From c1f1bd57f6d99bff890a412e850dac85fe921f38 Mon Sep 17 00:00:00 2001
From: Luca Burgazzoli <lburgazzoli@gmail.com>
Date: Thu, 4 May 2023 09:39:15 +0200
Subject: [PATCH] examples(allocation): ensures data used in host functions
 isn't freed before invocation

Signed-off-by: Luca Burgazzoli <lburgazzoli@gmail.com>
---
 examples/allocation/tinygo/testdata/greet.go  |  25 ++++++++++++++----
 .../allocation/tinygo/testdata/greet.wasm     | Bin 54254 -> 54228 bytes
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/examples/allocation/tinygo/testdata/greet.go b/examples/allocation/tinygo/testdata/greet.go
index 2eb717d5652..85c161850e7 100644
--- a/examples/allocation/tinygo/testdata/greet.go
+++ b/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
diff --git a/examples/allocation/tinygo/testdata/greet.wasm b/examples/allocation/tinygo/testdata/greet.wasm
index 6a7fa97869035863d6a1e667bc231392334ce9b9..cc2fe35ddac45efcaf5317667b5e270bcd31e632 100755
GIT binary patch
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delta 263
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DPf9IG