Clarify plugin_ref lifetimes in function comments
The mechanism in plugin_lock() et.al. which automatically unlocks plugins unless called with NULL THD is quite subtle, and deserves proper documentation. (This was found during MDEV-12179 development, but is independent of it, and so done in a separate commit).
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@ -938,6 +938,10 @@ SHOW_COMP_OPTION plugin_status(const char *name, size_t len, int type)
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}
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/*
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If LEX is passed non-NULL, an automatic unlock of the plugin will happen
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in the LEX destructor.
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*/
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static plugin_ref intern_plugin_lock(LEX *lex, plugin_ref rc)
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{
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st_plugin_int *pi= plugin_ref_to_int(rc);
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@ -981,6 +985,16 @@ static plugin_ref intern_plugin_lock(LEX *lex, plugin_ref rc)
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}
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/*
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Notes on lifetime:
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If THD is passed as non-NULL (and with a non-NULL thd->lex), an entry is made
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in the thd->lex which will cause an automatic unlock of the plugin in the LEX
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destructor. In this case, no manual unlock must be done.
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Otherwise, when passing a NULL THD, the caller must arrange that plugin
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unlock happens later.
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*/
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plugin_ref plugin_lock(THD *thd, plugin_ref ptr)
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{
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LEX *lex= thd ? thd->lex : 0;
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@ -1017,6 +1031,16 @@ plugin_ref plugin_lock(THD *thd, plugin_ref ptr)
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}
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/*
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Notes on lifetime:
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If THD is passed as non-NULL (and with a non-NULL thd->lex), an entry is made
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in the thd->lex which will cause an automatic unlock of the plugin in the LEX
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destructor. In this case, no manual unlock must be done.
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Otherwise, when passing a NULL THD, the caller must arrange that plugin
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unlock happens later.
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*/
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plugin_ref plugin_lock_by_name(THD *thd, const LEX_STRING *name, int type)
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{
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LEX *lex= thd ? thd->lex : 0;
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