Option for host managed memory (#1400)

* Option for host managed memory

* Rename Allocator to MemoryCreator

* Create LinearMemory and MemoryCreator traits in api

* Leave only one as_ptr function in LinearMemory trait

* Memory creator test

* Update comments/docs for LinearMemory and MemoryCreator traits

* Add guard page to the custom memory example

* Remove mut from LinearMemory trait as_ptr

* Host_memory_grow test

crates/runtime/src/memory.rs

@@ -1,6 +1,6 @@
 //! Memory management for linear memories.
 //!
-//! `LinearMemory` is to WebAssembly linear memories what `Table` is to WebAssembly tables.
+//! `RuntimeLinearMemory` is to WebAssembly linear memories what `Table` is to WebAssembly tables.
 
 use crate::mmap::Mmap;
 use crate::vmcontext::VMMemoryDefinition;
@@ -9,9 +9,40 @@ use std::cell::RefCell;
 use std::convert::TryFrom;
 use wasmtime_environ::{MemoryPlan, MemoryStyle, WASM_MAX_PAGES, WASM_PAGE_SIZE};
 
+/// A memory allocator
+pub trait RuntimeMemoryCreator: Send + Sync {
+    /// Create new RuntimeLinearMemory
+    fn new_memory(&self, plan: &MemoryPlan) -> Result<Box<dyn RuntimeLinearMemory>, String>;
+}
+
+/// A default memory allocator used by Wasmtime
+pub struct DefaultMemoryCreator;
+
+impl RuntimeMemoryCreator for DefaultMemoryCreator {
+    /// Create new MmapMemory
+    fn new_memory(&self, plan: &MemoryPlan) -> Result<Box<dyn RuntimeLinearMemory>, String> {
+        Ok(Box::new(MmapMemory::new(plan)?) as Box<dyn RuntimeLinearMemory>)
+    }
+}
+
+/// A linear memory
+pub trait RuntimeLinearMemory {
+    /// Returns the number of allocated wasm pages.
+    fn size(&self) -> u32;
+
+    /// Grow memory by the specified amount of wasm pages.
+    ///
+    /// Returns `None` if memory can't be grown by the specified amount
+    /// of wasm pages.
+    fn grow(&self, delta: u32) -> Option<u32>;
+
+    /// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm code.
+    fn vmmemory(&self) -> VMMemoryDefinition;
+}
+
 /// A linear memory instance.
 #[derive(Debug)]
-pub struct LinearMemory {
+pub struct MmapMemory {
     // The underlying allocation.
     mmap: RefCell<WasmMmap>,
 
@@ -35,7 +66,7 @@ struct WasmMmap {
     size: u32,
 }
 
-impl LinearMemory {
+impl MmapMemory {
     /// Create a new linear memory instance with specified minimum and maximum number of wasm pages.
     pub fn new(plan: &MemoryPlan) -> Result<Self, String> {
         // `maximum` cannot be set to more than `65536` pages.
@@ -77,9 +108,11 @@ impl LinearMemory {
             needs_signal_handlers,
         })
     }
+}
 
+impl RuntimeLinearMemory for MmapMemory {
     /// Returns the number of allocated wasm pages.
-    pub fn size(&self) -> u32 {
+    fn size(&self) -> u32 {
         self.mmap.borrow().size
     }
 
@@ -87,7 +120,7 @@ impl LinearMemory {
     ///
     /// Returns `None` if memory can't be grown by the specified amount
     /// of wasm pages.
-    pub fn grow(&self, delta: u32) -> Option<u32> {
+    fn grow(&self, delta: u32) -> Option<u32> {
         // Optimization of memory.grow 0 calls.
         let mut mmap = self.mmap.borrow_mut();
         if delta == 0 {
@@ -143,7 +176,7 @@ impl LinearMemory {
     }
 
     /// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm code.
-    pub fn vmmemory(&self) -> VMMemoryDefinition {
+    fn vmmemory(&self) -> VMMemoryDefinition {
         let mut mmap = self.mmap.borrow_mut();
         VMMemoryDefinition {
             base: mmap.alloc.as_mut_ptr(),