Reserve handle index 0 in the component model

This commit updates the allocation scheme for resources in the component
model to start at 1 instead of 0 when communicating with components.
This is an implementation of WebAssembly/component-model#284.

While this broke a number of tests we have this shouldn't actually break
any components in practice. The broken tests were all overly-precise in
their assertions and error messages and this shouldn't idiomatically
come up in any guest language, so this should not be a practically
breaking change.

This change additionally places an upper limit on the maximum
allocatable index at `1 << 30` which is also specified in the above PR.

crates/runtime/src/component.rs

@@ -531,7 +531,7 @@ impl ComponentInstance {
 
     /// Implementation of the `resource.new` intrinsic for `i32`
     /// representations.
-    pub fn resource_new32(&mut self, resource: TypeResourceTableIndex, rep: u32) -> u32 {
+    pub fn resource_new32(&mut self, resource: TypeResourceTableIndex, rep: u32) -> Result<u32> {
         self.resource_tables().resource_new(Some(resource), rep)
     }
 
@@ -600,7 +600,7 @@ impl ComponentInstance {
     ) -> Result<u32> {
         let mut tables = self.resource_tables();
         let rep = tables.resource_lift_own(Some(src), idx)?;
-        Ok(tables.resource_lower_own(Some(dst), rep))
+        tables.resource_lower_own(Some(dst), rep)
     }
 
     pub(crate) fn resource_transfer_borrow(
@@ -623,7 +623,7 @@ impl ComponentInstance {
         if dst_owns_resource {
             return Ok(rep);
         }
-        Ok(tables.resource_lower_borrow(Some(dst), rep))
+        tables.resource_lower_borrow(Some(dst), rep)
     }
 
     pub(crate) fn resource_enter_call(&mut self) {

crates/runtime/src/component/libcalls.rs

@@ -519,9 +519,7 @@ fn inflate_latin1_bytes(dst: &mut [u16], latin1_bytes_so_far: usize) -> &mut [u1
 
 unsafe fn resource_new32(vmctx: *mut VMComponentContext, resource: u32, rep: u32) -> Result<u32> {
     let resource = TypeResourceTableIndex::from_u32(resource);
-    Ok(ComponentInstance::from_vmctx(vmctx, |instance| {
-        instance.resource_new32(resource, rep)
-    }))
+    ComponentInstance::from_vmctx(vmctx, |instance| instance.resource_new32(resource, rep))
 }
 
 unsafe fn resource_rep32(vmctx: *mut VMComponentContext, resource: u32, idx: u32) -> Result<u32> {

crates/runtime/src/component/resources.rs

@@ -28,6 +28,11 @@ use std::mem;
 use wasmtime_environ::component::TypeResourceTableIndex;
 use wasmtime_environ::PrimaryMap;
 
+/// The maximum handle value is specified in
+/// <https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md>
+/// currently and keeps the upper bit free for use in the component.
+const MAX_RESOURCE_HANDLE: u32 = 1 << 30;
+
 /// Contextual state necessary to perform resource-related operations.
 ///
 /// This state a bit odd since it has a few optional bits, but the idea is that
@@ -125,7 +130,7 @@ impl ResourceTables<'_> {
     /// Implementation of the `resource.new` canonical intrinsic.
     ///
     /// Note that this is the same as `resource_lower_own`.
-    pub fn resource_new(&mut self, ty: Option<TypeResourceTableIndex>, rep: u32) -> u32 {
+    pub fn resource_new(&mut self, ty: Option<TypeResourceTableIndex>, rep: u32) -> Result<u32> {
         self.table(ty).insert(Slot::Own { rep, lend_count: 0 })
     }
 
@@ -173,7 +178,11 @@ impl ResourceTables<'_> {
     /// the same as `resource_new` implementation-wise.
     ///
     /// This is an implementation of the canonical ABI `lower_own` function.
-    pub fn resource_lower_own(&mut self, ty: Option<TypeResourceTableIndex>, rep: u32) -> u32 {
+    pub fn resource_lower_own(
+        &mut self,
+        ty: Option<TypeResourceTableIndex>,
+        rep: u32,
+    ) -> Result<u32> {
         self.table(ty).insert(Slot::Own { rep, lend_count: 0 })
     }
 
