mingw: replace MSVCRT's fstat() with a Win32-based implementation

fstat() is the only stat-related CRT function for which we don't have a
full replacement yet (and thus the only reason to stick with MSVCRT's
'struct stat' definition).

Fully implement fstat(), in preparation of implementing a POSIX 2013
compatible 'struct stat' with nanosecond-precision file times.

This allows us also to implement some clever code to handle pipes and
character devices in our own way.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -771,20 +771,31 @@ int mingw_stat(const char *file_name, struct stat *buf)
 int mingw_fstat(int fd, struct stat *buf)
 {
 	HANDLE fh = (HANDLE)_get_osfhandle(fd);
+	DWORD avail, type = GetFileType(fh) & ~FILE_TYPE_REMOTE;
 
-	if (fh == INVALID_HANDLE_VALUE) {
-		errno = EBADF;
-		return -1;
-	}
-	/* direct non-file handles to MS's fstat() */
-	if (GetFileType(fh) != FILE_TYPE_DISK)
-		return _fstati64(fd, buf);
+	switch (type) {
+	case FILE_TYPE_DISK:
+		return get_file_info_by_handle(fh, buf);
 
-	if (!get_file_info_by_handle(fh, buf))
+	case FILE_TYPE_CHAR:
+	case FILE_TYPE_PIPE:
+		/* initialize stat fields */
+		memset(buf, 0, sizeof(*buf));
+		buf->st_nlink = 1;
+
+		if (type == FILE_TYPE_CHAR) {
+			buf->st_mode = _S_IFCHR;
+		} else {
+			buf->st_mode = _S_IFIFO;
+			if (PeekNamedPipe(fh, NULL, 0, NULL, &avail, NULL))
+				buf->st_size = avail;
+		}
 		return 0;
 
-	errno = EBADF;
-	return -1;
+	default:
+		errno = EBADF;
+		return -1;
+	}
 }
 
 static inline void time_t_to_filetime(time_t t, FILETIME *ft)