Commitc1500a1

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Update header comment for lookup_type_cache()

Describe the way we handle concurrent invalidation messages.Discussion:https://postgr.es/m/CAPpHfdsQhwUrnB3of862j9RgHoJM--eRbifvBMvtQxpC57dxCA%40mail.gmail.comReviewed-by: Andrei Lepikhov, Artur Zakirov, Pavel Borisov

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Original file line number	Diff line number	Diff line change
`@@ -351,6 +351,15 @@ type_cache_syshash(const void *key, Size keysize)`
`351`	`351`	`* invalid. Note however that we may fail to find one or more of the`
`352`	`352`	`* values requested by 'flags'; the caller needs to check whether the fields`
`353`	`353`	`* are InvalidOid or not.`
	`354`	`+ *`
	`355`	`+ * Note that while filling TypeCacheEntry we might process concurrent`
	`356`	`+ * invalidation messages, causing our not-yet-filled TypeCacheEntry to be`
	`357`	`+ * invalidated. In this case, we typically only clear flags while values are`
	`358`	`+ * still available for the caller. It's expected that the caller holds`
	`359`	`+ * enough locks on type-depending objects that the values are still relevant.`
	`360`	`+ * It's also important that the tupdesc is filled after all other`
	`361`	`+ * TypeCacheEntry items for TYPTYPE_COMPOSITE. So, tupdesc can't get`
	`362`	`+ * invalidated during the lookup_type_cache() call.`
`354`	`363`	`*/`
`355`	`364`	`TypeCacheEntry*`
`356`	`365`	`lookup_type_cache(Oidtype_id,intflags)`

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