maint: port to Fedora 33

Fedora 33 uses GCC 10.2.1, which is a bit pickier.
* configure.ac: Do not use -Wsystem-headers, as this
runs afoul of netdb.h on Fedora 33.
* gnulib.modules: Add ‘attribute’.
* lib/wordsplit.c (wsnode_new): Return the newly allocated
pointer instead of a boolean, to pacify GCC 10.2.1 which otherwise
complains about use of possibly-null pointers.  All uses changed.
* src/buffer.c (try_new_volume): Don’t assume find_next_block succeeds.
(_write_volume_label): Pacify GCC 10.2.1 with an ‘assume’, since
LABEL must be nonnull here.
* src/common.h (FALLTHROUGH): Remove; now in attribute.h.
Include attribute.h, for ATTRIBUTE_NONNULL.
* src/misc.c (assign_string_or_null): New function,
taking over the old role of assign_string.
(assign_string): Assume VALUE is non-null.
(assign_null): New function, taking over the old
role of assign_string when its VALUE was nonnull.
All callers of assign_string changed to use these functions.
(assign_string_n): Clear *STRING if VALUE is null,
to fix a potential double-free.

src/buffer.c

@@ -28,6 +28,7 @@
 #include <fnmatch.h>
 #include <human.h>
 #include <quotearg.h>
+#include <verify.h>
 
 #include "common.h"
 #include <rmt.h>
@@ -1325,8 +1326,8 @@ new_volume (enum access_mode mode)
   if (verify_option)
     verify_volume ();
 
-  assign_string (&volume_label, NULL);
-  assign_string (&continued_file_name, NULL);
+  assign_null (&volume_label);
+  assign_null (&continued_file_name);
   continued_file_size = continued_file_offset = 0;
   current_block = record_start;
 
@@ -1505,7 +1506,7 @@ try_new_volume (void)
       ASSIGN_STRING_N (&volume_label, current_header->header.name);
       set_next_block_after (header);
       header = find_next_block ();
-      if (header->header.typeflag != GNUTYPE_MULTIVOL)
+      if (! (header && header->header.typeflag == GNUTYPE_MULTIVOL))
         break;
       FALLTHROUGH;
     case GNUTYPE_MULTIVOL:
@@ -1688,6 +1689,7 @@ _write_volume_label (const char *str)
     {
       union block *label = find_next_block ();
 
+      assume (label);
       memset (label, 0, BLOCKSIZE);
 
       strcpy (label->header.name, str);

src/common.h

@@ -43,18 +43,13 @@
 # define GLOBAL extern
 #endif
 
-#if 7 <= __GNUC__
-# define FALLTHROUGH __attribute__ ((__fallthrough__))
-#else
-# define FALLTHROUGH ((void) 0)
-#endif
-
 #define TAREXIT_SUCCESS PAXEXIT_SUCCESS
 #define TAREXIT_DIFFERS PAXEXIT_DIFFERS
 #define TAREXIT_FAILURE PAXEXIT_FAILURE
 
 
 #include "arith.h"
+#include <attribute.h>
 #include <backupfile.h>
 #include <exclude.h>
 #include <full-write.h>
@@ -633,7 +628,10 @@ void skip_member (void);
 #define max(a, b) ((a) < (b) ? (b) : (a))
 
 char const *quote_n_colon (int n, char const *arg);
-void assign_string (char **dest, const char *src);
+void assign_string_or_null (char **dest, const char *src)
+  ATTRIBUTE_NONNULL ((1));
+void assign_string (char **dest, const char *src) ATTRIBUTE_NONNULL ((1, 2));
+void assign_null (char **dest) ATTRIBUTE_NONNULL ((1));
 void assign_string_n (char **string, const char *value, size_t n);
 #define ASSIGN_STRING_N(s,v) assign_string_n (s, v, sizeof (v))
 int unquote_string (char *str);

src/extract.c

@@ -520,7 +520,7 @@ delay_set_stat (char const *file_name, struct tar_stat_info const *st,
   data->change_dir = chdir_current;
   data->cntx_name = NULL;
   if (st)
-    assign_string (&data->cntx_name, st->cntx_name);
+    assign_string_or_null (&data->cntx_name, st->cntx_name);
   if (st && st->acls_a_ptr)
     {
       data->acls_a_ptr = xmemdup (st->acls_a_ptr, st->acls_a_len + 1);
@@ -1329,7 +1329,7 @@ extract_file (char *file_name, int typeflag)
    first. If it doesn't exist, there is no matching entry in the list.
    Otherwise, look for the entry in list which has the matching dev
    and ino numbers.
-   
+
    This approach avoids scanning the singly-linked list in obvious cases
    and does not rely on comparing file names, which may differ for
    various reasons (e.g. relative vs. absolute file names).
@@ -1342,14 +1342,14 @@ find_delayed_link_source (char const *name)
 
