Delete historical interruptable support in Wasmtime (#3925)

* Delete historical interruptable support in Wasmtime

This commit removes the `Config::interruptable` configuration along with
the `InterruptHandle` type from the `wasmtime` crate. The original
support for adding interruption to WebAssembly was added pretty early on
in the history of Wasmtime when there was no other method to prevent an
infinite loop from the host. Nowadays, however, there are alternative
methods for interruption such as fuel or epoch-based interruption.

One of the major downsides of `Config::interruptable` is that even when
it's not enabled it forces an atomic swap to happen when entering
WebAssembly code. This technically could be a non-atomic swap if the
configuration option isn't enabled but that produces even more branch-y
code on entry into WebAssembly which is already something we try to
optimize. Calling into WebAssembly is on the order of a dozens of
nanoseconds at this time and an atomic swap, even uncontended, can add
up to 5ns on some platforms.

The main goal of this PR is to remove this atomic swap on entry into
WebAssembly. This is done by removing the `Config::interruptable` field
entirely, moving all existing consumers to epochs instead which are
suitable for the same purposes. This means that the stack overflow check
is no longer entangled with the interruption check and perhaps one day
we could continue to optimize that further as well.

Some consequences of this change are:

* Epochs are now the only method of remote-thread interruption.
* There are no more Wasmtime traps that produces the `Interrupted` trap
  code, although we may wish to move future traps to this so I left it
  in place.
* The C API support for interrupt handles was also removed and bindings
  for epoch methods were added.
* Function-entry checks for interruption are a tiny bit less efficient
  since one check is performed for the stack limit and a second is
  performed for the epoch as opposed to the `Config::interruptable`
  style of bundling the stack limit and the interrupt check in one. It's
  expected though that this is likely to not really be measurable.
* The old `VMInterrupts` structure is renamed to `VMRuntimeLimits`.

crates/environ/src/tunables.rs

@@ -24,14 +24,6 @@ pub struct Tunables {
     /// Whether or not to retain DWARF sections in compiled modules.
     pub parse_wasm_debuginfo: bool,
 
-    /// Whether or not to enable the ability to interrupt wasm code dynamically.
-    ///
-    /// More info can be found about the implementation in
-    /// crates/environ/src/cranelift.rs. Note that you can't interrupt host
-    /// calls and interrupts are implemented through the `VMInterrupts`
-    /// structure, or `InterruptHandle` in the `wasmtime` crate.
-    pub interruptable: bool,
-
     /// Whether or not fuel is enabled for generated code, meaning that fuel
     /// will be consumed every time a wasm instruction is executed.
     pub consume_fuel: bool,
@@ -89,7 +81,6 @@ impl Default for Tunables {
 
             generate_native_debuginfo: false,
             parse_wasm_debuginfo: true,
-            interruptable: false,
             consume_fuel: false,
             epoch_interruption: false,
             static_memory_bound_is_maximum: false,

crates/environ/src/vmoffsets.rs

@@ -4,7 +4,7 @@
 // Currently the `VMContext` allocation by field looks like this:
 //
 // struct VMContext {
-//      interrupts: *const VMInterrupts,
+//      runtime_limits: *const VMRuntimeLimits,
 //      externref_activations_table: *mut VMExternRefActivationsTable,
 //      store: *mut dyn Store,
 //      builtins: *mut VMBuiltinFunctionsArray,
@@ -74,7 +74,7 @@ pub struct VMOffsets<P> {
     pub num_escaped_funcs: u32,
 
     // precalculated offsets of various member fields
-    interrupts: u32,
+    runtime_limits: u32,
     epoch_ptr: u32,
     externref_activations_table: u32,
     store: u32,
@@ -221,7 +221,7 @@ impl<P: PtrSize> VMOffsets<P> {
             store: "jit store state",
             externref_activations_table: "jit host externref state",
             epoch_ptr: "jit current epoch state",
-            interrupts: "jit interrupt state",
+            runtime_limits: "jit runtime limits state",
         }
     }
 }
@@ -239,7 +239,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             num_defined_memories: fields.num_defined_memories,
             num_defined_globals: fields.num_defined_globals,
             num_escaped_funcs: fields.num_escaped_funcs,
-            interrupts: 0,
+            runtime_limits: 0,
             epoch_ptr: 0,
             externref_activations_table: 0,
             store: 0,
@@ -286,7 +286,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
         }
 
         fields! {
-            size(interrupts) = ret.ptr.size(),
+            size(runtime_limits) = ret.ptr.size(),
             size(epoch_ptr) = ret.ptr.size(),
             size(externref_activations_table) = ret.ptr.size(),
             size(store) = ret.ptr.size() * 2,
@@ -483,23 +483,23 @@ impl<P: PtrSize> VMOffsets<P> {
     }
 }
 
-/// Offsets for `VMInterrupts`.
+/// Offsets for `VMRuntimeLimits`.
 impl<P: PtrSize> VMOffsets<P> {
-    /// Return the offset of the `stack_limit` field of `VMInterrupts`
+    /// Return the offset of the `stack_limit` field of `VMRuntimeLimits`
     #[inline]
-    pub fn vminterrupts_stack_limit(&self) -> u8 {
+    pub fn vmruntime_limits_stack_limit(&self) -> u8 {
         0
     }
 
-    /// Return the offset of the `fuel_consumed` field of `VMInterrupts`
+    /// Return the offset of the `fuel_consumed` field of `VMRuntimeLimits`
     #[inline]
-    pub fn vminterrupts_fuel_consumed(&self) -> u8 {
+    pub fn vmruntime_limits_fuel_consumed(&self) -> u8 {
         self.pointer_size()
     }
 
-    /// Return the offset of the `epoch_deadline` field of `VMInterrupts`
+    /// Return the offset of the `epoch_deadline` field of `VMRuntimeLimits`
     #[inline]
-    pub fn vminterupts_epoch_deadline(&self) -> u8 {
+    pub fn vmruntime_limits_epoch_deadline(&self) -> u8 {
         self.pointer_size() + 8 // `stack_limit` is a pointer; `fuel_consumed` is an `i64`
     }
 }
@@ -535,10 +535,10 @@ impl<P: PtrSize> VMOffsets<P> {
 
 /// Offsets for `VMContext`.
 impl<P: PtrSize> VMOffsets<P> {
-    /// Return the offset to the `VMInterrupts` structure
+    /// Return the offset to the `VMRuntimeLimits` structure
     #[inline]
-    pub fn vmctx_interrupts(&self) -> u32 {
-        self.interrupts
+    pub fn vmctx_runtime_limits(&self) -> u32 {
+        self.runtime_limits
     }
 
     /// Return the offset to the `*const AtomicU64` epoch-counter