Make os.tempfolder work correctly for MinGW & MSVC (#7443)

diff --git a/builtin/mainmenu/common.lua b/builtin/mainmenu/common.lua
index cd4903f56ce8cde6c81ef6b1ecfa29a9605fbda0..cc61fe0adfc1e68c40d13f2132d1be015ee3ccfd 100644 (file)
--- a/builtin/mainmenu/common.lua
+++ b/builtin/mainmenu/common.lua
@@ -171,6 +171,23 @@ os.tempfolder = function()
        local filetocheck = os.tmpname()
        os.remove(filetocheck)
 
+       -- https://blogs.msdn.microsoft.com/vcblog/2014/06/18/c-runtime-crt-features-fixes-and-breaking-changes-in-visual-studio-14-ctp1/
+       --   The C runtime (CRT) function called by os.tmpname is tmpnam.
+       --   Microsofts tmpnam implementation in older CRT / MSVC releases is defective.
+       --   tmpnam return values starting with a backslash characterize this behavior.
+       -- https://sourceforge.net/p/mingw-w64/bugs/555/
+       --   MinGW tmpnam implementation is forwarded to the CRT directly.
+       -- https://sourceforge.net/p/mingw-w64/discussion/723797/thread/55520785/
+       --   MinGW links to an older CRT release (msvcrt.dll).
+       --   Due to legal concerns MinGW will never use a newer CRT.
+       --
+       --   Make use of TEMP to compose the temporary filename if an old
+       --   style tmpnam return value is detected.
+       if filetocheck:sub(1, 1) == "\\" then
+               local tempfolder = os.getenv("TEMP")
+               return tempfolder .. filetocheck
+       end
+
        local randname = "MTTempModFolder_" .. math.random(0,10000)
        local backstring = filetocheck:reverse()
        return filetocheck:sub(0, filetocheck:len() - backstring:find(DIR_DELIM) + 1) ..