Replace signals-based-traps with auto-detection

[alexcrichton]

This commit refactors the platform support of the wasmtime crate itself to remove the previously added signals-based-traps feature in favor of auto-detecting whether it's there or not. The build.rs script for the wasmtime crate will now detect the target platform and auto-enable this feature as necessary.

The signals-based-traps cargo feature is removed and split into two custom #[cfg] directives that the build script sets:

• has_virtual_memory - this is used to gate mmap implementations for example. This is enabled on unix || windows and will be off for no_std targets for example. This is split out of "signals-based-traps" to better handle platforms like iOS which have virtual memory but don't execute native code (removing the need for native signals).

• has_native_signals - gates signal handlers on Unix for example. This is disabled on MIRI but otherwise enabled for unix || windows. This is intended to in the future get disabled for iOS by default for example since it's not necessary when using Pulley. This is additionally off-by-default for no_std platforms.

Two new crate features were added for no_std or "custom" platforms to opt-in to the wasmtime-platform.h C APIs for implementing virtual memory and signals. These are used in the min-platform embedding example.

This commit additionally updates some various documentation here and there to be more up-to-date.

[alexcrichton]

One of the rough high-level goals here is that in conjunction with #9837 I hope to disable has_native_signals on Pulley-only platforms in the near future to statically remove all that code. That should make Wasmtime more portable by default since when there's no Cranelift backend there's no need to install signal handlers which often has lots of platform/architecture-specific implications.

[github-actions]

Subscribe to Label Action

cc @fitzgen

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

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

• fitzgen: fuzzing

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.

  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.

  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.

---

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.

[bjorn3]

wasm32-unknown-emscripten is marked as unix, but has neither signals, nor virtual memory. I guess it is not all too important that Wasmtime works on Emscripten though.

[alexcrichton]

That's a good point yeah, although this unix || windows logic matches what's currently in the sys module so we technically already didn't support Emscripten. It should be reasonable though to add support here though in the future.