win32: work around problem with stderr in MSVCRT

Try to run a non-existent command with standard error closed:

   xyz 2>&-

In recent versions of busybox-w32 this resulted in problems with
moving through history (either using up/down keys or ctrl-r) and
tab completion.  In all cases the order of the prompt and the
command were reversed.

Bisection showed the problem was first seen in PRE-5396 which
merged some commits from upstream, including fd47f0567 (lineedit:
print prompt and editing operations to stderr).

This (eventually) called to mind a previous problem with stderr
in xargs which was fixed by commit f192e6539 (xargs: fix 'xargs
-sNUM' tests).  In both cases it seemed that mixing calls to
bb_putchar_stderr() and fprintf(stderr, ...) was at fault.  The
former uses a file descriptor while the latter uses a stream.  It
was almost as if the stream was buffered.

- The problem with xargs affected 32-bit and 64-bit builds with
  MSVCRT.

- The problem with '2>&-' only affected 32-bit builds with MSVCRT.

- Neither problem was present with UCRT builds.

As a workaround change bb_putchar_stderr() to use the stderr stream
in builds for MSVCRT.

Saves 16 bytes in the 32-bit build.

(GitHub issue #460)

1 files changed, 5 insertions, 0 deletions

diff --git a/libbb/xfuncs.c b/libbb/xfuncs.c
index c365fb8be..7df1a4cd3 100644
--- a/libbb/xfuncs.c
+++ b/libbb/xfuncs.c
@@ -221,7 +221,12 @@ off_t FAST_FUNC fdlength(int fd)
 
 int FAST_FUNC bb_putchar_stderr(char ch)
 {
+#if ENABLE_PLATFORM_MINGW32 && !defined(_UCRT)
+        // Workaround for problems with stderr in MSVCRT
+        return fputc(ch, stderr);
+#else
         return write(STDERR_FILENO, &ch, 1);
+#endif
 }
 
 ssize_t FAST_FUNC full_write1_str(const char *str)