mingw: emulate write(2) that fails with a EPIPE

On Windows, when writing to a pipe fails, errno is always
EINVAL. However, Git expects it to be EPIPE.

According to the documentation, there are two cases in which write()
triggers EINVAL: the buffer is NULL, or the length is odd but the mode
is 16-bit Unicode (the broken pipe is not mentioned as possible cause).
Git never sets the file mode to anything but binary, therefore we know
that errno should actually be EPIPE if it is EINVAL and the buffer is
not NULL.

See https://msdn.microsoft.com/en-us/library/1570wh78.aspx for more
details.

This works around t5571.11 failing with v2.6.4 on Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -394,6 +394,23 @@ int mingw_fflush(FILE *stream)
 	return ret;
 }
 
+#undef write
+ssize_t mingw_write(int fd, const void *buf, size_t len)
+{
+	ssize_t result = write(fd, buf, len);
+
+	if (result < 0 && errno == EINVAL && buf) {
+		/* check if fd is a pipe */
+		HANDLE h = (HANDLE) _get_osfhandle(fd);
+		if (GetFileType(h) == FILE_TYPE_PIPE)
+			errno = EPIPE;
+		else
+			errno = EINVAL;
+	}
+
+	return result;
+}
+
 int mingw_access(const char *filename, int mode)
 {
 	wchar_t wfilename[MAX_PATH];

compat/mingw.h

@@ -210,6 +210,9 @@ FILE *mingw_freopen (const char *filename, const char *otype, FILE *stream);
 int mingw_fflush(FILE *stream);
 #define fflush mingw_fflush
 
+ssize_t mingw_write(int fd, const void *buf, size_t len);
+#define write mingw_write
+
 int mingw_access(const char *filename, int mode);
 #undef access
 #define access mingw_access