std: thread: Return error if setting thread stack size fails

Currently, when setting the thread stack size fails, it would be rounded
up to the nearest multiple of the page size and the code asserts that
the next call to pthread_attr_setstacksize succeeds.

This may be true for glibc, but it isn't true for musl, which not only
enforces a minimum stack size, but also a maximum stack size of
usize::MAX / 4 - PTHREAD_STACK_MIN [1], triggering the assert rather
than erroring gracefully.

There isn't any way to handle this properly other than bailing out and
letting the user know it didn't succeed.

[1]: https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_attr_setstacksize.c#n5

Signed-off-by: Jens Reidel <[email protected]>

Author: Gelbpunkt

library/std/src/sys/pal/unix/thread.rs

@@ -68,7 +68,15 @@ impl Thread {
                     let page_size = os::page_size();
                     let stack_size =
                         (stack_size + page_size - 1) & (-(page_size as isize - 1) as usize - 1);
-                    assert_eq!(libc::pthread_attr_setstacksize(attr.as_mut_ptr(), stack_size), 0);
+
+                    // Some libc implementations, e.g. musl, place an upper bound
+                    // on the stack size, in which case we can only gracefully return
+                    // an error here.
+                    if libc::pthread_attr_setstacksize(attr.as_mut_ptr(), stack_size) != 0 {
+                        assert_eq!(libc::pthread_attr_destroy(attr.as_mut_ptr()), 0);
+                        drop(Box::from_raw(p));
+                        return Err(io::Error::other("invalid stack size"));
+                    }
                 }
             };
         }