Fix MemoryType::default_value with shared memories (#12029)

* Fix `MemoryType::default_value` with shared memories

This fixes fallout from #12022 which was detected during fuzzing which
tried creating a shared memory.

* Fix tests

Author: alexcrichton

crates/fuzzing/src/oracles/dummy.rs

@@ -59,7 +59,11 @@ mod tests {
     fn dummy_memory_import() {
         let mut store = store();
         let memory_type = MemoryType::new(1, None);
-        let memory = memory_type.default_value(&mut store).unwrap();
+        let memory = memory_type
+            .default_value(&mut store)
+            .unwrap()
+            .into_memory()
+            .unwrap();
         assert_eq!(memory.size(&store), 1);
     }
 

crates/wasmtime/src/runtime/types.rs

@@ -1,5 +1,4 @@
 use crate::prelude::*;
-use crate::runtime::Memory as RuntimeMemory;
 use crate::runtime::externals::Global as RuntimeGlobal;
 use crate::runtime::externals::Table as RuntimeTable;
 use crate::runtime::externals::Tag as RuntimeTag;
@@ -1526,7 +1525,7 @@ impl ExternType {
             ExternType::Func(func_ty) => func_ty.default_value(store).map(Extern::Func),
             ExternType::Global(global_ty) => global_ty.default_value(store).map(Extern::Global),
             ExternType::Table(table_ty) => table_ty.default_value(store).map(Extern::Table),
-            ExternType::Memory(mem_ty) => mem_ty.default_value(store).map(Extern::Memory),
+            ExternType::Memory(mem_ty) => mem_ty.default_value(store),
             ExternType::Tag(tag_ty) => tag_ty.default_value(store).map(Extern::Tag),
         }
     }
@@ -3494,12 +3493,26 @@ impl MemoryType {
     pub(crate) fn wasmtime_memory(&self) -> &Memory {
         &self.ty
     }
-    /// Construct a new memory import initialized to this memory type’s default state
+    /// Construct a new memory import initialized to this memory type’s default
+    /// state.
     ///
-    /// Returns a host `Memory` in the given store with the configured initial
-    /// page size and zeroed contents.
-    pub fn default_value(&self, store: impl AsContextMut) -> Result<RuntimeMemory> {
-        RuntimeMemory::new(store, self.clone())
+    /// Returns a host `Memory` or `SharedMemory` depending on if this is a
+    /// shared memory type or not. The memory's type will have the same type as
+    /// `self` and the initial contents of the memory, if any, will be all zero.
+    pub fn default_value(&self, store: impl AsContextMut) -> Result<Extern> {
+        Ok(if self.is_shared() {
+            #[cfg(feature = "threads")]
+            {
+                let store = store.as_context();
+                Extern::SharedMemory(crate::SharedMemory::new(store.engine(), self.clone())?)
+            }
+            #[cfg(not(feature = "threads"))]
+            {
+                bail!("creation of shared memories disabled at compile time")
+            }
+        } else {
+            Extern::Memory(crate::Memory::new(store, self.clone())?)
+        })
     }
 }