handle drive-qualified nōn-absolute pathnames in do_realpath(), untested

the idea here is that:

- /foo/bar and a:/foo/bar are absolute
- foo/bar is relative
- a:foo/bar needs to be handled specially,
  mostly per making it into an absolute (“a:/” + getcwd(a:) + “foo/bar”)

os2.c

@@ -21,16 +21,19 @@
  */
 
 #define INCL_DOS
+#define INCL_DOSFILEMGR
+#define INCL_DOSMISC
 #include <os2.h>
 
 #include "sh.h"
 
 #include <klibc/startup.h>
+#include <errno.h>
 #include <io.h>
 #include <unistd.h>
 #include <process.h>
 
-__RCSID("$MirOS: src/bin/mksh/os2.c,v 1.2 2017/04/29 22:04:29 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/os2.c,v 1.3 2017/10/11 23:23:03 tg Exp $");
 
 static char *remove_trailing_dots(char *);
 static int access_stat_ex(int (*)(), const char *, void *);
@@ -557,3 +560,52 @@ cleanup(void)
 {
 	cleanup_temps();
 }
+
+int
+getdrvwd(char **cpp, unsigned int drvltr)
+{
+	PBYTE *cp;
+	ULONG sz;
+	APIRET rc;
+	ULONG drvno;
+
+	if (DosQuerySysInfo(QSV_MAX_PATH_LENGTH, QSV_MAX_PATH_LENGTH,
+	    &sz, sizeof(sz)) != 0) {
+		errno = EDOOFUS;
+		return (-1);
+	}
+
+	/* allocate 'X:/' plus sz plus NUL */
+	checkoktoadd((size_t)sz, (size_t)4);
+	cp = aresize(*cpp, (size_t)sz + (size_t)4, ATEMP);
+	cp[0] = drvltr;
+	cp[1] = ':';
+	cp[2] = '/';
+	drvno = (rtt2asc(drvltr) | 0x20U) - rtt2asc('a') + 1;
+	/* NUL is part of space within buffer passed */
+	++sz;
+	if ((rc = DosQueryCurrentDir(drvno, cp + 3, &sz)) == 0) {
+		/* success! */
+		*cpp = cp;
+		return (0);
+	}
+	afree(cp, ATEMP);
+	*cpp = NULL;
+	switch (rc) {
+	case 15: /* invalid drive */
+		errno = ENOTBLK;
+		break;
+	case 26: /* not dos disk */
+		errno = ENODEV;
+		break;
+	case 108: /* drive locked */
+		errno = EDEADLK;
+		break;
+	case 111: /* buffer overflow */
+		errno = ENAMETOOLONG;
+		break;
+	default:
+		errno = EINVAL;
+	}
+	return (-1);
+}