Test eol used by lua io.write() under different OSs

[muzimuzhi]

Backgrounds

• l3build save <name> uses CRLF on Win and LF on *nix. Since test output is always rewritten rather than incrementally appended/edited, this causes unnecessary EOL changes.
  See https://github.com/lvjr/tabularray/pull/385#issuecomment-1474855272
• l3build already tries to normalize EOLs in rewrite() using gsub(file_content, "\r\n", "\n"), but has no effect.

  local function rewrite(source,result,processor,...)
    local file = assert(open(source,"rb"))
    local content = gsub(file:read("*all") .. "\n","\r\n","\n")
    close(file)
    local new_content = processor(content,...)
    local newfile = open(result,"w")
    output(newfile)
    write(new_content)
    close(newfile)
  end

[muzimuzhi]

First test: How does lua io.write() behave under different OSs?

On Win, io.write("abc\nabc\n") still writes CRLF (\x0d\x0a)

$ xxd eol-lf.txt
00000000: 6162 630d 0a61 6263 0d0a                 abc..abc..

From mailing list thread http://lua-users.org/lists/lua-l/2015-05/msg00348.html,

Open the file in binary mode.
http://lua-users.org/lists/lua-l/2015-05/msg00349.html

It's CRT (not lua) that does the translation.
In C, one can call _setmode( _fileno( stdout ), _O_BINARY ) to set output to binary mode.
I don't see how it can be done directly in lua.
http://lua-users.org/lists/lua-l/2015-05/msg00358.html

[muzimuzhi]

Open the file in binary mode.
http://lua-users.org/lists/lua-l/2015-05/msg00349.html

Test 2: Try with io.open(fname, "wb")

Succeeded

On Win,

$ xxd eol-crlf.txt
00000000: 6162 630d 0a61 6263 0d0a                 abc..abc..
$ xxd eol-lf.txt
00000000: 6162 630a 6162 630a                      abc.abc.
$ xxd eol-mixed.txt
00000000: 6162 630d 0a61 6263 0a                   abc..abc.

[muzimuzhi]

Also see this thread http://lua-users.org/lists/lua-l/2009-11/msg00815.html

In windows if you open with "r" it will translate LF to CRLF
Try using "rb" on the open and mac and win should give the same result
David Burgess
http://lua-users.org/lists/lua-l/2009-11/msg00817.html

So my question is, why the script get incorrect(?) result in windows?
Is the issue in windows's fwrite function?

Yes. When you open in text mode, "\n" is written as CRLF instead of LF.
That's what text mode means.

In text mode, when you write "\r\n", it is written as CRCRLF. The first
CR is for the "\r" and the CRLF is for the "\n".
--
roger ivie
[email protected]
http://lua-users.org/lists/lua-l/2009-11/msg00819.html

This answer https://stackoverflow.com/a/43967013 quoted PIL

The simple model functions io.input and io.output always open a file in text mode (the default). In Unix, there is no difference between binary files and text files. But in some systems, notably Windows, binary files must be opened with a special flag. To handle such binary files, you must use io.open, with the letter `b´ in the mode string.
https://www.lua.org/pil/21.2.2.html