Make grub-probe work with symbolic links under /dev/mapper as well

as with real block devices.  The Linux world seems to be (at best)
in transition here, and GRUB shouldn't get caught in the middle.
* kern/emu/getroot.c (find_root_device): Follow symbolic links under
/dev/mapper.

ChangeLog.mapper-symlink

@@ -0,0 +1,8 @@
+2010-05-28  Colin Watson  <cjwatson@ubuntu.com>
+
+	Make grub-probe work with symbolic links under /dev/mapper as well
+	as with real block devices.  The Linux world seems to be (at best)
+	in transition here, and GRUB shouldn't get caught in the middle.
+
+	* kern/emu/getroot.c (find_root_device): Follow symbolic links under
+	/dev/mapper.

kern/emu/getroot.c

@@ -126,9 +126,20 @@ find_root_device (const char *dir, dev_t dev)
 	/* Ignore any error.  */
 	continue;
 
-      if (S_ISLNK (st.st_mode))
+      if (S_ISLNK (st.st_mode)) {
-	/* Don't follow symbolic links.  */
+#ifdef __linux__
+	if (strcmp (dir, "mapper") == 0) {
+	  /* Follow symbolic links under /dev/mapper/; the canonical name
+	     may be something like /dev/dm-0, but the names under
+	     /dev/mapper/ are more human-readable and so we prefer them if
+	     we can get them.  */
+	  if (stat (ent->d_name, &st) < 0)
+	    continue;
+	} else
+#endif /* __linux__ */
+	/* Don't follow other symbolic links.  */
 	continue;
+      }
 
       if (S_ISDIR (st.st_mode))
 	{