fix(gnolang): allow comparisons using uninitialized string values (#1132

)

The provided additional test, without the patch, fails with the
following error:

```
=== RUN   TestFiles/comp3.gno
Machine.RunMain() panic: interface conversion: gnolang.Value is nil, not gnolang.StringValue
[...]
goroutine 180 [running]:
runtime/debug.Stack()
        /usr/lib/go/src/runtime/debug/stack.go:24 +0x65
[...]
panic({0xb91680, 0xc001fa7bf0})
        /usr/lib/go/src/runtime/panic.go:884 +0x213
github.com/gnolang/gno/gnovm/pkg/gnolang.isLss(0xc0054b4050, 0xc0054b4078)
        /home/howl/oc/gno2/gnovm/pkg/gnolang/op_binary.go:492 +0x431
```

This seems to be because internally, the string value is "uninitialized"
(hopefully right word here). This is in opposition to an initialised
string, as would happen for the statement `x := ""`.

This PR changes the behaviour for comparisons inside of `op_binary` (<,
>, <=, >=) to use `GetString` instead of a type assertion of
`TypedValue.V` to a `StringValue`. This is in line with what is already
done inside of `isEql`, introduced in this commit:


da6520f#diff-7cc5c6bc5496b6ad9ed55e04e1cdf2f0d1d5954af21be5bc38ef3c46389149a9L358

`git blame` points instead this part of code inside of the comparisons
functions to go back to the initial commit.

Additionally, I looked for other cases where we are currently doing type
assertions directly into a `StringValue`, and there was one in the
implementation of `append`. Since this is a special case and requires
having a native value as the first argument, I haven't written a test
for it, but the change should be safe as `GetString()` internally just
does the type assertion, but checks for `tv.V == nil` first.

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Manfred Touron <[email protected]>

gnovm/pkg/gnolang/op_binary.go

@@ -489,7 +489,7 @@ func isEql(store Store, lv, rv *TypedValue) bool {
 func isLss(lv, rv *TypedValue) bool {
 	switch lv.T.Kind() {
 	case StringKind:
-		return (lv.V.(StringValue) < rv.V.(StringValue))
+		return (lv.GetString() < rv.GetString())
 	case IntKind:
 		return (lv.GetInt() < rv.GetInt())
 	case Int8Kind:
@@ -533,7 +533,7 @@ func isLss(lv, rv *TypedValue) bool {
 func isLeq(lv, rv *TypedValue) bool {
 	switch lv.T.Kind() {
 	case StringKind:
-		return (lv.V.(StringValue) <= rv.V.(StringValue))
+		return (lv.GetString() <= rv.GetString())
 	case IntKind:
 		return (lv.GetInt() <= rv.GetInt())
 	case Int8Kind:
@@ -577,7 +577,7 @@ func isLeq(lv, rv *TypedValue) bool {
 func isGtr(lv, rv *TypedValue) bool {
 	switch lv.T.Kind() {
 	case StringKind:
-		return (lv.V.(StringValue) > rv.V.(StringValue))
+		return (lv.GetString() > rv.GetString())
 	case IntKind:
 		return (lv.GetInt() > rv.GetInt())
 	case Int8Kind:
@@ -621,7 +621,7 @@ func isGtr(lv, rv *TypedValue) bool {
 func isGeq(lv, rv *TypedValue) bool {
 	switch lv.T.Kind() {
 	case StringKind:
-		return (lv.V.(StringValue) >= rv.V.(StringValue))
+		return (lv.GetString() >= rv.GetString())
 	case IntKind:
 		return (lv.GetInt() >= rv.GetInt())
 	case Int8Kind:

gnovm/pkg/gnolang/uverse.go

@@ -550,7 +550,7 @@ func UverseNode() *PackageNode {
 					if xt.Elem().Kind() == Uint8Kind {
 						// TODO this might be faster if reflect supports
 						// appending this way without first converting to a slice.
-						argrv := reflect.ValueOf([]byte(arg1.TV.V.(StringValue)))
+						argrv := reflect.ValueOf([]byte(arg1.TV.GetString()))
 						resrv := reflect.AppendSlice(sv, argrv)
 						m.PushValue(TypedValue{
 							T: xt,

gnovm/tests/files/append5.gno

@@ -0,0 +1,10 @@
+package main
+
+func main() {
+	var x string
+	y := append([]byte{'X'}, x...)
+	println(string(y))
+}
+
+// Output:
+// X

gnovm/tests/files/comp3.gno

@@ -0,0 +1,21 @@
+package main
+
+func main() {
+	// test against uninitialized value: https://github.com/gnolang/gno/pull/1132
+	var x string
+	y := "Hello"
+	results := [...]bool{
+		x < y,
+		x <= y,
+		x >= y,
+		x > y,
+
+		x == y,
+		x == "",
+		y == "",
+	}
+	println(results)
+}
+
+// Output:
+// array[(true bool),(true bool),(false bool),(false bool),(false bool),(true bool),(false bool)]