_hook;
+
 ////////////////////////////////////////////////////////////////////////////////
 // Thread-local storage
 ////////////////////////////////////////////////////////////////////////////////
@@ -485,6 +490,9 @@ impl Builder {
             Some(name) => Thread::new(id, name.into()),
             None => Thread::new_unnamed(id),
         };
+
+        let hooks = spawnhook::run_spawn_hooks(&my_thread)?;
+
         let their_thread = my_thread.clone();
 
         let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
@@ -535,6 +543,9 @@ impl Builder {
             }
 
             crate::io::set_output_capture(output_capture);
+            for hook in hooks {
+                hook();
+            }
 
             let f = f.into_inner();
             let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {

library/std/src/thread/spawnhook.rs

@@ -0,0 +1,92 @@
+use crate::io;
+use crate::sync::RwLock;
+use crate::thread::Thread;
+
+static SPAWN_HOOKS: RwLock<
+    Vec<&'static (dyn Fn(&Thread) -> io::Result<Box<dyn FnOnce() + Send>> + Sync)>,
+> = RwLock::new(Vec::new());
+
+/// Registers a function to run for every new thread spawned.
+///
+/// The hook is executed in the parent thread, and returns a function
+/// that will be executed in the new thread.
+///
+/// The hook is called with the `Thread` handle for the new thread.
+///
+/// If the hook returns an `Err`, thread spawning is aborted. In that case, the
+/// function used to spawn the thread (e.g. `std::thread::spawn`) will return
+/// the error returned by the hook.
+///
+/// Hooks can only be added, not removed.
+///
+/// The hooks will run in order, starting with the most recently added.
+///
+/// # Usage
+///
+/// ```
+/// #![feature(thread_spawn_hook)]
+///
+/// std::thread::add_spawn_hook(|_| {
+///     ..; // This will run in the parent (spawning) thread.
+///     Ok(move || {
+///         ..; // This will run it the child (spawned) thread.
+///     })
+/// });
+/// ```
+///
+/// # Example
+///
+/// A spawn hook can be used to initialize thread locals from the parent thread:
+///
+/// ```
+/// #![feature(thread_spawn_hook)]
+///
+/// use std::cell::Cell;
+///
+/// thread_local! {
+///     static X: Cell<u32> = Cell::new(0);
+/// }
+///
+/// std::thread::add_spawn_hook(|_| {
+///     // Get the value of X in the spawning thread.
+///     let value = X.get();
+///     // Set the value of X in the newly spawned thread.
+///     Ok(move || {
+///         X.set(value);
+///     })
+/// });
+///
+/// X.set(123);
+///
+/// std::thread::spawn(|| {
+///     assert_eq!(X.get(), 123);
+/// }).join().unwrap();
+/// ```
+#[unstable(feature = "thread_spawn_hook", issue = "none")]
+pub fn add_spawn_hook<F, G>(hook: F)
+where
+    F: 'static + Sync + Fn(&Thread) -> io::Result<G>,
+    G: 'static + Send + FnOnce(),
+{
+    SPAWN_HOOKS.write().unwrap_or_else(|e| e.into_inner()).push(Box::leak(Box::new(
+        move |thread: &Thread| -> io::Result<_> {
+            let f: Box<dyn FnOnce() + Send> = Box::new(hook(thread)?);
+            Ok(f)
+        },
+    )));
+}
+
+/// Runs all the spawn hooks.
+///
+/// Called on the parent thread.
+///
+/// Returns the functions to be called on the newly spawned thread.
+pub(super) fn run_spawn_hooks(thread: &Thread) -> io::Result<Vec<Box<dyn FnOnce() + Send>>> {
+    SPAWN_HOOKS
+        .read()
+        .unwrap_or_else(|e| e.into_inner())
+        .iter()
+        .rev()
+        .map(|hook| hook(thread))
+        .collect()
+}