MDEV-22387: Do not violate __attribute__((nonnull))

Passing a null pointer to a nonnull argument is not only undefined
behaviour, but it also grants the compiler the permission to optimize
away further checks whether the pointer is null. GCC -O2 at least
starting with version 8 may do that, potentially causing SIGSEGV.

sql/protocol.cc

 /* Copyright (c) 2000, 2012, Oracle and/or its affiliates.
-   Copyright (c) 2008, 2012, Monty Program Ab
+   Copyright (c) 2008, 2020, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -58,7 +58,8 @@ bool Protocol_binary::net_store_data(const uchar *from, size_t length)
       packet->realloc(packet_length+9+length))
     return 1;
   uchar *to= net_store_length((uchar*) packet->ptr()+packet_length, length);
-  memcpy(to,from,length);
+  if (length)
+    memcpy(to,from,length);
   packet->length((uint) (to+length-(uchar*) packet->ptr()));
   return 0;
 }
@@ -715,7 +716,8 @@ void net_send_progress_packet(THD *thd)
 uchar *net_store_data(uchar *to, const uchar *from, size_t length)
 {
   to=net_store_length_fast(to,length);
-  memcpy(to,from,length);
+  if (length)
+    memcpy(to,from,length);
   return to+length;
 }
 

sql/sql_string.cc

@@ -826,7 +826,7 @@ int sortcmp(const String *s,const String *t, CHARSET_INFO *cs)
 int stringcmp(const String *s,const String *t)
 {
   uint32 s_len=s->length(),t_len=t->length(),len=MY_MIN(s_len,t_len);
-  int cmp= memcmp(s->ptr(), t->ptr(), len);
+  int cmp= len ? memcmp(s->ptr(), t->ptr(), len) : 0;
   return (cmp) ? cmp : (int) (s_len - t_len);
 }
 

strings/ctype-mb.c

 /* Copyright (c) 2000, 2014, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2014, SkySQL Ab.
+   Copyright (c) 2009, 2020, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -407,7 +407,9 @@ my_copy_fix_mb(CHARSET_INFO *cs,
                                                         src, src + src_length,
                                                         nchars, status);
   DBUG_ASSERT(well_formed_nchars <= nchars);
-  memmove(dst, src, (well_formed_length= status->m_source_end_pos - src));
+  well_formed_length= status->m_source_end_pos - src;
+  if (well_formed_length)
+    memmove(dst, src, well_formed_length);
   if (!status->m_well_formed_error_pos)
     return well_formed_length;