also ignore 'thread leaks' with -Zmiri-ignore-leaks

README.md

@@ -230,7 +230,8 @@ environment variable:
   the host so that it cannot be accessed by the program.  Can be used multiple
   times to exclude several variables.  On Windows, the `TERM` environment
   variable is excluded by default.
-* `-Zmiri-ignore-leaks` disables the memory leak checker.
+* `-Zmiri-ignore-leaks` disables the memory leak checker, and also allows some
+  remaining threads to exist when the main thread exits.
 * `-Zmiri-measureme=<name>` enables `measureme` profiling for the interpreted program.
    This can be used to find which parts of your program are executing slowly under Miri.
    The profile is written out to a file with the prefix `<name>`, and can be processed

src/eval.rs

@@ -300,10 +300,20 @@ pub fn eval_main<'tcx>(tcx: TyCtxt<'tcx>, main_id: DefId, config: MiriConfig) ->
     match res {
         Ok(return_code) => {
             if !ignore_leaks {
+                // Check for thread leaks.
+                if !ecx.have_all_terminated() {
+                    tcx.sess.err(
+                        "the main thread terminated without waiting for all remaining threads",
+                    );
+                    tcx.sess.note_without_error("pass `-Zmiri-ignore-leaks` to disable this check");
+                    return None;
+                }
+                // Check for memory leaks.
                 info!("Additonal static roots: {:?}", ecx.machine.static_roots);
                 let leaks = ecx.memory.leak_report(&ecx.machine.static_roots);
                 if leaks != 0 {
                     tcx.sess.err("the evaluated program leaked memory");
+                    tcx.sess.note_without_error("pass `-Zmiri-ignore-leaks` to disable this check");
                     // Ignore the provided return code - let the reported error
                     // determine the return code.
                     return None;

src/thread.rs

@@ -302,6 +302,11 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
         self.threads[thread_id].state == ThreadState::Terminated
     }
 
+    /// Have all threads terminated?
+    fn have_all_terminated(&self) -> bool {
+        self.threads.iter().all(|thread| thread.state == ThreadState::Terminated)
+    }
+
     /// Enable the thread for execution. The thread must be terminated.
     fn enable_thread(&mut self, thread_id: ThreadId) {
         assert!(self.has_terminated(thread_id));
@@ -491,15 +496,7 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
         // If we get here again and the thread is *still* terminated, there are no more dtors to run.
         if self.threads[MAIN_THREAD].state == ThreadState::Terminated {
             // The main thread terminated; stop the program.
-            if self.threads.iter().any(|thread| thread.state != ThreadState::Terminated) {
-                // FIXME: This check should be either configurable or just emit
-                // a warning. For example, it seems normal for a program to
-                // terminate without waiting for its detached threads to
-                // terminate. However, this case is not trivial to support
-                // because we also probably do not want to consider the memory
-                // owned by these threads as leaked.
-                throw_unsup_format!("the main thread terminated without waiting for other threads");
-            }
+            // We do *not* run TLS dtors of remaining threads, which seems to match rustc behavior.
             return Ok(SchedulingAction::Stop);
         }
         // This thread and the program can keep going.
@@ -645,6 +642,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.machine.threads.has_terminated(thread_id)
     }
 
+    #[inline]
+    fn have_all_terminated(&self) -> bool {
+        let this = self.eval_context_ref();
+        this.machine.threads.have_all_terminated()
+    }
+
     #[inline]
     fn enable_thread(&mut self, thread_id: ThreadId) {
         let this = self.eval_context_mut();

tests/compile-fail/concurrency/libc_pthread_create_main_terminate.rs

@@ -1,5 +1,5 @@
 // ignore-windows: No libc on Windows
-// error-pattern: unsupported operation: the main thread terminated without waiting for other threads
+// error-pattern: the main thread terminated without waiting for all remaining threads
 
 // Check that we terminate the program when the main thread terminates.
 
@@ -10,7 +10,7 @@ extern crate libc;
 use std::{mem, ptr};
 
 extern "C" fn thread_start(_null: *mut libc::c_void) -> *mut libc::c_void {
-    ptr::null_mut()
+    loop {}
 }
 
 fn main() {

tests/run-pass/threadleak_ignored.rs

@@ -0,0 +1,25 @@
+// ignore-windows: Concurrency on Windows is not supported yet.
+// compile-flags: -Zmiri-ignore-leaks
+
+//! Test that leaking threads works, and that their destructors are not executed.
+
+use std::cell::RefCell;
+
+struct LoudDrop(i32);
+impl Drop for LoudDrop {
+    fn drop(&mut self) {
+        eprintln!("Dropping {}", self.0);
+    }
+}
+
+thread_local! {
+    static X: RefCell<Option<LoudDrop>> = RefCell::new(None);
+}
+
+fn main() {
+    X.with(|x| *x.borrow_mut() = Some(LoudDrop(0)));
+
+    let _detached = std::thread::spawn(|| {
+        X.with(|x| *x.borrow_mut() = Some(LoudDrop(1)));
+    });
+}

tests/run-pass/threadleak_ignored.stderr

@@ -0,0 +1,3 @@
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
+
+Dropping 0