Bug#35981: ALTER EVENT causes the server to change the PRESERVE option.

If [NOT] PRESERVE was not given, parser always defaulted to NOT
PRESERVE, making it impossible for the "not given = no change"
rule to work in ALTER EVENT. Leaving out the PRESERVE-clause
defaults to NOT PRESERVE on CREATE now, and to "no change" in
ALTER.

mysql-test/r/events_2.result:
  show that giving no PRESERVE-clause to ALTER EVENT
  results in no change. show that giving no PRESERVE-clause
  to CREATE EVENT defaults to NOT PRESERVE as per the docs.
  Show specifically that this is also handled correctly when
  trying to ALTER EVENTs into the past.
mysql-test/t/events_2.test:
  show that giving no PRESERVE-clause to ALTER EVENT
  results in no change. show that giving no PRESERVE-clause
  to CREATE EVENT defaults to NOT PRESERVE as per the docs.
  Show specifically that this is also handled correctly when
  trying to ALTER EVENTs into the past.
sql/event_db_repository.cc:
  If ALTER EVENT was given no PRESERVE-clause (meaning "no change"),
  we don't know the previous PRESERVE-setting by the time we check
  the parse-data. If ALTER EVENT was given dates that are in the past,
  we don't know how to react, lacking the PRESERVE-setting. Heal this
  by running the check later when we have actually read the previous
  EVENT-data.
sql/event_parse_data.cc:
  Change default for ON COMPLETION to indicate, "not specified."
  Also defer throwing errors when ALTER EVENT is given dates in
  the past but not PRESERVE-clause until we know the previous
  PRESERVE-value.
sql/event_parse_data.h:
  Add third state for ON COMPLETION [NOT] PRESERVE (preserve,
  don't, not specified).
  
  Make check_dates() public so we can defer this check until
  deeper in the callstack where we have all the required data.
sql/sql_yacc.yy:
  If CREATE EVENT is not given ON COMPLETION [NOT] PRESERVE,
  we default to NOT, as per the docs.

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/event_parse_data.h b/sql/event_parse_data.h
index 221bf92664f..87a800c2078 100644
--- a/sql/event_parse_data.h
+++ b/sql/event_parse_data.h
@@ -38,7 +38,12 @@ public:
 
   enum enum_on_completion
   {
-    ON_COMPLETION_DROP = 1,
+    /*
+      On CREATE EVENT, DROP is the DEFAULT as per the docs.
+      On ALTER  EVENT, "no change" is the DEFAULT.
+    */
+    ON_COMPLETION_DEFAULT = 0,
+    ON_COMPLETION_DROP,
     ON_COMPLETION_PRESERVE
   };
 
@@ -80,6 +85,9 @@ public:
   bool
   check_parse_data(THD *thd);
 
+  bool
+  check_dates(THD *thd, int previous_on_completion);
+
 private:
 
   void