MDEV-28131 Unexpected warning while selecting from information_schema.processlist

Problem:

DECIMAL columns in I_S must be explicitly set of some value.

I_S columns do not have `DEFAULT 0` (after MDEV-18918), so during
restore_record() their record fragments pointed by Field::ptr are
initialized to zero bytes 0x00.
But an array of 0x00's is not a valid binary DECIMAL value.
So val_decimal() called for such Field_new_decimal generated a warning
when seeing a wrong binary encoded DECIMAL value in the record.

Fix:

Explicitly setting INFORMATION_SCHEMA.PROCESSLIST.PROGRESS
to the decimal value of 0 if no progress information is available.

mysql-test/main/processlist.result

@@ -40,3 +40,23 @@ utf8mb4_string	xxx😎yyy
 #
 # End of 10.1 tests
 #
+#
+# Start of 10.3 tests
+#
+#
+# MDEV-28131 Unexpected warning while selecting from information_schema.processlist
+#
+connect  conn1, localhost, root,,;
+connection conn1;
+SELECT SLEEP(1000);
+connection default;
+SELECT progress FROM information_schema.processlist WHERE info='SELECT SLEEP(1000)';
+progress
+0.000
+connection conn1;
+Got one of the listed errors
+connection default;
+disconnect conn1;
+#
+# End of 10.3 tests
+#

mysql-test/main/processlist.test

@@ -70,3 +70,38 @@ SELECT INFO, INFO_BINARY, 'xxx😎yyy' AS utf8mb4_string FROM INFORMATION_SCHEMA
 --echo #
 --echo # End of 10.1 tests
 --echo #
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-28131 Unexpected warning while selecting from information_schema.processlist
+--echo #
+
+connect (conn1, localhost, root,,);
+connection conn1;
+let $ID= `select connection_id()`;
+send SELECT SLEEP(1000);
+connection default;
+let $wait_timeout= 10;
+let $wait_condition=select count(*)=1 from information_schema.processlist
+where state='User sleep' and info='SELECT SLEEP(1000)';
+--source include/wait_condition.inc
+SELECT progress FROM information_schema.processlist WHERE info='SELECT SLEEP(1000)';
+disable_query_log;
+eval kill $ID;
+enable_query_log;
+let $wait_timeout= 10;
+let $wait_condition=select count(*)=0 from information_schema.processlist
+where state='User sleep' and info='SELECT SLEEP(1000)';
+--source include/wait_condition.inc
+connection conn1;
+--error 2013,ER_CONNECTION_KILLED
+reap;
+connection default;
+disconnect conn1;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/sql_show.cc

@@ -3351,6 +3351,16 @@ int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
           table->field[11]->store((double) tmp->progress.counter /
                                   (double) max_counter*100.0);
         }
+        else
+        {
+          /*
+            This is a DECIMAL column without DEFAULT.
+            restore_record() fills its Field::ptr to zero bytes,
+            according to pack_length(). But an array of zero bytes
+            is not a valid decimal. Set it explicitly to 0.
+          */
+          table->field[11]->store((longlong) 0, true);
+        }
         mysql_mutex_unlock(&tmp->LOCK_thd_data);
       }