not a flaw but it seems like exactly what would cause corruption of the first argument

i don't know if editing that file causes recompilation of cargo, it might but then it might get overwritten again

can try to add something hacky there to see if extending the size of

a

in

extern_get_union_for

unconditionally fixes the error

@hundreds-breakfast-49010: would timebox getting this particular strategy working, and swap to another one as in https://pantsbuild.slack.com/archives/C0D7TNJHL/p1561584224108300

am trying to logically bisect the changes

fwiw this is my current working diff

diff --git a/src/python/pants/engine/native.py b/src/python/pants/engine/native.py
index 055764c..d96ca2b 100644
--- a/src/python/pants/engine/native.py
+++ b/src/python/pants/engine/native.py
@@ -290,16 +290,14 @@ class _FFISpecification(object):
   def extern_get_union_for(self, context_handle, type_id):
     """Return an optional union if the type belongs to one"""
 
+    <http://logger.info|logger.info>('h: {}'.format(context_handle))
     c = self._ffi.from_handle(context_handle)
     input_type = c.from_id(type_id.tup_0)
 
     build_configuration = self.build_configuration
     print("BUILD: {}".format(build_configuration))
 
-    response = self._ffi.new('OptionalTypeId*')
-    response.tag = self._lib.NoTypeId
-    response.no_type_id = ()
-    return response[0]
+    return OptionalTypeId(type_id=type_id.tup_0, is_some=False)
 
   @_extern_decl('Ident', ['ExternContext*', 'Handle*'])
   def extern_identify(self, context_handle, val):
@@ -335,6 +333,7 @@ class _FFISpecification(object):
   @_extern_decl('Buffer', ['ExternContext*', 'TypeId'])
   def extern_type_to_str(self, context_handle, type_id):
     """Given a TypeId, write type.__name__ and return it."""
+    <http://logger.info|logger.info>('i: {}'.format(context_handle))
     c = self._ffi.from_handle(context_handle)
     return c.utf8_buf(text_type(c.from_id(type_id.tup_0).__name__))
 
@@ -488,6 +487,10 @@ class TypeId(datatype(['tup_0'])):
   """Corresponds to the native object of the same name."""
 
 
+class OptionalTypeId(datatype(['type_id', 'is_some'])):
+  """Corresponds to the native object of the same name."""
+
+
 class PyResult(datatype(['is_throw', 'handle'])):
   """Corresponds to the native object of the same name."""
 
diff --git a/src/rust/engine/src/externs.rs b/src/rust/engine/src/externs.rs
index f8799fb..818cf2e 100644
--- a/src/rust/engine/src/externs.rs
+++ b/src/rust/engine/src/externs.rs
@@ -26,8 +26,7 @@ pub fn get_type_for(val: &Value) -> TypeId {
 
 pub fn get_union_for(ty: TypeId) -> Option<TypeId> {
   with_externs(|e| match (e.get_union_for)(e.context, ty) {
-    OptionalTypeId::NoTypeId => None,
-    OptionalTypeId::SomeTypeId(type_id) => Some(type_id),
+    OptionalTypeId(type_id, is_some) => if is_some { Some(type_id) } else { None },
   })
 }
 
@@ -448,10 +447,7 @@ impl From<Result<(), String>> for PyResult {
 ///This is basically an Option<TypeId>, but manually-monomorphized
 ///to make it work more smoothly with the Python FFI
 #[repr(C)]
-pub enum OptionalTypeId {
-  NoTypeId,
-  SomeTypeId(TypeId),
-}
+pub struct OptionalTypeId(TypeId, bool);
 
 ///
 /// The response from a call to extern_generator_send. Gets include Idents for their Handles
diff --git a/src/rust/engine/src/nodes.rs b/src/rust/engine/src/nodes.rs
index a73fed2..7217492 100644
--- a/src/rust/engine/src/nodes.rs
+++ b/src/rust/engine/src/nodes.rs
@@ -864,7 +864,8 @@ impl Task {
           .ok_or_else(|| throw(&format!("no edges for task {:?} exist!", entry)))
           .and_then(|edges| {
             edges.entry_for(&dependency_key).cloned().ok_or_else(|| {
-              let declared_type_union = get.declared_subject.and_then(|ty| externs::get_union_for(ty));
+              let declared_type_union = get.declared_subject
+                .and_then(|ty| externs::get_union_for(ty) );
               match declared_type_union {
                 Some(union_ty) => {
                   let description = "foo";

yeah this is really sad but i might use

pub struct OptionalTypeId(TypeId, bool);

as above because it's not immediately clear to me why the

enum

approach is failing

i tried having

native.py

regex-replace the

char a[...]

array for

extern_get_union_for()

in

native_engine.c

by adding a line to the

_hackily_recreate_includes_for_bindings()

method in

native.py

changing the size of any arrays i tried to modify in the generated code did not work

this would possibly be a good issue to make against the cffi project, but would really want to understand just how the enum is laid out in memory (and then whether the hypothesis is correct that it's clobbering the context somehow)

no worries if you want to keep digging, just want to unblock you if that's the case

could also just go with specifying a particular

TypeId

value (0?) to represent the "null" case as @witty-crayon-22786 suggested, which would be "idiomatic" enough if we were writing C. risk of collision seems small / can maybe be avoided entirely since i think we generate the ids ourself somewhere in native.py?

that would also be not as large aka more efficient but like unclear if that's an issue at the FFI right now