Refine recursion check to handle child import/export forwarding

design/mvp/CanonicalABI.md

@@ -130,8 +130,11 @@ class Store:
   def __init__(self):
     self.pending = []
 
-  def invoke(self, f: FuncInst, caller, on_start, on_resolve) -> Call:
-    return f(caller, on_start, on_resolve)
+  def invoke(self, f: FuncInst, caller: Optional[Supertask], on_start, on_resolve) -> Call:
+    host_caller = Supertask()
+    host_caller.inst = None
+    host_caller.supertask = caller
+    return f(host_caller, on_start, on_resolve)
 
   def tick(self):
     random.shuffle(self.pending)
@@ -167,7 +170,7 @@ OnStart = Callable[[], list[any]]
 OnResolve = Callable[[Optional[list[any]]], None]
 
 class Supertask:
-  inst: ComponentInstance
+  inst: Optional[ComponentInstance]
   supertask: Optional[Supertask]
 
 class Call:
@@ -190,6 +193,14 @@ However, as described in the [concurrency explainer], an async call's
 (currently) that the caller can know or do about it (hence there are
 currently no other methods on `Call`).
 
+The optional `Supertask.inst` field either points to the `ComponentInstance`
+containing the supertask or, if `None`, indicates that the supertask is a host
+function. Because `Store.invoke` unconditionally appends a host `Supertask`,
+every callstack is rooted by a host `Supertask`. There is no prohibition on
+component-to-host-to-component calls (as long as the conditions checked by
+`call_might_be_recursive` are satisfied) and thus host `Supertask`s may also
+appear anywhere else in the callstack.
+
 
 ## Supporting definitions
 
@@ -280,24 +291,147 @@ behavior and enforce invariants.
 ```python
 class ComponentInstance:
   store: Store
+  parent: Optional[ComponentInstance]
   table: Table
   may_leave: bool
   backpressure: int
   exclusive: bool
   num_waiting_to_enter: int
 
-  def __init__(self, store):
+  def __init__(self, store, parent = None):
+    assert(parent is None or parent.store is store)
     self.store = store
+    self.parent = parent
     self.table = Table()
     self.may_leave = True
     self.backpressure = 0
     self.exclusive = False
     self.num_waiting_to_enter = 0
 ```
 Components are always instantiated in the context of a `Store` which is saved
-immutably in the `store` field. The other fields are described below as they
-are used.
+immutably in the `store` field.
+
+If a component is instantiated by an `instantiate` expression in a "parent"
+component, the parent's `ComponentInstance` is immutably saved in the `parent`
+field of the child's `ComponentInstance`. If instead a component is
+instantiated directly by the host, the `parent` field is `None`. Thus, the set
+of component instances in a store forms a forest rooted by the component
+instances that were instantiated directly by the host.
+
+Based on this, the "reflexive ancestors" of a component (i.e., all parent
+component instances up to the root component including the component itself) can
+be enumerated and tested via these two helper functions:
+```python
+  def reflexive_ancestors(self) -> set[ComponentInstance]:
+    s = set()
+    inst = self
+    while inst is not None:
+      s.add(inst)
+      inst = inst.parent
+    return s
+
+  def is_reflexive_ancestor_of(self, other):
+    while other is not None:
+      if self is other:
+        return True
+      other = other.parent
+    return False
+```
 
