Allow foreign exceptions to unwind through Rust code and Rust panics to unwind through FFI

src/doc/unstable-book/src/language-features/lang-items.md

@@ -249,11 +249,11 @@ the source code.
 - Runtime
   - `start`: `libstd/rt.rs`
   - `eh_personality`: `libpanic_unwind/emcc.rs` (EMCC)
-  - `eh_personality`: `libpanic_unwind/seh64_gnu.rs` (SEH64 GNU)
+  - `eh_personality`: `libpanic_unwind/gcc.rs` (GNU)
   - `eh_personality`: `libpanic_unwind/seh.rs` (SEH)
-  - `eh_unwind_resume`: `libpanic_unwind/seh64_gnu.rs` (SEH64 GNU)
   - `eh_unwind_resume`: `libpanic_unwind/gcc.rs` (GCC)
-  - `msvc_try_filter`: `libpanic_unwind/seh.rs` (SEH)
+  - `eh_catch_typeinfo`: `libpanic_unwind/seh.rs` (SEH)
+  - `eh_catch_typeinfo`: `libpanic_unwind/emcc.rs` (EMCC)
   - `panic`: `libcore/panicking.rs`
   - `panic_bounds_check`: `libcore/panicking.rs`
   - `panic_impl`: `libcore/panicking.rs`

src/libpanic_unwind/dwarf/eh.rs

@@ -51,7 +51,7 @@ pub enum EHAction {
 
 pub const USING_SJLJ_EXCEPTIONS: bool = cfg!(all(target_os = "ios", target_arch = "arm"));
 
-pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>)
+pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>, foreign_exception: bool)
     -> Result<EHAction, ()>
 {
     if lsda.is_null() {
@@ -96,7 +96,7 @@ pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>)
                     return Ok(EHAction::None)
                 } else {
                     let lpad = lpad_base + cs_lpad;
-                    return Ok(interpret_cs_action(cs_action, lpad))
+                    return Ok(interpret_cs_action(cs_action, lpad, foreign_exception))
                 }
             }
         }
@@ -121,16 +121,23 @@ pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>)
                 // Can never have null landing pad for sjlj -- that would have
                 // been indicated by a -1 call site index.
                 let lpad = (cs_lpad + 1) as usize;
-                return Ok(interpret_cs_action(cs_action, lpad))
+                return Ok(interpret_cs_action(cs_action, lpad, foreign_exception))
             }
         }
     }
 }
 
-fn interpret_cs_action(cs_action: u64, lpad: usize) -> EHAction {
+fn interpret_cs_action(cs_action: u64, lpad: usize, foreign_exception: bool) -> EHAction {
     if cs_action == 0 {
+        // If cs_action is 0 then this is a cleanup (Drop::drop). We run these
+        // for both Rust panics and foriegn exceptions.
         EHAction::Cleanup(lpad)
+    } else if foreign_exception {
+        // catch_unwind should not catch foreign exceptions, only Rust panics.
+        // Instead just continue unwinding.
+        EHAction::None
     } else {
+        // Stop unwinding Rust panics at catch_unwind.
         EHAction::Catch(lpad)
     }
 }

src/libpanic_unwind/emcc.rs

@@ -15,14 +15,48 @@ use alloc::boxed::Box;
 use libc::{self, c_int};
 use unwind as uw;
 
+// This matches the layout of std::type_info in C++
+#[repr(C)]
+struct TypeInfo {
+    vtable: *const usize,
+    name: *const u8,
+}
+unsafe impl Sync for TypeInfo {}
+
+extern "C" {
+    // The leading `\x01` byte here is actually a magical signal to LLVM to
+    // *not* apply any other mangling like prefixing with a `_` character.
+    //
+    // This symbol is the vtable used by C++'s `std::type_info`. Objects of type
+    // `std::type_info`, type descriptors, have a pointer to this table. Type
+    // descriptors are referenced by the C++ EH structures defined above and
+    // that we construct below.
+    //
+    // Note that the real size is larger than 3 usize, but we only need our
+    // vtable to point to the third element.
+    #[link_name = "\x01_ZTVN10__cxxabiv117__class_type_infoE"]
+    static CLASS_TYPE_INFO_VTABLE: [usize; 3];
+}
+
+// std::type_info for a rust_panic class
+#[lang = "eh_catch_typeinfo"]
+static EXCEPTION_TYPE_INFO: TypeInfo = TypeInfo {
+    // Normally we would use .as_ptr().add(2) but this doesn't work in a const context.
+    vtable: unsafe { &CLASS_TYPE_INFO_VTABLE[2] },
+    // This intentionally doesn't use the normal name mangling scheme because
+    // we don't want C++ to be able to produce or catch Rust panics.
+    name: b"rust_panic\0".as_ptr(),
+};
+
 pub fn payload() -> *mut u8 {
     ptr::null_mut()
 }
 
 pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
     assert!(!ptr.is_null());
-    let ex = ptr::read(ptr as *mut _);
-    __cxa_free_exception(ptr as *mut _);
+    let adjusted_ptr = __cxa_begin_catch(ptr as *mut libc::c_void);
+    let ex = ptr::read(adjusted_ptr as *mut _);
+    __cxa_end_catch();
     ex
 }
 
@@ -32,11 +66,8 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     if exception == ptr::null_mut() {
         return uw::_URC_FATAL_PHASE1_ERROR as u32;
     }
-    let exception = exception as *mut Box<dyn Any + Send>;
-    ptr::write(exception, data);
-    __cxa_throw(exception as *mut _, ptr::null_mut(), ptr::null_mut());
-
-    unreachable!()
+    ptr::write(exception as *mut _, data);
+    __cxa_throw(exception as *mut _, &EXCEPTION_TYPE_INFO, ptr::null_mut());
 }
 
 #[lang = "eh_personality"]
@@ -52,10 +83,11 @@ unsafe extern "C" fn rust_eh_personality(version: c_int,
 
 extern "C" {
     fn __cxa_allocate_exception(thrown_size: libc::size_t) -> *mut libc::c_void;
-    fn __cxa_free_exception(thrown_exception: *mut libc::c_void);
+    fn __cxa_begin_catch(thrown_exception: *mut libc::c_void) -> *mut libc::c_void;
+    fn __cxa_end_catch();
     fn __cxa_throw(thrown_exception: *mut libc::c_void,
-                   tinfo: *mut libc::c_void,
-                   dest: *mut libc::c_void);
+                   tinfo: *const TypeInfo,
+                   dest: *mut libc::c_void) -> !;
     fn __gxx_personality_v0(version: c_int,
                             actions: uw::_Unwind_Action,
                             exception_class: uw::_Unwind_Exception_Class,