Add support for minimum field width for strings to my_vsnprintf()

This patch adds support for right aligned strings and numbers.
Left alignment is left as an exercise for anyone needing it.

MDEV-25612 "Assertion `to <= end' failed in process_args" fixed.
(Was caused by the original version of this patch)

mysql-test/main/strings.result

+#
+# MDEV-25612 Assertion `to <= end' failed in process_args
+#
+SET SESSION default_master_connection=REPEAT('a',190),lc_messages=ru_ru;
+CHANGE MASTER TO master_host='dummy';
+START SLAVE sql_thread;
+CHANGE MASTER TO master_user='user',master_password='pwd';
+ERROR HY000: \042D\0442\0443 \043E\043F\0435\0440\0430\0446\0438\044E \043D\0435\0432\043E\0437\043C\043E\0436\043D\043E \0432\044B\043F\043E\043B\043D\0438\0442\044C \043F\0440\0438 \0440\0430\0431\043E\0442\0430\044E\0449\0435\043C \043F\043E\0442\043E\043A\0435 \043F\043E\0434\0447\0438\043D\0435\043D\043D\043E\0433\043E \0441\0435\0440\0432\0435\0440\0430 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+STOP SLAVE;

mysql-test/main/strings.test

+--source include/not_windows.inc
+--source include/not_embedded.inc
+
+#
+# Test that exposes problems in string functions like my_vsnprintf
+#
+
+--echo #
+--echo # MDEV-25612 Assertion `to <= end' failed in process_args
+--echo #
+SET SESSION default_master_connection=REPEAT('a',190),lc_messages=ru_ru;
+CHANGE MASTER TO master_host='dummy';
+START SLAVE sql_thread;
+--error ER_SLAVE_MUST_STOP
+CHANGE MASTER TO master_user='user',master_password='pwd';
+STOP SLAVE;

strings/my_vsnprintf.c

@@ -224,12 +224,27 @@ static char *backtick_string(CHARSET_INFO *cs, char *to, const char *end,
 */
 
 static char *process_str_arg(CHARSET_INFO *cs, char *to, const char *end,
-                             size_t width, char *par, uint print_type,
-                             my_bool nice_cut)
+                             longlong length_arg, size_t width, char *par,
+                             uint print_type, my_bool nice_cut)
 {
   int well_formed_error;
   uint dots= 0;
   size_t plen, left_len= (size_t) (end - to) + 1, slen=0;
+  my_bool left_fill= 1;
+  size_t length;
+
+  /*
+    The sign of the length argument specific the string should be right
+    or left adjusted
+  */
+  if (length_arg < 0)
+  {
+    length= (size_t) -length_arg;
+    left_fill= 0;
+  }
+  else
+    length= (size_t) length_arg;
+
   if (!par)
     par = (char*) "(null)";
 
@@ -239,7 +254,10 @@ static char *process_str_arg(CHARSET_INFO *cs, char *to, const char *end,
     if (plen > width)
       plen= width;
     if (left_len <= plen)
+    {
       plen = left_len - 1;
+      length= plen;
+    }
     if ((slen > plen))
     {
       if (plen < 3)
@@ -259,24 +277,34 @@ static char *process_str_arg(CHARSET_INFO *cs, char *to, const char *end,
     plen= slen= strnlen(par, width);
     dots= 0;
     if (left_len <= plen)
+    {
       plen = left_len - 1;
+      length= plen;
+    }
   }
 
   plen= my_well_formed_length(cs, par, par + plen, width, &well_formed_error);
   if (print_type & ESCAPED_ARG)
   {
+    const char *org_to= to;
     to= backtick_string(cs, to, end, par, plen + dots, '`', MY_TEST(dots));
+    plen= (size_t) (to - org_to);
     dots= 0;
   }
   else
+  {
+    if (left_fill)
+    {
+      if (plen + dots < length)
+        to= strfill(to, length - plen - dots, ' ');
+    }
     to= strnmov(to,par,plen);
-
     if (dots)
-  {
-    for (; dots; dots--)
-      *(to++)= '.';
-    *(to)= 0;
+      to= strfill(to, dots, '.');
   }
+
+  if (!left_fill && plen + dots < length)
+    to= strfill(to, length - plen - dots, ' ');
   return to;
 }
 
@@ -494,11 +522,16 @@ static char *process_args(CHARSET_INFO *cs, char *to, char *end,
       case 's':
       case 'T':
       {
+        longlong min_field_width;
         char *par= args_arr[print_arr[i].arg_idx].str_arg;
         width= (print_arr[i].flags & WIDTH_ARG)
           ? (size_t)args_arr[print_arr[i].width].longlong_arg
           : print_arr[i].width;
-        to= process_str_arg(cs, to, end, width, par, print_arr[i].flags,
+        min_field_width= (print_arr[i].flags & LENGTH_ARG)
+          ? args_arr[print_arr[i].length].longlong_arg
+          : (longlong) print_arr[i].length;
+        to= process_str_arg(cs, to, end, min_field_width, width, par,
+                            print_arr[i].flags,
                             (print_arr[i].arg_type == 'T'));
         break;
       }
@@ -565,7 +598,7 @@ static char *process_args(CHARSET_INFO *cs, char *to, char *end,
           *to++= ' ';
           *to++= '"';
           my_strerror(errmsg_buff, sizeof(errmsg_buff), (int) larg);
-          to= process_str_arg(cs, to, real_end, width, errmsg_buff,
+          to= process_str_arg(cs, to, real_end, 0, width, errmsg_buff,
                               print_arr[i].flags, 1);
           if (real_end > to) *to++= '"';
         }
@@ -693,7 +726,8 @@ size_t my_vsnprintf_ex(CHARSET_INFO *cs, char *to, size_t n,
     if (*fmt == 's' || *fmt == 'T')			/* String parameter */
     {
       reg2 char *par= va_arg(ap, char *);
-      to= process_str_arg(cs, to, end, width, par, print_type, (*fmt == 'T'));
+      to= process_str_arg(cs, to, end, (longlong) length, width, par,
+                          print_type, (*fmt == 'T'));
       continue;
     }
     else if (*fmt == 'b')				/* Buffer parameter */
@@ -751,7 +785,7 @@ size_t my_vsnprintf_ex(CHARSET_INFO *cs, char *to, size_t n,
         *to++= ' ';
         *to++= '"';
         my_strerror(errmsg_buff, sizeof(errmsg_buff), (int) larg);
-        to= process_str_arg(cs, to, real_end, width, errmsg_buff,
+        to= process_str_arg(cs, to, real_end, 0, width, errmsg_buff,
                             print_type, 1);
         if (real_end > to) *to++= '"';
       }

unittest/mysys/my_vsnprintf-t.c

@@ -61,7 +61,7 @@ static void test_many(const char **res, const char *fmt, ...)
 
 int main(void)
 {
-  plan(47);
+  plan(48);
 
   test1("Constant string",
         "Constant string");
@@ -96,8 +96,11 @@ int main(void)
   test1("Flag '0' works <0006>",
         "Flag '0' works <%04d>", 6);
 
-  test1("Width is ignored for strings <x> <y>",
-        "Width is ignored for strings <%04s> <%5s>", "x", "y");
+  test1("Width for strings <   x> <    y>",
+        "Width for strings <%04s> <%5s>", "x", "y");
+
+  test1("Negative width is ignored for strings <   x> <    y>",
+        "Negative width is ignored for strings <%-4s> <%-5s>", "x", "y");
 
   test1("Precision works for strings <abcde>",
         "Precision works for strings <%.5s>", "abcdef!");