+How the host instantiates and invokes root components is up to the host and not
+specified by the Component Model. Exports of previously-instantiated root
+components *may* be supplied as the imports of subsequently-instantiated root
+components. Due to the ordered nature of instantiation, root components cannot
+directly import each others' exports in cyclic manner. However, the host *may*
+attempt to perform cyclic component-to-host-to-component calls using host
+powers.
+
+Because a child component is fully encapsulated by its parent component (with
+all child imports specified by the parent's `instantiate` expression and access
+to all child exports controlled by the parent through its private instance index
+space), the host does not have direct control over how a child component is
+instantiated or invoked. However, if a child's ancestors transitively forward
+the root component's host-supplied imports to the child, direct child-to-host
+calls are possible. Symmetrically, if a child's ancestors transitively
+re-export the child's exports from the root component, direct host-to-child
+calls are possible.
+
+Recursive component calls are technically possible using either host powers (as
+mentioned above) or via a parent component lowering a child component's export
+to a `funcref` and then recursively calling this `funcref` from a lifted parent
+function passed as an import to the child. However, for the time being, both
+cases are prevented via trap for several reasons:
+* automatic [backpressure] would otherwise deadlock in unpredictable and
+  surprising ways;
+* by default, most code does not expect [recursive reentrance] and will break
+  in subtle and potentially security sensitive ways if allowed;
+* to properly handle recursive reentrance, an extra ABI parameter is required
+  to link recursive calls and this requires opting in via some
+  [TBD](Concurrency.md#TODO) function effect type or canonical ABI option.
+
+The `call_might_be_recursive` predicate is used by `canon_lift` and
+`canon_resource_drop` (defined below) to conservatively detect recursive
+reentrance and subsequently trap.
+```python
+def call_might_be_recursive(caller: Supertask, callee_inst: ComponentInstance):
+  if caller.inst is None:
+    while caller is not None:
+      if caller.inst and caller.inst.reflexive_ancestors() & callee_inst.reflexive_ancestors():
+        return True
+      caller = caller.supertask
+    return False
+  else:
+    return (caller.inst.is_reflexive_ancestor_of(callee_inst) or
+            callee_inst.is_reflexive_ancestor_of(caller.inst))
+```
+The first case (where `caller.inst` is `None`) covers host-to-component calls.
+By testing whether any of the callers' reflexive anecestor sets intersect the
+callee's ancestor set, the following case is considered recursive:
+```
+     +-------+
+     |   A   |<-.
+     | +---+ |  |
+host-->| B |-->host
+     | +---+ |
+     +-------+
+```
+Here, when attempting to recursively call back into `A`, `caller` points to the
+following stack:
+```
+|inst=None| --supertask--> |inst=B| --supertask--> |inst=None| --supertask--> None
+```
+while `A` does not appear as the `inst` of any `Supertask` on this stack,
+`B.reflexive_ancestors()` is `{ B, A }`, so the loop correctly determines that
+`A` is being reentered. This ensures that child components are kept an
+encapsulated detail of the parent.
+
+The second case (where `caller.inst` is not `None`) covers component-to-
+component calls by conservatively rejecting any call from a component to its
+anecestor or descendant (thereby preventing any possible recursion via ancestor
+`funcref`). Thus, the following sibling-to-sibling component call is allowed:
+```
+     +----------------+
+     |      P         |
+     | +----+  +----+ |
+host-->| C1 |->| C2 | |
+     | +----+  +----+ |
+     +----------------+
+```
+while the following child-to-parent and parent-to-child calls are disallowed:
+```
+     +----------+        +----------+
+     | +---+    |        |    +---+ |
+host-->| C |->P |  host->| P->| C | |
+     | +---+    |        |    +---+ |
+     +----------+        +----------+
+```
+This conservative approximation allows `call_might_be_recursive` to be computed
+ahead-of-time when compiling a fused component-to-component adapter (where both
+caller and callee intances and their relationship are statically known). In the
+future this check will be relaxed and more sophisticated optimizations can be
+used to statically eliminate the check in common cases.
+
+The other fields of `ComponentInstance` are described below as they are used.
 