   if (!delayed_link_head)
     return NULL;
-  
+
   if (fstatat (chdir_fd, name, &st, AT_SYMLINK_NOFOLLOW))
     {
       if (errno != ENOENT)
 	stat_error (name);
       return NULL;
     }
-  
+
   for (dl = delayed_link_head; dl; dl = dl->next)
     {
       if (dl->dev == st.st_dev && dl->ino == st.st_ino)
@@ -1357,7 +1357,7 @@ find_delayed_link_source (char const *name)
     }
   return dl;
 }
-  
+
 /* Create a placeholder file with name FILE_NAME, which will be
    replaced after other extraction is done by a symbolic link if
    IS_SYMLINK is true, and by a hard link otherwise.  Set
@@ -1385,7 +1385,7 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made,
 	   */
 	  return 0;
 	}
-      
+
       switch (maybe_recoverable (file_name, false, interdir_made))
 	{
 	case RECOVER_OK:
@@ -1442,7 +1442,7 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made,
       p->sources->next = 0;
       strcpy (p->sources->string, file_name);
       p->cntx_name = NULL;
-      assign_string (&p->cntx_name, current_stat_info.cntx_name);
+      assign_string_or_null (&p->cntx_name, current_stat_info.cntx_name);
       p->acls_a_ptr = NULL;
       p->acls_a_len = 0;
       p->acls_d_ptr = NULL;
@@ -1467,7 +1467,7 @@ extract_link (char *file_name, int typeflag)
   char const *link_name;
   int rc;
   struct delayed_link *dl;
-  
+
   link_name = current_stat_info.link_name;
 
   if (! absolute_names_option && contains_dot_dot (link_name))
@@ -1475,7 +1475,7 @@ extract_link (char *file_name, int typeflag)
   dl = find_delayed_link_source (link_name);
   if (dl)
     return create_placeholder_file (file_name, false, &interdir_made, dl);
-  
+
   do
     {
       struct stat st1, st2;
@@ -1697,7 +1697,7 @@ prepare_to_extract (char const *file_name, int typeflag, tar_extractor_t *fun)
 
     case GNUTYPE_VOLHDR:
       return false;
-      
+
     case GNUTYPE_MULTIVOL:
       ERROR ((0, 0,
 	      _("%s: Cannot extract -- file is continued from another volume"),
@@ -1753,7 +1753,7 @@ prepare_to_extract (char const *file_name, int typeflag, tar_extractor_t *fun)
 	}
     }
   *fun = extractor;
-  
+
   return true;
 }
 

src/misc.c

@@ -42,11 +42,28 @@ quote_n_colon (int n, char const *arg)
 
 /* Assign STRING to a copy of VALUE if not zero, or to zero.  If
    STRING was nonzero, it is freed first.  */
+void
+assign_string_or_null (char **string, const char *value)
+{
+  if (value)
+    assign_string (string, value);
+  else
+    assign_null (string);
+}
+
 void
 assign_string (char **string, const char *value)
 {
   free (*string);
-  *string = value ? xstrdup (value) : 0;
+  *string = xstrdup (value);
+}
+
+void
+assign_null (char **string)
+{
+  char *old = *string;
+  *string = NULL;
+  free (old);
 }
 
 void
@@ -61,6 +78,8 @@ assign_string_n (char **string, const char *value, size_t n)
       p[l] = 0;
       *string = p;
     }
+  else
+    *string = NULL;
 }
 
 #if 0
@@ -715,7 +734,7 @@ maybe_backup_file (const char *file_name, bool this_is_the_archive)
      possible, real problems are unlikely.  Doing any better would require a
      convention, GNU-wide, for all programs doing backups.  */
 
-  assign_string (&after_backup_name, 0);
+  assign_null (&after_backup_name);
 
   /* Check if we really need to backup the file.  */
 
@@ -758,7 +777,7 @@ maybe_backup_file (const char *file_name, bool this_is_the_archive)
       ERROR ((0, e, _("%s: Cannot rename to %s"),
 	      quotearg_colon (before_backup_name),
 	      quote_n (1, after_backup_name)));
-      assign_string (&after_backup_name, 0);
+      assign_null (&after_backup_name);
       return false;
     }
 }
@@ -782,7 +801,7 @@ undo_last_backup (void)
 	fprintf (stdlis, _("Renaming %s back to %s\n"),
 		 quote_n (0, after_backup_name),
 		 quote_n (1, before_backup_name));
-      assign_string (&after_backup_name, 0);
+      assign_null (&after_backup_name);
     }
 }
 
@@ -1041,11 +1060,11 @@ tar_getcdpath (int idx)
     {
       int i;
       int save_cwdi = chdir_current;
-      
+
       for (i = idx; i >= 0; i--)
 	if (wd[i].abspath)
 	  break;
-      
+
       while (++i <= idx)
 	{
 	  chdir_do (i);
@@ -1069,7 +1088,7 @@ tar_getcdpath (int idx)
 
       chdir_do (save_cwdi);
     }
-	   
+
   return wd[idx].abspath;
 }