BUG#22216779: 5.6 CRASHES ON A 5.7 DATADIR

karthik-kamath · karthik-kamath · commit 52284173ab80 · 2016-02-03T11:54:58.000+05:30
ANALYSIS:
=========
Server startup on 5.6 with the 5.7 data directory results in
server crash. This is basically an attempt to downgrade, which
is unsupported.
The system table 'plugin' uses the InnoDB storage engine in 5.7
whereas it uses MyISAM in 5.6. So during 5.6 server startup,
the 'plugin' table cannot be opened which results in the 'hton'
pointer set to NULL. Access to this pointer results in the
server crash.

FIX:
====
A check has been introduced to verify if the 'hton' pointer contains
a non NULL value. If not, safe server shutdown is ensured.
diff --git a/sql/handler.cc b/sql/handler.cc
@@ -337,7 +337,7 @@ plugin_ref ha_resolve_by_name(THD *thd, const LEX_STRING *name,
   if ((plugin= my_plugin_lock_by_name(thd, name, MYSQL_STORAGE_ENGINE_PLUGIN)))
   {
     handlerton *hton= plugin_data(plugin, handlerton *);
-    if (!(hton->flags & HTON_NOT_USER_SELECTABLE))
+    if (hton && !(hton->flags & HTON_NOT_USER_SELECTABLE))
       return plugin;
       
     /*

Original file line number	Diff line number	Diff line change
`@@ -337,7 +337,7 @@ plugin_ref ha_resolve_by_name(THD thd, const LEX_STRING name,`
`337`	`337`	`if ((plugin= my_plugin_lock_by_name(thd, name, MYSQL_STORAGE_ENGINE_PLUGIN)))`
`338`	`338`	`{`
`339`	`339`	`handlerton hton= plugin_data(plugin, handlerton );`
`340`		`- if (!(hton->flags & HTON_NOT_USER_SELECTABLE))`
	`340`	`+ if (hton && !(hton->flags & HTON_NOT_USER_SELECTABLE))`
`341`	`341`	`return plugin;`
`342`	`342`
`343`	`343`	`/*`