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.

configure.ac

@@ -162,6 +162,7 @@ if test "$gl_gcc_warnings" = yes; then
   nw="$nw -Winline"                 # It's OK to not inline.
   nw="$nw -Wstrict-overflow"	    # It's OK to optimize strictly.
   nw="$nw -Wsuggest-attribute=pure" # Too many warnings for now.
+  nw="$nw -Wsystem-headers"         # Don't let system headers trigger warnings
   nw="$nw -Wstack-protector"
 
   gl_MANYWARN_ALL_GCC([ws])

gnulib.modules

@@ -23,6 +23,7 @@ areadlinkat-with-size
 argmatch
 argp
 argp-version-etc
+attribute
 backupfile
 closeout
 configmake

lib/wordsplit.c

@@ -424,14 +424,13 @@ wsnode_len (struct wordsplit_node *p)
     return p->v.segm.end - p->v.segm.beg;
 }
 
-static int
-wsnode_new (struct wordsplit *wsp, struct wordsplit_node **pnode)
+static struct wordsplit_node *
+wsnode_new (struct wordsplit *wsp)
 {
   struct wordsplit_node *node = calloc (1, sizeof (*node));
   if (!node)
-    return _wsplt_nomem (wsp);
-  *pnode = node;
-  return 0;
+    _wsplt_nomem (wsp);
+  return node;
 }
 
 static void
@@ -527,14 +526,11 @@ wsnode_insert (struct wordsplit *wsp, struct wordsplit_node *node,
 static int
 wordsplit_add_segm (struct wordsplit *wsp, size_t beg, size_t end, int flg)
 {
-  struct wordsplit_node *node;
-  int rc;
-
   if (end == beg && !(flg & _WSNF_EMPTYOK))
     return 0;
-  rc = wsnode_new (wsp, &node);
-  if (rc)
-    return rc;
+  struct wordsplit_node *node = wsnode_new (wsp);
+  if (!node)
+    return 1;
   node->flags = flg & ~(_WSNF_WORD | _WSNF_EMPTYOK);
   node->v.segm.beg = beg;
   node->v.segm.end = end;
@@ -900,11 +896,11 @@ node_split_prefix (struct wordsplit *wsp,
 		   struct wordsplit_node *node,
 		   size_t beg, size_t len, int flg)
 {
-  struct wordsplit_node *newnode;
 
   if (len == 0)
     return 0;
-  if (wsnode_new (wsp, &newnode))
+  struct wordsplit_node *newnode = wsnode_new (wsp);
+  if (!newnode)
     return 1;
   wsnode_insert (wsp, newnode, *ptail, 0);
   if (node->flags & _WSNF_WORD)
@@ -1195,7 +1191,8 @@ expvar (struct wordsplit *wsp, const char *str, size_t len,
     }
   else
     {
-      if (wsnode_new (wsp, &newnode))
+      newnode = wsnode_new (wsp);
+      if (!newnode)
 	return 1;
       wsnode_insert (wsp, newnode, *ptail, 0);
       *ptail = newnode;
@@ -1357,7 +1354,8 @@ expvar (struct wordsplit *wsp, const char *str, size_t len,
     {
       if (flg & _WSNF_QUOTE)
 	{
-	  if (wsnode_new (wsp, &newnode))
+	  newnode = wsnode_new (wsp);
+	  if (!newnode)
 	    {
 	      free (value);
 	      return 1;
@@ -1371,7 +1369,8 @@ expvar (struct wordsplit *wsp, const char *str, size_t len,
 	{
 	  free (value);
 	  /* Empty string is a special case */
-	  if (wsnode_new (wsp, &newnode))
+	  newnode = wsnode_new (wsp);
+	  if (!newnode)
 	    return 1;
 	  wsnode_insert (wsp, newnode, *ptail, 0);
 	  *ptail = newnode;
@@ -1404,7 +1403,8 @@ expvar (struct wordsplit *wsp, const char *str, size_t len,
     {
       size_t size = *pend - start + 1;
 
-      if (wsnode_new (wsp, &newnode))
+      newnode = wsnode_new (wsp);
+      if (!newnode)
 	return 1;
       wsnode_insert (wsp, newnode, *ptail, 0);
       *ptail = newnode;
@@ -1417,7 +1417,8 @@ expvar (struct wordsplit *wsp, const char *str, size_t len,
     }
   else
     {
-      if (wsnode_new (wsp, &newnode))
+      newnode = wsnode_new (wsp);
+      if (!newnode)
 	return 1;
       wsnode_insert (wsp, newnode, *ptail, 0);
       *ptail = newnode;
@@ -1585,7 +1586,8 @@ expcmd (struct wordsplit *wsp, const char *str, size_t len,
     {
       if (flg & _WSNF_QUOTE)
 	{
-	  if (wsnode_new (wsp, &newnode))
+	  newnode = wsnode_new (wsp);
+	  if (!newnode)
 	    return 1;
 	  wsnode_insert (wsp, newnode, *ptail, 0);
 	  *ptail = newnode;
@@ -1596,7 +1598,8 @@ expcmd (struct wordsplit *wsp, const char *str, size_t len,
 	{
 	  free (value);
 	  /* Empty string is a special case */
-	  if (wsnode_new (wsp, &newnode))
+	  newnode = wsnode_new (wsp);
+	  if (!newnode)
 	    return 1;
 	  wsnode_insert (wsp, newnode, *ptail, 0);
 	  *ptail = newnode;
@@ -1627,7 +1630,8 @@ expcmd (struct wordsplit *wsp, const char *str, size_t len,
     }
   else
     {
-      if (wsnode_new (wsp, &newnode))
+      newnode = wsnode_new (wsp);
+      if (!newnode)
 	return 1;
       wsnode_insert (wsp, newnode, *ptail, 0);
       *ptail = newnode;
@@ -1855,10 +1859,10 @@ wordsplit_pathexpand (struct wordsplit *wsp)
 	  prev = p;
 	  for (i = 0; i < g.gl_pathc; i++)
 	    {
-	      struct wordsplit_node *newnode;
+	      struct wordsplit_node *newnode = wsnode_new (wsp);
 	      char *newstr;
 
-	      if (wsnode_new (wsp, &newnode))
+	      if (!newnode)
 		return 1;
 	      newstr = strdup (g.gl_pathv[i]);
 	      if (!newstr)
@@ -2559,4 +2563,3 @@ wordsplit_perror (struct wordsplit *wsp)
       wsp->ws_error ("%s", wordsplit_strerror (wsp));
     }
 }
-

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);
@@ -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;

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);
     }
 }