runtime: temporarily store waspi2 memory allocations from cabi_realloc

This is an initial take at solving the interaction between the TinyGo GC and the Component Model allocation scheme (cabi_realloc).

Author: ydnar

src/runtime/runtime_wasip2.go

@@ -30,8 +30,21 @@ func os_runtime_args() []string {
 }
 
 //export cabi_realloc
-func cabi_realloc(ptr, oldsize, align, newsize unsafe.Pointer) unsafe.Pointer {
-	return realloc(ptr, uintptr(newsize))
+func cabi_realloc(ptr unsafe.Pointer, oldSize, align, newSize uintptr) unsafe.Pointer {
+	if newSize == 0 {
+		return nil
+	}
+	newPtr := realloc(ptr, newSize)
+	if ptr != nil {
+		for i := range wasmAllocs {
+			if wasmAllocs[i] == ptr {
+				wasmAllocs[i] = newPtr
+				return newPtr
+			}
+		}
+	}
+	wasmAllocs = append(wasmAllocs, newPtr)
+	return newPtr
 }
 
 func ticksToNanoseconds(ticks timeUnit) int64 {

src/runtime/runtime_wasmentry.go

@@ -96,8 +96,15 @@ func wasmExportExit() {
 	// //go:wasmexport function has exited.
 	schedulerExit = true
 
+	// Clear wasm allocations.
+	wasmAllocs = nil
+
 	task.Pause()
 
 	// TODO: we could cache the allocated stack so we don't have to keep
 	// allocating a new stack on every //go:wasmexport call.
 }
+
+// wasmAllocs holds memory allocated by the host in guest memory.
+// See func cabi_realloc for more information.
+var wasmAllocs []unsafe.Pointer