examples(allocation): free memory after unmarshalling a result from the guest (#1390)

Signed-off-by: Luca Burgazzoli <lburgazzoli@gmail.com>

examples/allocation/tinygo/README.md

@@ -12,6 +12,6 @@ go >> Hello, wazero!
 Under the covers, [greet.go](testdata/greet.go) does a few things of interest:
 * Uses `unsafe.Pointer` to change a Go pointer to a numeric type.
 * Uses `reflect.StringHeader` to build back a string from a pointer, len pair.
-* Relies on TinyGo not eagerly freeing pointers returned.
+* Relies on CGO to allocate memory used to pass data from TinyGo to host.
 
 See https://wazero.io/languages/tinygo/ for more tips.

examples/allocation/tinygo/greet.go

@@ -90,9 +90,21 @@ func main() {
 	if err != nil {
 		log.Panicln(err)
 	}
-	// Note: This pointer is still owned by TinyGo, so don't try to free it!
+
 	greetingPtr := uint32(ptrSize[0] >> 32)
 	greetingSize := uint32(ptrSize[0])
+
+	// This pointer is managed by TinyGo, but TinyGo is unaware of external usage.
+	// So, we have to free it when finished
+	if greetingPtr != 0 {
+		defer func() {
+			_, err := free.Call(ctx, uint64(greetingPtr))
+			if err != nil {
+				log.Panicln(err)
+			}
+		}()
+	}
+
 	// The pointer is a linear memory offset, which is where we write the name.
 	if bytes, ok := mod.Memory().Read(greetingPtr, greetingSize); !ok {
 		log.Panicf("Memory.Read(%d, %d) out of range of memory size %d",

examples/allocation/tinygo/testdata/greet.go

@@ -1,5 +1,8 @@
 package main
 
+// #include <stdlib.h>
+import "C"
+
 import (
 	"fmt"
 	"reflect"
@@ -53,7 +56,7 @@ func _greet(ptr, size uint32) {
 func _greeting(ptr, size uint32) (ptrSize uint64) {
 	name := ptrToString(ptr, size)
 	g := greeting(name)
-	ptr, size = stringToPtr(g)
+	ptr, size = stringToLeakedPtr(g)
 	return (uint64(ptr) << uint64(32)) | uint64(size)
 }
 
@@ -77,3 +80,15 @@ func stringToPtr(s string) (uint32, uint32) {
 	unsafePtr := uintptr(unsafe.Pointer(ptr))
 	return uint32(unsafePtr), uint32(len(buf))
 }
+
+// stringToLeakedPtr returns a pointer and size pair for the given string in a way
+// compatible with WebAssembly numeric types. The pointer is not automatically
+// managed by TinyGo hence it must be freed by the host.
+func stringToLeakedPtr(s string) (uint32, uint32) {
+	size := C.ulong(len(s))
+	ptr := unsafe.Pointer(C.malloc(size))
+
+	copy(unsafe.Slice((*byte)(ptr), size), []byte(s))
+
+	return uint32(uintptr(ptr)), uint32(len(s))
+}