Add support for coordinating record typmods among parallel workers.

Tuples can have type RECORDOID and a typmod number that identifies a blessed
TupleDesc in a backend-private cache.  To support the sharing of such tuples
through shared memory and temporary files, provide a typmod registry in
shared memory.

To achieve that, introduce per-session DSM segments, created on demand when a
backend first runs a parallel query.  The per-session DSM segment has a
table-of-contents just like the per-query DSM segment, and initially the
contents are a shared record typmod registry and a DSA area to provide the
space it needs to grow.

State relating to the current session is accessed via a Session object
reached through global variable CurrentSession that may require significant
redesign further down the road as we figure out what else needs to be shared
or remodelled.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/backend/utils/cache/typcache.c

@@ -46,6 +46,8 @@
 #include "access/heapam.h"
 #include "access/htup_details.h"
 #include "access/nbtree.h"
+#include "access/parallel.h"
+#include "access/session.h"
 #include "catalog/indexing.h"
 #include "catalog/pg_am.h"
 #include "catalog/pg_constraint.h"
@@ -55,7 +57,9 @@
 #include "catalog/pg_type.h"
 #include "commands/defrem.h"
 #include "executor/executor.h"
+#include "lib/dshash.h"
 #include "optimizer/planner.h"
+#include "storage/lwlock.h"
 #include "utils/builtins.h"
 #include "utils/catcache.h"
 #include "utils/fmgroids.h"
@@ -142,6 +146,117 @@ typedef struct RecordCacheEntry
 	TupleDesc	tupdesc;
 } RecordCacheEntry;
 
+/*
+ * To deal with non-anonymous record types that are exchanged by backends
+ * involved in a parallel query, we also need a shared verion of the above.
+ */
+struct SharedRecordTypmodRegistry
+{
+	/* A hash table for finding a matching TupleDesc. */
+	dshash_table_handle record_table_handle;
+	/* A hash table for finding a TupleDesc by typmod. */
+	dshash_table_handle typmod_table_handle;
+	/* A source of new record typmod numbers. */
+	pg_atomic_uint32 next_typmod;
+};
+
+/*
+ * When using shared tuple descriptors as hash table keys we need a way to be
+ * able to search for an equal shared TupleDesc using a backend-local
+ * TupleDesc.  So we use this type which can hold either, and hash and compare
+ * functions that know how to handle both.
+ */
+typedef struct SharedRecordTableKey
+{
+	union
+	{
+		TupleDesc	local_tupdesc;
+		dsa_pointer shared_tupdesc;
+	};
+	bool		shared;
+} SharedRecordTableKey;
+
+/*
+ * The shared version of RecordCacheEntry.  This lets us look up a typmod
+ * using a TupleDesc which may be in local or shared memory.
+ */
+typedef struct SharedRecordTableEntry
+{
+	SharedRecordTableKey key;
+} SharedRecordTableEntry;
+
+/*
+ * An entry in SharedRecordTypmodRegistry's typmod table.  This lets us look
+ * up a TupleDesc in shared memory using a typmod.
+ */
+typedef struct SharedTypmodTableEntry
+{
+	uint32		typmod;
+	dsa_pointer shared_tupdesc;
+} SharedTypmodTableEntry;
+
+/*
+ * A comparator function for SharedTupleDescTableKey.
+ */
+static int
+shared_record_table_compare(const void *a, const void *b, size_t size,
+							void *arg)
+{
+	dsa_area   *area = (dsa_area *) arg;
+	SharedRecordTableKey *k1 = (SharedRecordTableKey *) a;
+	SharedRecordTableKey *k2 = (SharedRecordTableKey *) b;
+	TupleDesc	t1;
+	TupleDesc	t2;
+
+	if (k1->shared)
+		t1 = (TupleDesc) dsa_get_address(area, k1->shared_tupdesc);
+	else
+		t1 = k1->local_tupdesc;
+
+	if (k2->shared)
+		t2 = (TupleDesc) dsa_get_address(area, k2->shared_tupdesc);
+	else
+		t2 = k2->local_tupdesc;
+
+	return equalTupleDescs(t1, t2) ? 0 : 1;
+}
+
+/*
+ * A hash function for SharedRecordTableKey.
+ */
+static uint32
+shared_record_table_hash(const void *a, size_t size, void *arg)
+{
+	dsa_area   *area = (dsa_area *) arg;
+	SharedRecordTableKey *k = (SharedRecordTableKey *) a;
+	TupleDesc	t;
+
+	if (k->shared)
+		t = (TupleDesc) dsa_get_address(area, k->shared_tupdesc);
+	else
+		t = k->local_tupdesc;
+
+	return hashTupleDesc(t);
+}
+
+/* Parameters for SharedRecordTypmodRegistry's TupleDesc table. */
+static const dshash_parameters srtr_record_table_params = {
+	sizeof(SharedRecordTableKey),	/* unused */
+	sizeof(SharedRecordTableEntry),
+	shared_record_table_compare,
+	shared_record_table_hash,
+	LWTRANCHE_SESSION_RECORD_TABLE
+};
+
+/* Parameters for SharedRecordTypmodRegistry's typmod hash table. */
+static const dshash_parameters srtr_typmod_table_params = {
+	sizeof(uint32),
+	sizeof(SharedTypmodTableEntry),
+	dshash_memcmp,
+	dshash_memhash,
+	LWTRANCHE_SESSION_TYPMOD_TABLE
+};
+
 static HTAB *RecordCacheHash = NULL;
 
