examples(allocation): ensures data used in host functions isn't freed…

… before invocation

Signed-off-by: Luca Burgazzoli <[email protected]>

examples/allocation/tinygo/testdata/greet.go

@@ -6,6 +6,7 @@ import "C"
 import (
 	"fmt"
 	"reflect"
+	"runtime"
 	"unsafe"
 )
 
@@ -21,6 +22,11 @@ func greet(name string) {
 func log(message string) {
 	ptr, size := stringToPtr(message)
 	_log(ptr, size)
+
+	// the stringToPtr returns a pointer to the underlying bytes
+	// of the message it must be ensured that the string is not
+	// freed
+	runtime.KeepAlive(message)
 }
 
 // _log is a WebAssembly import which prints a string (linear memory offset,
@@ -72,13 +78,22 @@ func ptrToString(ptr uint32, size uint32) string {
 	}))
 }
 
-// stringToPtr returns a pointer and size pair for the given string in a way
-// compatible with WebAssembly numeric types.
+// stringToPtr returns a pointer and size pair to the underlying bytes of the given
+// string in a way compatible with WebAssembly numeric types. Since Go strings are
+// immutable, the pointer returned by stringToPtr must not be modified.
 func stringToPtr(s string) (uint32, uint32) {
-	buf := []byte(s)
-	ptr := &buf[0]
+	// for an empty string the return value by TinyGo 0.27 is nil, however the
+	// subsequent are still safe and result in the method returning pointer = 0
+	// and size = 0
+	//
+	// Note that:
+	// - we assume that the given string is never empty so there is no special
+	//   handling for such case
+	// - this method does not keep the given string alive, hence it is the
+	//   responsibility of the caller to ensure the string is kept alive
+	ptr := unsafe.StringData(s)
 	unsafePtr := uintptr(unsafe.Pointer(ptr))
-	return uint32(unsafePtr), uint32(len(buf))
+	return uint32(unsafePtr), uint32(len(s))
 }
 
 // stringToLeakedPtr returns a pointer and size pair for the given string in a way