efivars: deal with uncommitted efi variables

Unfortunately kernel reports EOF if there's an inconsistency between efivarfs var list
and what's actually stored in firmware, c.f. #34304. A zero size env var is not allowed in
efi and hence the variable doesn't really exist in the backing store as long as it is zero
sized, and the kernel calls this "uncommitted". Hence we translate EOF back to ENOENT here,
as with kernel behavior before
https://github.com/torvalds/linux/commit/3fab70c165795431f00ddf9be8b84ddd07bd1f8f

If the kernel changes behaviour (to flush dentries on resume), we can drop
this at some point in the future. But note that the commit is 11
years old at this point so we'll need to deal with the current behaviour for
a long time.

Fix #34304.

src/basic/efivars.c

@@ -96,6 +96,22 @@ int efi_get_variable(
                                 (void) usleep_safe(EFI_RETRY_DELAY);
                 }
 
+                /* Unfortunately kernel reports EOF if there's an inconsistency between efivarfs var list
+                 * and what's actually stored in firmware, c.f. #34304. A zero size env var is not allowed in
+                 * efi and hence the variable doesn't really exist in the backing store as long as it is zero
+                 * sized, and the kernel calls this "uncommitted". Hence we translate EOF back to ENOENT here,
+                 * as with kernel behavior before
+                 * https://github.com/torvalds/linux/commit/3fab70c165795431f00ddf9be8b84ddd07bd1f8f
+                 *
+                 * If the kernel changes behaviour (to flush dentries on resume), we can drop
+                 * this at some point in the future. But note that the commit is 11
+                 * years old at this point so we'll need to deal with the current behaviour for
+                 * a long time.
+                 */
+                if (n == 0)
+                        return log_debug_errno(SYNTHETIC_ERRNO(ENOENT),
+                                               "EFI variable %s is uncommitted", p);
+
                 if (n != sizeof(a))
                         return log_debug_errno(SYNTHETIC_ERRNO(EIO),
                                                "Read %zi bytes from EFI variable %s, expected %zu.",  n, p, sizeof(a));