@@ -237,7 +246,11 @@ impl ResourceTables<'_> {
     /// function. The other half of this implementation is located on
     /// `VMComponentContext` which handles the special case of avoiding borrow
     /// tracking entirely.
-    pub fn resource_lower_borrow(&mut self, ty: Option<TypeResourceTableIndex>, rep: u32) -> u32 {
+    pub fn resource_lower_borrow(
+        &mut self,
+        ty: Option<TypeResourceTableIndex>,
+        rep: u32,
+    ) -> Result<u32> {
         let scope = self.calls.scopes.len() - 1;
         let borrow_count = &mut self.calls.scopes.last_mut().unwrap().borrow_count;
         *borrow_count = borrow_count.checked_add(1).unwrap();
@@ -278,12 +291,8 @@ impl ResourceTables<'_> {
 }
 
 impl ResourceTable {
-    fn next(&self) -> usize {
-        self.next as usize
-    }
-
-    fn insert(&mut self, new: Slot) -> u32 {
-        let next = self.next();
+    fn insert(&mut self, new: Slot) -> Result<u32> {
+        let next = self.next as usize;
         if next == self.slots.len() {
             self.slots.push(Slot::Free {
                 next: self.next.checked_add(1).unwrap(),
@@ -294,36 +303,49 @@ impl ResourceTable {
             Slot::Free { next } => next,
             _ => unreachable!(),
         };
-        u32::try_from(ret).unwrap()
+
+        // The component model reserves index 0 as never allocatable so add one
+        // to the table index to start the numbering at 1 instead. Also note
+        // that the component model places an upper-limit per-table on the
+        // maximum allowed index.
+        let ret = ret + 1;
+        if ret >= MAX_RESOURCE_HANDLE {
+            bail!("cannot allocate another handle: index overflow");
+        }
+        Ok(ret)
+    }
+
+    fn handle_index_to_table_index(&self, idx: u32) -> Option<usize> {
+        // NB: `idx` is decremented by one to account for the `+1` above during
+        // allocation.
+        let idx = idx.checked_sub(1)?;
+        usize::try_from(idx).ok()
     }
 
     fn rep(&self, idx: u32) -> Result<u32> {
-        match usize::try_from(idx).ok().and_then(|i| self.slots.get(i)) {
+        let slot = self
+            .handle_index_to_table_index(idx)
+            .and_then(|i| self.slots.get(i));
+        match slot {
             None | Some(Slot::Free { .. }) => bail!("unknown handle index {idx}"),
             Some(Slot::Own { rep, .. } | Slot::Borrow { rep, .. }) => Ok(*rep),
         }
     }
 
     fn get_mut(&mut self, idx: u32) -> Result<&mut Slot> {
-        match usize::try_from(idx)
-            .ok()
-            .and_then(|i| self.slots.get_mut(i))
-        {
+        let slot = self
+            .handle_index_to_table_index(idx)
+            .and_then(|i| self.slots.get_mut(i));
+        match slot {
             None | Some(Slot::Free { .. }) => bail!("unknown handle index {idx}"),
             Some(other) => Ok(other),
         }
     }
 
     fn remove(&mut self, idx: u32) -> Result<Slot> {
-        match usize::try_from(idx).ok().and_then(|i| self.slots.get(i)) {
-            Some(Slot::Own { .. }) | Some(Slot::Borrow { .. }) => {}
-            _ => bail!("unknown handle index {idx}"),
-        };
-        let ret = mem::replace(
-            &mut self.slots[idx as usize],
-            Slot::Free { next: self.next },
-        );
-        self.next = idx;
+        let to_fill = Slot::Free { next: self.next };
+        let ret = mem::replace(self.get_mut(idx)?, to_fill);
+        self.next = idx - 1;
         Ok(ret)
     }
 }

crates/wasmtime/src/runtime/component/func/options.rs

@@ -289,13 +289,21 @@ impl<'a, T> LowerContext<'a, T> {
     /// into a guest-local index.
     ///
     /// The `ty` provided is which table to put this into.
-    pub fn guest_resource_lower_own(&mut self, ty: TypeResourceTableIndex, rep: u32) -> u32 {
+    pub fn guest_resource_lower_own(
+        &mut self,
+        ty: TypeResourceTableIndex,
+        rep: u32,
+    ) -> Result<u32> {
         self.resource_tables().guest_resource_lower_own(rep, ty)
     }
 
     /// Lowers a `borrow` resource into the guest, converting the `rep` to a
     /// guest-local index in the `ty` table specified.
-    pub fn guest_resource_lower_borrow(&mut self, ty: TypeResourceTableIndex, rep: u32) -> u32 {
+    pub fn guest_resource_lower_borrow(
+        &mut self,
+        ty: TypeResourceTableIndex,
+        rep: u32,
+    ) -> Result<u32> {
         // Implement `lower_borrow`'s special case here where if a borrow is
         // inserted into a table owned by the instance which implemented the
         // original resource then no borrow tracking is employed and instead the
@@ -308,7 +316,7 @@ impl<'a, T> LowerContext<'a, T> {
         // Note that the unsafety here should be valid given the contract of
         // `LowerContext::new`.
         if unsafe { (*self.instance).resource_owned_by_own_instance(ty) } {
-            return rep;
+            return Ok(rep);
         }
         self.resource_tables().guest_resource_lower_borrow(rep, ty)
     }
@@ -330,7 +338,7 @@ impl<'a, T> LowerContext<'a, T> {
     ///
     /// Note that this is a special case for `Resource<T>`. Most of the time a
     /// host value shouldn't be lowered with a lowering context.
-    pub fn host_resource_lower_own(&mut self, rep: u32) -> HostResourceIndex {
+    pub fn host_resource_lower_own(&mut self, rep: u32) -> Result<HostResourceIndex> {
         self.resource_tables().host_resource_lower_own(rep)
     }
 