 #### Table State
 
@@ -804,7 +938,7 @@ class Task(Call, Supertask):
   opts: CanonicalOptions
   inst: ComponentInstance
   ft: FuncType
-  supertask: Optional[Task]
+  supertask: Supertask
   on_resolve: OnResolve
   num_borrows: int
   threads: list[Thread]
@@ -838,37 +972,6 @@ called (by the `Task.return_` and `Task.cancel` methods, defined below).
       assert(self.num_borrows == 0)
 ```
 
-The `Task.trap_if_on_the_stack` method checks for unintended reentrance,
-enforcing a [component invariant]. This guard uses the `Supertask` defined by
-the [Embedding](#embedding) interface to walk up the async call tree defined as
-part of [structured concurrency]. The async call tree is necessary to
-distinguish between the deadlock-hazardous kind of reentrance (where the new
-task is a transitive subtask of a task already running in the same component
-instance) and the normal kind of async reentrance (where the new task is just a
-sibling of any existing tasks running in the component instance). Note that, in
-the [future](Concurrency.md#TODO), there will be a way for a function to opt in
-(via function type attribute) to the hazardous kind of reentrance, which will
-nuance this test.
-```python
-  def trap_if_on_the_stack(self, inst):
-    c = self.supertask
-    while c is not None:
-      trap_if(c.inst is inst)
-      c = c.supertask
-```
-An optimizing implementation can avoid the O(n) loop in `trap_if_on_the_stack`
-in several ways:
-* Reentrance by a child component can (often) be statically ruled out when the
-  parent component doesn't both lift and lower the child's imports and exports
-  (i.e., "donut wrapping").
-* Reentrance of the root component by the host can either be asserted not to
-  happen or be tracked in a per-root-component-instance flag.
-* When a potentially-reenterable child component only lifts and lowers
-  synchronously, reentrance can be tracked in a per-component-instance flag.
-* For the remaining cases, the live instances on the stack can be maintained in
-  a packed bit-vector (assigning each potentially-reenterable async component
-  instance a static bit position) that is passed by copy from caller to callee.
-
 The `Task.needs_exclusive` predicate returns whether the Canonical ABI options
 indicate that the core wasm being executed does not expect to be reentered
 (e.g., because the code is using a single global linear memory shadow stack).
@@ -3132,8 +3235,8 @@ Based on this, `canon_lift` is defined in chunks as follows, starting with how
 a `lift`ed function starts executing:
 ```python
 def canon_lift(opts, inst, ft, callee, caller, on_start, on_resolve) -> Call:
+  trap_if(call_might_be_recursive(caller, inst))
   task = Task(opts, inst, ft, caller, on_resolve)
-  task.trap_if_on_the_stack(inst)
   def thread_func(thread):
     if not task.enter(thread):
       return
@@ -3147,16 +3250,16 @@ def canon_lift(opts, inst, ft, callee, caller, on_start, on_resolve) -> Call:
     flat_ft = flatten_functype(opts, ft, 'lift')
     assert(types_match_values(flat_ft.params, flat_args))
 ```
-Each call starts by immediately checking for unexpected reentrance using
-`Task.trap_if_on_the_stack`.
+Each lifted function call starts by immediately trapping on possible recursive
+reentrance (as defined by `call_might_be_recursive` above).
 
 The `thread_func` is immediately called from a new `Thread` created and resumed
-at the end of `canon_lift` and so control flow proceeds directly from the
-`trap_if_on_stack` to the `enter`. `Task.enter` (defined above) suspends the
-newly-created `Thread` if there is backpressure until the backpressure is
-resolved. If the caller cancels the new `Task` while the `Task` is still
-waiting to `enter`, the call is aborted before the arguments are lowered (which
-means that owned-handle arguments are not transferred).
+at the end of `canon_lift` and so control flow proceeds directly to the `enter`.
+`Task.enter` (defined above) suspends the newly-created `Thread` if there is
+backpressure until the backpressure is resolved. If the caller cancels the new
+`Task` while the `Task` is still waiting to `enter`, the call is aborted before
+the arguments are lowered (which means that owned-handle arguments are not
+transferred).
 
 Once the backpressure gate is cleared, the `Thread` is added to the callee's
 component instance's table (storing the index for later retrieval by the
@@ -3541,7 +3644,7 @@ def canon_resource_drop(rt, thread, i):
         callee = partial(canon_lift, callee_opts, rt.impl, ft, rt.dtor)
         [] = canon_lower(caller_opts, ft, callee, thread, [h.rep])
       else:
-        thread.task.trap_if_on_the_stack(rt.impl)
+        trap_if(call_might_be_recursive(thread.task, rt.impl))
   else:
     h.borrow_scope.num_borrows -= 1
   return []
@@ -3558,9 +3661,9 @@ reentrance guard of `Task.enter`, an exception is made when the resource type's
 implementation-instance is the same as the current instance (which is
 statically known for any given `canon resource.drop`).
 
-When a destructor isn't present, the rules still perform a reentrance check
+When a destructor isn't present, there is still a trap on recursive reentrance
 since this is the caller's responsibility and the presence or absence of a
-destructor is an encapsualted implementation detail of the resource type.
+destructor is an encapsulated implementation detail of the resource type.
 
 
 ### `canon resource.rep`
@@ -4780,6 +4883,7 @@ def canon_thread_available_parallelism():
 [Concurrency Explainer]: Concurrency.md
 [Suspended]: Concurrency#thread-built-ins
 [Structured Concurrency]: Concurrency.md#subtasks-and-supertasks
+[Recursive Reentrance]: Concurrency.md#subtasks-and-supertasks
 [Backpressure]: Concurrency.md#backpressure
 [Current Thread]: Concurrency.md#current-thread-and-task
 [Current Task]: Concurrency.md#current-thread-and-task

design/mvp/Explainer.md

@@ -2881,7 +2881,7 @@ three runtime invariants:
    component instance.
 2. The Component Model disallows reentrance by trapping if a callee's
    component-instance is already on the stack when the call starts.
-   (For details, see [`trap_if_on_the_stack`](CanonicalABI.md#task-state)
+   (For details, see [`call_might_be_recursive`](CanonicalABI.md#component-instance-state)
    in the Canonical ABI explainer.) This default prevents obscure
    composition-time bugs and also enables more-efficient non-reentrant
    runtime glue code. This rule will be relaxed by an opt-in