Repository: yuin/gopher-lua (go)

Issue:
io.open(filename, "w") should truncate file
You **must** post issues only here. Questions, ideas **must** be posted in [discussions](https://github.com/yuin/gopher-lua/discussions). 

- [x] GopherLua is a Lua5.1 implementation. You should be familiar with Lua programming language. Have you read [Lua 5.1 reference manual](http://www.lua.org/manual/5.1/) carefully?
- [x] GopherLua is a Lua5.1 implementation.  In Lua, to keep it simple, **it is more important to remove functionalities rather than to add functionalities unlike other languages** . If you are going to introduce some new cool functionalities into the GopherLua code base and the functionalities can be implemented by existing APIs, It should be implemented as a library. 

Please answer the following before submitting your issue:

1. What version of GopherLua are you using? : 1.1.0 (fa815b5cd712a146016c373261cda69942ec74bb)
2. What version of Go are you using? : go version go1.19.2 linux/amd64
3. What operating system and processor architecture are you using? : x86_64 GNU/Linux
4. What did you do? :

I use this test script (test.lua):
```lua
local function write(filename, content)
local f = assert(io.open(filename, "w"))
  f:write(content)
  assert(f:close())
end

local filename = "test.txt"
os.remove(filename)
write(filename, "abcdef\n")
write(filename, "g\n")
```

5. What did you expect to see? :

If I use 5.1 lua of system I get this output:
```
$ lua -v
Lua 5.1.5  Copyright (C) 1994-2012 Lua.org, PUC-Rio
$ lua test.lua && cat test.txt 
g
```

Unfortunately the manual does not explicitly tells that file must be truncated: https://www.lua.org/manual/5.1/manual.html#5.7 But for fopen this is standard: https://man7.org/linux/man-pages/man3/fopen.3.html Programming in lua mention also erase of "w" https://www.lua.org/pil/21.2.html:  "`w´ for writing (which also erases any previous content of the file)"

6. What did you see instead? :

```
$ ./glua test.lua && cat test.txt 
g
cdef
```

Pull request description:
Write trunc
Fixes #426

Changes proposed in this pull request:

- add truncation to flags for open with mode `"w"`
- add test for testing this behavior

Patch:
diff --git a/iolib.go b/iolib.go
index 4a86f89..b59e82b 100644
--- a/iolib.go
+++ b/iolib.go
@@ -629,7 +629,7 @@ func ioOpenFile(L *LState) int {
 		mode = os.O_RDONLY
 		writable = false
 	case "w", "wb":
-		mode = os.O_WRONLY | os.O_CREATE
+		mode = os.O_WRONLY | os.O_TRUNC | os.O_CREATE
 		readable = false
 	case "a", "ab":
 		mode = os.O_WRONLY | os.O_APPEND | os.O_CREATE


Test patch:
diff --git a/oslib_test.go b/oslib_test.go
new file mode 100644
index 0000000..5af9262
--- /dev/null
+++ b/oslib_test.go
@@ -0,0 +1,33 @@
+package lua
+
+import (
+	"testing"
+)
+
+// correctly gc-ed. There was a bug in gopher lua where local vars were not being gc-ed in all circumstances.
+func TestOsWrite(t *testing.T) {
+	s := `
+		local function write(filename, content)
+		local f = assert(io.open(filename, "w"))
+		  f:write(content)
+		  assert(f:close())
+		end
+
+		local filename = os.tmpname()
+		write(filename, "abc")
+		write(filename, "d")
+		local f = assert(io.open(filename, "r"))
+		local content = f:read("*all"):gsub("%s+", "")
+		f:close()
+		os.remove(filename)
+		local expected = "d"
+		if content ~= expected then
+			error(string.format("Invalid content: Expecting \"%s\", got \"%s\"", expected, content))
+		end
+`
+	L := NewState()
+	defer L.Close()
+	if err := L.DoString(s); err != nil {
+		t.Error(err)
+	}
+}