 static TupleDesc *RecordCacheArray = NULL;
@@ -168,6 +283,13 @@ static void TypeCacheConstrCallback(Datum arg, int cacheid, uint32 hashvalue);
 static void load_enum_cache_data(TypeCacheEntry *tcache);
 static EnumItem *find_enumitem(TypeCacheEnumData *enumdata, Oid arg);
 static int	enum_oid_cmp(const void *left, const void *right);
+static void shared_record_typmod_registry_detach(dsm_segment *segment,
+									 Datum datum);
+static void shared_record_typmod_registry_worker_detach(dsm_segment *segment,
+											Datum datum);
+static TupleDesc find_or_make_matching_shared_tupledesc(TupleDesc tupdesc);
+static dsa_pointer share_tupledesc(dsa_area *area, TupleDesc tupdesc,
+				uint32 typmod);
 
 
 /*
@@ -377,8 +499,8 @@ lookup_type_cache(Oid type_id, int flags)
 
 		/*
 		 * Reset info about hash functions whenever we pick up new info about
-		 * equality operator.  This is so we can ensure that the hash functions
-		 * match the operator.
+		 * equality operator.  This is so we can ensure that the hash
+		 * functions match the operator.
 		 */
 		typentry->flags &= ~(TCFLAGS_CHECKED_HASH_PROC);
 		typentry->flags &= ~(TCFLAGS_CHECKED_HASH_EXTENDED_PROC);
@@ -1243,6 +1365,33 @@ cache_record_field_properties(TypeCacheEntry *typentry)
 	typentry->flags |= TCFLAGS_CHECKED_FIELD_PROPERTIES;
 }
 
+/*
+ * Make sure that RecordCacheArray is large enough to store 'typmod'.
+ */
+static void
+ensure_record_cache_typmod_slot_exists(int32 typmod)
+{
+	if (RecordCacheArray == NULL)
+	{
+		RecordCacheArray = (TupleDesc *)
+			MemoryContextAllocZero(CacheMemoryContext, 64 * sizeof(TupleDesc));
+		RecordCacheArrayLen = 64;
+	}
+
+	if (typmod >= RecordCacheArrayLen)
+	{
+		int32		newlen = RecordCacheArrayLen * 2;
+
+		while (typmod >= newlen)
+			newlen *= 2;
+
+		RecordCacheArray = (TupleDesc *) repalloc(RecordCacheArray,
+												  newlen * sizeof(TupleDesc));
+		memset(RecordCacheArray + RecordCacheArrayLen, 0,
+			   (newlen - RecordCacheArrayLen) * sizeof(TupleDesc *));
+		RecordCacheArrayLen = newlen;
+	}
+}
 