@@ -483,13 +491,13 @@ impl<'a> LiftContext<'a> {
 
     /// Lowers a resource into the host-owned table, returning the index it was
     /// inserted at.
-    pub fn host_resource_lower_own(&mut self, rep: u32) -> HostResourceIndex {
+    pub fn host_resource_lower_own(&mut self, rep: u32) -> Result<HostResourceIndex> {
         self.resource_tables().host_resource_lower_own(rep)
     }
 
     /// Lowers a resource into the host-owned table, returning the index it was
     /// inserted at.
-    pub fn host_resource_lower_borrow(&mut self, rep: u32) -> HostResourceIndex {
+    pub fn host_resource_lower_borrow(&mut self, rep: u32) -> Result<HostResourceIndex> {
         self.resource_tables().host_resource_lower_borrow(rep)
     }
 

crates/wasmtime/src/runtime/component/resources.rs

@@ -397,15 +397,15 @@ impl<'a> HostResourceTables<'a> {
     /// will point to the `rep` specified as well as recording that it has the
     /// `ty` specified. The returned index is suitable for conversion into
     /// either [`Resource`] or [`ResourceAny`].
-    pub fn host_resource_lower_own(&mut self, rep: u32) -> HostResourceIndex {
-        let idx = self.tables.resource_lower_own(None, rep);
-        self.new_host_index(idx)
+    pub fn host_resource_lower_own(&mut self, rep: u32) -> Result<HostResourceIndex> {
+        let idx = self.tables.resource_lower_own(None, rep)?;
+        Ok(self.new_host_index(idx))
     }
 
     /// See [`HostResourceTables::host_resource_lower_own`].
-    pub fn host_resource_lower_borrow(&mut self, rep: u32) -> HostResourceIndex {
-        let idx = self.tables.resource_lower_borrow(None, rep);
-        self.new_host_index(idx)
+    pub fn host_resource_lower_borrow(&mut self, rep: u32) -> Result<HostResourceIndex> {
+        let idx = self.tables.resource_lower_borrow(None, rep)?;
+        Ok(self.new_host_index(idx))
     }
 
     /// Validates that `idx` is still valid for the host tables, notably
@@ -453,6 +453,12 @@ impl<'a> HostResourceTables<'a> {
         match list.get_mut(idx as usize) {
             Some(slot) => *slot = gen,
             None => {
+                // Resource handles start at 1, not zero, so push two elements
+                // for the first resource handle.
+                if list.is_empty() {
+                    assert_eq!(idx, 1);
+                    list.push(0);
+                }
                 assert_eq!(idx as usize, list.len());
                 list.push(gen);
             }
@@ -482,7 +488,11 @@ impl<'a> HostResourceTables<'a> {
     /// into a guest-local index.
     ///
     /// The `ty` provided is which table to put this into.
-    pub fn guest_resource_lower_own(&mut self, rep: u32, ty: TypeResourceTableIndex) -> u32 {
+    pub fn guest_resource_lower_own(
+        &mut self,
+        rep: u32,
+        ty: TypeResourceTableIndex,
+    ) -> Result<u32> {
         self.tables.resource_lower_own(Some(ty), rep)
     }
 
@@ -495,7 +505,11 @@ impl<'a> HostResourceTables<'a> {
     /// into a guest has a special case where `rep` is returned directly if `ty`
     /// belongs to the component being lowered into. That property must be
     /// handled by the caller of this function.
-    pub fn guest_resource_lower_borrow(&mut self, rep: u32, ty: TypeResourceTableIndex) -> u32 {
+    pub fn guest_resource_lower_borrow(
+        &mut self,
+        rep: u32,
+        ty: TypeResourceTableIndex,
+    ) -> Result<u32> {
         self.tables.resource_lower_borrow(Some(ty), rep)
     }
 
