Add shared memories (#4187)

* Add shared memories

This change adds the ability to use shared memories in Wasmtime when the
[threads proposal] is enabled. Shared memories are annotated as `shared`
in the WebAssembly syntax, e.g., `(memory 1 1 shared)`, and are
protected from concurrent access during `memory.size` and `memory.grow`.

[threads proposal]: https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md

In order to implement this in Wasmtime, there are two main cases to
cover:
    - a program may simply create a shared memory and possibly export it;
    this means that Wasmtime itself must be able to create shared
    memories
    - a user may create a shared memory externally and pass it in as an
    import during instantiation; this is the case when the program
    contains code like `(import "env" "memory" (memory 1 1
    shared))`--this case is handled by a new Wasmtime API
    type--`SharedMemory`

Because of the first case, this change allows any of the current
memory-creation mechanisms to work as-is. Wasmtime can still create
either static or dynamic memories in either on-demand or pooling modes,
and any of these memories can be considered shared. When shared, the
`Memory` runtime container will lock appropriately during `memory.size`
and `memory.grow` operations; since all memories use this container, it
is an ideal place for implementing the locking once and once only.

The second case is covered by the new `SharedMemory` structure. It uses
the same `Mmap` allocation under the hood as non-shared memories, but
allows the user to perform the allocation externally to Wasmtime and
share the memory across threads (via an `Arc`). The pointer address to
the actual memory is carefully wired through and owned by the
`SharedMemory` structure itself. This means that there are differing
views of where to access the pointer (i.e., `VMMemoryDefinition`): for
owned memories (the default), the `VMMemoryDefinition` is stored
directly by the `VMContext`; in the `SharedMemory` case, however, this
`VMContext` must point to this separate structure.

To ensure that the `VMContext` can always point to the correct
`VMMemoryDefinition`, this change alters the `VMContext` structure.
Since a `SharedMemory` owns its own `VMMemoryDefinition`, the
`defined_memories` table in the `VMContext` becomes a sequence of
pointers--in the shared memory case, they point to the
`VMMemoryDefinition` owned by the `SharedMemory` and in the owned memory
case (i.e., not shared) they point to `VMMemoryDefinition`s stored in a
new table, `owned_memories`.

This change adds an additional indirection (through the `*mut
VMMemoryDefinition` pointer) that could add overhead. Using an imported
memory as a proxy, we measured a 1-3% overhead of this approach on the
`pulldown-cmark` benchmark. To avoid this, Cranelift-generated code will
special-case the owned memory access (i.e., load a pointer directly to
the `owned_memories` entry) for `memory.size` so that only
shared memories (and imported memories, as before) incur the indirection
cost.

* review: remove thread feature check

* review: swap wasmtime-types dependency for existing wasmtime-environ use

* review: remove unused VMMemoryUnion

* review: reword cross-engine error message

* review: improve tests

* review: refactor to separate prevent Memory <-> SharedMemory conversion

* review: into_shared_memory -> as_shared_memory

* review: remove commented out code

* review: limit shared min/max to 32 bits

* review: skip imported memories

* review: imported memories are not owned

* review: remove TODO

* review: document unsafe send + sync

* review: add limiter assertion

* review: remove TODO

* review: improve tests

* review: fix doc test

* fix: fixes based on discussion with Alex

This changes several key parts:
 - adds memory indexes to imports and exports
 - makes `VMMemoryDefinition::current_length` an atomic usize

* review: add `Extern::SharedMemory`

* review: remove TODO

* review: atomically load from VMMemoryDescription in JIT-generated code

* review: add test probing the last available memory slot across threads

* fix: move assertion to new location due to rebase

* fix: doc link

* fix: add TODOs to c-api

* fix: broken doc link

* fix: modify pooling allocator messages in tests

* review: make owned_memory_index panic instead of returning an option

* review: clarify calculation of num_owned_memories

* review: move 'use' to top of file

* review: change '*const [u8]' to '*mut [u8]'

* review: remove TODO

* review: avoid hard-coding memory index

* review: remove 'preallocation' parameter from 'Memory::_new'

* fix: component model memory length

* review: check that shared memory plans are static

* review: ignore growth limits for shared memory

* review: improve atomic store comment

* review: add FIXME for memory growth failure

* review: add comment about absence of bounds-checked 'memory.size'

* review: make 'current_length()' doc comment more precise

* review: more comments related to memory.size non-determinism

* review: make 'vmmemory' unreachable for shared memory

* review: move code around

* review: thread plan through to 'wrap()'

* review: disallow shared memory allocation with the pooling allocator

crates/runtime/src/vmcontext.rs

@@ -7,7 +7,9 @@ use std::any::Any;
 use std::cell::UnsafeCell;
 use std::marker;
 use std::ptr::NonNull;
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::u32;
+use wasmtime_environ::DefinedMemoryIndex;
 
 pub const VMCONTEXT_MAGIC: u32 = u32::from_le_bytes(*b"core");
 
@@ -129,6 +131,9 @@ pub struct VMMemoryImport {
 
     /// A pointer to the `VMContext` that owns the memory description.
     pub vmctx: *mut VMContext,
+
+    /// The index of the memory in the containing `vmctx`.
+    pub index: DefinedMemoryIndex,
 }
 
 // Declare that this type is send/sync, it's the responsibility of users of
@@ -205,14 +210,41 @@ mod test_vmglobal_import {
 /// The fields compiled code needs to access to utilize a WebAssembly linear
 /// memory defined within the instance, namely the start address and the
 /// size in bytes.
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug)]
 #[repr(C)]
 pub struct VMMemoryDefinition {
     /// The start address.
     pub base: *mut u8,
 
     /// The current logical size of this linear memory in bytes.
-    pub current_length: usize,
+    ///
+    /// This is atomic because shared memories must be able to grow their length
+    /// atomically. For relaxed access, see
+    /// [`VMMemoryDefinition::current_length()`].
+    pub current_length: AtomicUsize,
+}
+
+impl VMMemoryDefinition {
+    /// Return the current length of the [`VMMemoryDefinition`] by performing a
+    /// relaxed load; do not use this function for situations in which a precise
+    /// length is needed. Owned memories (i.e., non-shared) will always return a
+    /// precise result (since no concurrent modification is possible) but shared
+    /// memories may see an imprecise value--a `current_length` potentially
+    /// smaller than what some other thread observes. Since Wasm memory only
+    /// grows, this under-estimation may be acceptable in certain cases.
+    pub fn current_length(&self) -> usize {
+        self.current_length.load(Ordering::Relaxed)
+    }
+
+    /// Return a copy of the [`VMMemoryDefinition`] using the relaxed value of
+    /// `current_length`; see [`VMMemoryDefinition::current_length()`].
+    pub unsafe fn load(ptr: *mut Self) -> Self {
+        let other = &*ptr;
+        VMMemoryDefinition {
+            base: other.base,
+            current_length: other.current_length().into(),
+        }
+    }
 }
 
 #[cfg(test)]