[PATCHv2] grub-install: Add backup and restore

[Dimitri John Ledkov]

Refactor clean_grub_dir to create a backup of all the files, instead
of just irrevocably removing them as the first action. If available,
register on_exit handle to restore the backup if any errors occur, or
remove the backup if everything was successful. If on_exit is not
available, the backup remains on disk for manual recovery.

This allows safer upgrades of MBR & modules, such that
modules/images/fonts/translations are consistent with MBR in case of
errors. For example accidental grub-install /dev/non-existent-disk
currently clobbers and upgrades modules in /boot/grub, despite not
actually updating any MBR. This increases peak disk-usage slightly, by
requiring temporarily twice the disk space to complete grub-install.

Also add modinfo.sh to the cleanup/backup/restore codepath, to ensure
it is also cleaned / backed up / restored.

Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
---

 Changes since v1: as reported on linux-ext4 mailing list and debugged
 by Colin Watson, the grub_util_path_concat_ext call was incorrect in
 the restore backup case as there was no extention needed. Instead the
 call is corrected to just grub_util_path_concat.

 This patch is now shipped in Ubuntu & Debian in multiple series. It
 would be nice to have this merged upstream, as it greatly improves
 grub upgrades and prevents missmatches of MBR & modules.

 configure.ac               |   2 +-
 util/grub-install-common.c | 105 +++++++++++++++++++++++++++++++------
 2 files changed, 91 insertions(+), 16 deletions(-)

diff --git a/configure.ac b/configure.ac
index 7c10a4db7..71cd414c3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -419,7 +419,7 @@ else
 fi
 
 # Check for functions and headers.
-AC_CHECK_FUNCS(posix_memalign memalign getextmntent)
+AC_CHECK_FUNCS(posix_memalign memalign getextmntent on_exit)
 AC_CHECK_HEADERS(sys/param.h sys/mount.h sys/mnttab.h limits.h)
 
 # glibc 2.25 still includes sys/sysmacros.h in sys/types.h but emits 
deprecation
diff --git a/util/grub-install-common.c b/util/grub-install-common.c
index 277eaf4e2..74584b92b 100644
--- a/util/grub-install-common.c
+++ b/util/grub-install-common.c
@@ -185,38 +185,113 @@ grub_install_mkdir_p (const char *dst)
   free (t);
 }
 
+static int
+strcmp_ext (const char *a, const char *b, const char *ext)
+{
+  char *bsuffix = grub_util_path_concat_ext (1, b, ext);
+  int r = strcmp (a, bsuffix);
+  free (bsuffix);
+  return r;
+}
+
+enum clean_grub_dir_mode
+{
+  CLEAN = 0,
+  CLEAN_BACKUP = 1,
+  CREATE_BACKUP = 2,
+  RESTORE_BACKUP = 3,
+};
+
 static void
-clean_grub_dir (const char *di)
+clean_grub_dir_real (const char *di, enum clean_grub_dir_mode mode)
 {
   grub_util_fd_dir_t d;
   grub_util_fd_dirent_t de;
+  char suffix[2] = "";
+
+  if ((mode == CLEAN_BACKUP) || (mode == RESTORE_BACKUP))
+    {
+      strcpy (suffix, "-");
+    }
 
   d = grub_util_fd_opendir (di);
   if (!d)
-    grub_util_error (_("cannot open directory `%s': %s"),
-                    di, grub_util_fd_strerror ());
+    {
+      if (mode == CLEAN_BACKUP)
+       return;
+      grub_util_error (_("cannot open directory `%s': %s"),
+                      di, grub_util_fd_strerror ());
+    }
 
   while ((de = grub_util_fd_readdir (d)))
     {
       const char *ext = strrchr (de->d_name, '.');
-      if ((ext && (strcmp (ext, ".mod") == 0
-                  || strcmp (ext, ".lst") == 0
-                  || strcmp (ext, ".img") == 0
-                  || strcmp (ext, ".mo") == 0)
-          && strcmp (de->d_name, "menu.lst") != 0)
-         || strcmp (de->d_name, "efiemu32.o") == 0
-         || strcmp (de->d_name, "efiemu64.o") == 0)
+      if ((ext && (strcmp_ext (ext, ".mod", suffix) == 0
+                  || strcmp_ext (ext, ".lst", suffix) == 0
+                  || strcmp_ext (ext, ".img", suffix) == 0
+                  || strcmp_ext (ext, ".mo", suffix) == 0)
+          && strcmp_ext (de->d_name, "menu.lst", suffix) != 0)
+         || strcmp_ext (de->d_name, "modinfo.sh", suffix) == 0
+         || strcmp_ext (de->d_name, "efiemu32.o", suffix) == 0
+         || strcmp_ext (de->d_name, "efiemu64.o", suffix) == 0)
        {
-         char *x = grub_util_path_concat (2, di, de->d_name);
-         if (grub_util_unlink (x) < 0)
-           grub_util_error (_("cannot delete `%s': %s"), x,
-                            grub_util_fd_strerror ());
-         free (x);
+         char *srcf = grub_util_path_concat (2, di, de->d_name);
+
+         if (mode == CREATE_BACKUP)
+           {
+             char *dstf = grub_util_path_concat_ext (2, di, de->d_name, "-");
+             if (grub_util_rename (srcf, dstf) < 0)
+               grub_util_error (_("cannot backup `%s': %s"), srcf,
+                                grub_util_fd_strerror ());
+             free (dstf);
+           }
+         else if (mode == RESTORE_BACKUP)
+           {
+             char *dstf = grub_util_path_concat (2, di, de->d_name);
+             dstf[strlen (dstf) - 1] = 0;
+             if (grub_util_rename (srcf, dstf) < 0)
+               grub_util_error (_("cannot restore `%s': %s"), dstf,
+                                grub_util_fd_strerror ());
+             free (dstf);
+           }
+         else
+           {
+             if (grub_util_unlink (srcf) < 0)
+               grub_util_error (_("cannot delete `%s': %s"), srcf,
+                                grub_util_fd_strerror ());
+           }
+         free (srcf);
        }
     }
   grub_util_fd_closedir (d);
 }
 
+static void
+restore_backup_on_exit (int status, void *arg)
+{
+  if (status == 0)
+    {
+      clean_grub_dir_real ((char *) arg, CLEAN_BACKUP);
+    }
+  else
+    {
+      clean_grub_dir_real ((char *) arg, CLEAN);
+      clean_grub_dir_real ((char *) arg, RESTORE_BACKUP);
+    }
+  free (arg);
+  arg = NULL;
+}
+
+static void
+clean_grub_dir (const char *di)
+{
+  clean_grub_dir_real (di, CLEAN_BACKUP);
+  clean_grub_dir_real (di, CREATE_BACKUP);
+#if defined(HAVE_ON_EXIT)
+  on_exit (restore_backup_on_exit, strdup (di));
+#endif
+}
+
 struct install_list
 {
   int is_default;
-- 
2.27.0