Auto merge of #135359 - RalfJung:lang-start-unwind, r=joboet

use a single large catch_unwind in lang_start

I originally planned to use `abort_unwind` but reading the comment in `thread_cleanup` it seems we are deliberately going for slightly nicer error messages here, so this preserves that. It still seems nice to not repeat `catch_unwind` so often.

1 files changed, 30 insertions, 17 deletions

diff --git a/library/std/src/rt.rs b/library/std/src/rt.rs
index b2492238bd3..24a362072ab 100644
--- a/library/std/src/rt.rs
+++ b/library/std/src/rt.rs
@@ -67,7 +67,7 @@ macro_rules! rtunwrap {
     };
 }
 
-fn handle_rt_panic(e: Box<dyn Any + Send>) {
+fn handle_rt_panic<T>(e: Box<dyn Any + Send>) -> T {
     mem::forget(e);
     rtabort!("initialization or cleanup bug");
 }
@@ -157,7 +157,7 @@ fn lang_start_internal(
     argc: isize,
     argv: *const *const u8,
     sigpipe: u8,
-) -> Result<isize, !> {
+) -> isize {
     // Guard against the code called by this function from unwinding outside of the Rust-controlled
     // code, which is UB. This is a requirement imposed by a combination of how the
     // `#[lang="start"]` attribute is implemented as well as by the implementation of the panicking
@@ -168,19 +168,33 @@ fn lang_start_internal(
     // panic is a std implementation bug. A quite likely one too, as there isn't any way to
     // prevent std from accidentally introducing a panic to these functions. Another is from
     // user code from `main` or, more nefariously, as described in e.g. issue #86030.
-    // SAFETY: Only called once during runtime initialization.
-    panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) })
-        .unwrap_or_else(handle_rt_panic);
-    let ret_code = panic::catch_unwind(move || panic::catch_unwind(main).unwrap_or(101) as isize)
-        .map_err(move |e| {
-            mem::forget(e);
-            rtabort!("drop of the panic payload panicked");
+    //
+    // We use `catch_unwind` with `handle_rt_panic` instead of `abort_unwind` to make the error in
+    // case of a panic a bit nicer.
+    panic::catch_unwind(move || {
+        // SAFETY: Only called once during runtime initialization.
+        unsafe { init(argc, argv, sigpipe) };
+
+        let ret_code = panic::catch_unwind(main).unwrap_or_else(move |payload| {
+            // Carefully dispose of the panic payload.
+            let payload = panic::AssertUnwindSafe(payload);
+            panic::catch_unwind(move || drop({ payload }.0)).unwrap_or_else(move |e| {
+                mem::forget(e); // do *not* drop the 2nd payload
+                rtabort!("drop of the panic payload panicked");
+            });
+            // Return error code for panicking programs.
+            101
         });
-    panic::catch_unwind(cleanup).unwrap_or_else(handle_rt_panic);
-    // Guard against multiple threads calling `libc::exit` concurrently.
-    // See the documentation for `unique_thread_exit` for more information.
-    panic::catch_unwind(crate::sys::exit_guard::unique_thread_exit).unwrap_or_else(handle_rt_panic);
-    ret_code
+        let ret_code = ret_code as isize;
+
+        cleanup();
+        // Guard against multiple threads calling `libc::exit` concurrently.
+        // See the documentation for `unique_thread_exit` for more information.
+        crate::sys::exit_guard::unique_thread_exit();
+
+        ret_code
+    })
+    .unwrap_or_else(handle_rt_panic)
 }
 
 #[cfg(not(any(test, doctest)))]
@@ -191,11 +205,10 @@ fn lang_start<T: crate::process::Termination + 'static>(
     argv: *const *const u8,
     sigpipe: u8,
 ) -> isize {
-    let Ok(v) = lang_start_internal(
+    lang_start_internal(
         &move || crate::sys::backtrace::__rust_begin_short_backtrace(main).report().to_i32(),
         argc,
         argv,
         sigpipe,
-    );
-    v
+    )
 }