@@ -612,7 +626,7 @@ where
                     // can move the rep into the guest table.
                     ResourceState::Index(idx) => cx.host_resource_lift_own(idx)?,
                 };
-                Ok(cx.guest_resource_lower_own(t, rep))
+                cx.guest_resource_lower_own(t, rep)
             }
             InterfaceType::Borrow(t) => {
                 let rep = match self.state.get() {
@@ -632,7 +646,7 @@ where
                     //
                     // Afterwards this is the same as the `idx` case below.
                     ResourceState::NotInTable => {
-                        let idx = cx.host_resource_lower_own(self.rep);
+                        let idx = cx.host_resource_lower_own(self.rep)?;
                         let prev = self.state.swap(ResourceState::Index(idx));
                         assert_eq!(prev, ResourceState::NotInTable);
                         cx.host_resource_lift_borrow(idx)?
@@ -642,7 +656,7 @@ where
                     // out of the table with borrow-tracking employed.
                     ResourceState::Index(idx) => cx.host_resource_lift_borrow(idx)?,
                 };
-                Ok(cx.guest_resource_lower_borrow(t, rep))
+                cx.guest_resource_lower_borrow(t, rep)
             }
             _ => bad_type_info(),
         }
@@ -879,9 +893,9 @@ impl ResourceAny {
 
         let mut tables = HostResourceTables::new_host(store.0);
         let (idx, own_state) = match state.get() {
-            ResourceState::Borrow => (tables.host_resource_lower_borrow(rep), None),
+            ResourceState::Borrow => (tables.host_resource_lower_borrow(rep)?, None),
             ResourceState::NotInTable => {
-                let idx = tables.host_resource_lower_own(rep);
+                let idx = tables.host_resource_lower_own(rep)?;
                 (
                     idx,
                     Some(OwnState {
@@ -1025,14 +1039,14 @@ impl ResourceAny {
                     bail!("mismatched resource types");
                 }
                 let rep = cx.host_resource_lift_own(self.idx)?;
-                Ok(cx.guest_resource_lower_own(t, rep))
+                cx.guest_resource_lower_own(t, rep)
             }
             InterfaceType::Borrow(t) => {
                 if cx.resource_type(t) != self.ty {
                     bail!("mismatched resource types");
                 }
                 let rep = cx.host_resource_lift_borrow(self.idx)?;
-                Ok(cx.guest_resource_lower_borrow(t, rep))
+                cx.guest_resource_lower_borrow(t, rep)
             }
             _ => bad_type_info(),
         }
@@ -1043,7 +1057,7 @@ impl ResourceAny {
             InterfaceType::Own(t) => {
                 let ty = cx.resource_type(t);
                 let (rep, dtor, flags) = cx.guest_resource_lift_own(t, index)?;
-                let idx = cx.host_resource_lower_own(rep);
+                let idx = cx.host_resource_lower_own(rep)?;
                 Ok(ResourceAny {
                     idx,
                     ty,
@@ -1057,7 +1071,7 @@ impl ResourceAny {
             InterfaceType::Borrow(t) => {
                 let ty = cx.resource_type(t);
                 let rep = cx.guest_resource_lift_borrow(t, index)?;
-                let idx = cx.host_resource_lower_borrow(rep);
+                let idx = cx.host_resource_lower_borrow(rep)?;
                 Ok(ResourceAny {
                     idx,
                     ty,

tests/all/component_model/resources.rs

@@ -233,7 +233,7 @@ fn mismatch_intrinsics() -> Result<()> {
     let ctor = i.get_typed_func::<(u32,), (ResourceAny,)>(&mut store, "ctor")?;
     assert_eq!(
         ctor.call(&mut store, (100,)).unwrap_err().to_string(),
-        "unknown handle index 0"
+        "unknown handle index 1"
     );
 
     Ok(())
@@ -371,7 +371,7 @@ fn drop_guest_twice() -> Result<()> {
 
     assert_eq!(
         dtor.call(&mut store, (&t,)).unwrap_err().to_string(),
-        "unknown handle index 0"
+        "unknown handle index 1"
     );
 
     Ok(())
@@ -1250,7 +1250,7 @@ fn pass_guest_back_as_borrow() -> Result<()> {
 
     // Should not be valid to use `resource` again
     let err = take.call(&mut store, (&resource,)).unwrap_err();
-    assert_eq!(err.to_string(), "unknown handle index 0");
+    assert_eq!(err.to_string(), "unknown handle index 1");
 
     Ok(())
 }
@@ -1412,9 +1412,9 @@ fn guest_different_host_same() -> Result<()> {
                     (import "" "drop2" (func $drop2 (param i32)))
 
                     (func (export "f") (param i32 i32)
-                        ;; separate tables both have initial index of 0
-                        (if (i32.ne (local.get 0) (i32.const 0)) (then (unreachable)))
-                        (if (i32.ne (local.get 1) (i32.const 0)) (then (unreachable)))
+                        ;; separate tables both have initial index of 1
+                        (if (i32.ne (local.get 0) (i32.const 1)) (then (unreachable)))
+                        (if (i32.ne (local.get 1) (i32.const 1)) (then (unreachable)))
 
                         ;; host should end up getting the same resource
                         (call $f (local.get 0) (local.get 1))