Delete historical interruptable support in Wasmtime

[alexcrichton]

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.

[github-actions]

Subscribe to Label Action

cc @fitzgen, @peterhuene

Details

This issue or pull request has been labeled: "fuzzing", "wasmtime:api", "wasmtime:c-api", "wasmtime:config", "wasmtime:docs"

Thus the following users have been cc'd because of the following labels:

• fitzgen: fuzzing
• peterhuene: wasmtime:api, wasmtime:c-api

To subscribe or unsubscribe from this label, edit the .github/subscribe-to-label.json configuration file.

Learn more.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

  Details

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
  ensured that this configuration is exercised by the fuzz targets.

  Details

  For example, if you expose a new strategy for allocating the next instance
  slot inside the pooling allocator, you should ensure that at least one of our
  fuzz targets exercises that new strategy.

  Often, all that is required of you is to ensure that there is a knob for this
  configuration option in wasmtime_fuzzing::Config (or one
  of its nested structs).

  Rarely, this may require authoring a new fuzz target to specifically test this
  configuration. See our docs on fuzzing for more details.

• If you are enabling a configuration option by default, make sure that it
  has been fuzzed for at least two weeks before turning it on by default.

---

Details

To modify this label's message, edit the .github/label-messager/wasmtime-config.md file.

To add new label messages or remove existing label messages, edit the
.github/label-messager.json configuration file.

Learn more.

[cfallin]

LGTM -- this is a nice simplification!

[theduke]

I would appreciate some guidance on how to achieve interruption support with async yielding now.

Before 0.35 I could use epoch_deadline_async_yield_and_update and use the InterruptHandle to do a hard abort.

The only idea I came up with is wrapping the call_async future with futures::future::Abortable, which would stop execution on the next epoch yield.

But maybe I'm missing a better solution?

[alexcrichton]

Yes if you're using the epoch support for async yielding then you'll want to use a timeout-like approach to drop the future at an await point (or abortable as you mentioned) to stop execution of wasm altogether.