MDEV-20466: SHOW PROCESSLIST truncates query text on \0 bytes

Data should be sent with length.

mysql-test/r/processlist.result

@@ -17,3 +17,4 @@ select command, time < 5 from information_schema.processlist where id != connect
 command	time < 5
 Sleep	1
 set debug_sync='reset';
+End of 5.5 tests

mysql-test/t/processlist.test

@@ -50,3 +50,5 @@ select command, time < 5 from information_schema.processlist where id != connect
 
 disconnect con1;
 set debug_sync='reset';
+
+--echo End of 5.5 tests

mysql-test/t/processlist_MDEV-20466.test

@@ -0,0 +1,35 @@
+--echo #  This test has a result that includes \0 byte.
+--echo #  Such a byte makes diff to treat the whole file as binary,
+--echo #  and to refuse to diff it.
+--echo #  That's why this test is put in a separate file,
+--echo #  so that diff would still work on results of other tests.
+
+source include/have_debug.inc;
+source include/have_debug_sync.inc;
+
+--echo #
+--echo # MDEV-20466: SHOW PROCESSLIST truncates query text on \0 bytes
+--echo #
+
+connect (con1,localhost,root,,);
+
+#select * from information_schema.processlist;
+connection con1;
+
+let $q= `select CONCAT("SELECT user FROM mysql.user WHERE user ='some", CHAR(0), "' or sleep (3)")`;
+
+SET DEBUG_SYNC= 'before_join_optimize SIGNAL in_sync';
+--send_eval $q;
+
+connection default;
+
+SET DEBUG_SYNC= 'now WAIT_FOR in_sync';
+
+--replace_column 1 ID 3 HOST_NAME 6 TIME 9 TIME_MS
+SHOW PROCESSLIST;
+
+disconnect con1;
+
+SET DEBUG_SYNC = 'RESET';
+
+--echo End of 5.5 tests

sql/sql_show.cc

@@ -2278,8 +2278,12 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
     else
       protocol->store_null();
     protocol->store(thd_info->state_info, system_charset_info);
-    protocol->store(thd_info->query_string.str(),
+    if (thd_info->query_string.length())
-                    thd_info->query_string.charset());
+      protocol->store(thd_info->query_string.str(),
+                      thd_info->query_string.length(),
+                      thd_info->query_string.charset());
+    else
+      protocol->store_null();
     if (!thd->variables.old_mode &&
         !(thd->variables.old_behavior & OLD_MODE_NO_PROGRESS_INFO))
       protocol->store(thd_info->progress, 3, &store_buffer);