 /*
  * lookup_rowtype_tupdesc_internal --- internal routine to lookup a rowtype
@@ -1273,15 +1422,53 @@ lookup_rowtype_tupdesc_internal(Oid type_id, int32 typmod, bool noError)
 		/*
 		 * It's a transient record type, so look in our record-type table.
 		 */
-		if (typmod < 0 || typmod >= NextRecordTypmod)
+		if (typmod >= 0)
 		{
-			if (!noError)
-				ereport(ERROR,
-						(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-						 errmsg("record type has not been registered")));
-			return NULL;
+			/* It is already in our local cache? */
+			if (typmod < RecordCacheArrayLen &&
+				RecordCacheArray[typmod] != NULL)
+				return RecordCacheArray[typmod];
+
+			/* Are we attached to a shared record typmod registry? */
+			if (CurrentSession->shared_typmod_registry != NULL)
+			{
+				SharedTypmodTableEntry *entry;
+
+				/* Try to find it in the shared typmod index. */
+				entry = dshash_find(CurrentSession->shared_typmod_table,
+									&typmod, false);
+				if (entry != NULL)
+				{
+					TupleDesc	tupdesc;
+
+					tupdesc = (TupleDesc)
+						dsa_get_address(CurrentSession->area,
+										entry->shared_tupdesc);
+					Assert(typmod == tupdesc->tdtypmod);
+
+					/* We may need to extend the local RecordCacheArray. */
+					ensure_record_cache_typmod_slot_exists(typmod);
+
+					/*
+					 * Our local array can now point directly to the TupleDesc
+					 * in shared memory.
+					 */
+					RecordCacheArray[typmod] = tupdesc;
+					Assert(tupdesc->tdrefcount == -1);
+
+					dshash_release_lock(CurrentSession->shared_typmod_table,
+										entry);
+
+					return RecordCacheArray[typmod];
+				}
+			}
 		}
-		return RecordCacheArray[typmod];
+
+		if (!noError)
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("record type has not been registered")));
+		return NULL;
 	}
 }
 
@@ -1303,7 +1490,7 @@ lookup_rowtype_tupdesc(Oid type_id, int32 typmod)
 	TupleDesc	tupDesc;
 
 	tupDesc = lookup_rowtype_tupdesc_internal(type_id, typmod, false);
-	IncrTupleDescRefCount(tupDesc);
+	PinTupleDesc(tupDesc);
 	return tupDesc;
 }
 
@@ -1321,7 +1508,7 @@ lookup_rowtype_tupdesc_noerror(Oid type_id, int32 typmod, bool noError)
 
 	tupDesc = lookup_rowtype_tupdesc_internal(type_id, typmod, noError);
 	if (tupDesc != NULL)
-		IncrTupleDescRefCount(tupDesc);
+		PinTupleDesc(tupDesc);
 	return tupDesc;
 }
 
@@ -1376,7 +1563,6 @@ assign_record_type_typmod(TupleDesc tupDesc)
 	RecordCacheEntry *recentry;
 	TupleDesc	entDesc;
 	bool		found;
-	int32		newtypmod;
 	MemoryContext oldcxt;
 
 	Assert(tupDesc->tdtypeid == RECORDOID);
@@ -1414,36 +1600,210 @@ assign_record_type_typmod(TupleDesc tupDesc)
 	recentry->tupdesc = NULL;
 	oldcxt = MemoryContextSwitchTo(CacheMemoryContext);
 
-	if (RecordCacheArray == NULL)
+	/* Look in the SharedRecordTypmodRegistry, if attached */
+	entDesc = find_or_make_matching_shared_tupledesc(tupDesc);
+	if (entDesc == NULL)
 	{
-		RecordCacheArray = (TupleDesc *) palloc(64 * sizeof(TupleDesc));
-		RecordCacheArrayLen = 64;
+		/* Reference-counted local cache only. */
+		entDesc = CreateTupleDescCopy(tupDesc);
+		entDesc->tdrefcount = 1;
+		entDesc->tdtypmod = NextRecordTypmod++;
 	}
-	else if (NextRecordTypmod >= RecordCacheArrayLen)
-	{
-		int32		newlen = RecordCacheArrayLen * 2;
-
-		RecordCacheArray = (TupleDesc *) repalloc(RecordCacheArray,
-												  newlen * sizeof(TupleDesc));
-		RecordCacheArrayLen = newlen;
-	}
-
-	/* if fail in subrs, no damage except possibly some wasted memory... */
-	entDesc = CreateTupleDescCopy(tupDesc);
+	ensure_record_cache_typmod_slot_exists(entDesc->tdtypmod);
+	RecordCacheArray[entDesc->tdtypmod] = entDesc;
 	recentry->tupdesc = entDesc;
-	/* mark it as a reference-counted tupdesc */
-	entDesc->tdrefcount = 1;
-	/* now it's safe to advance NextRecordTypmod */
-	newtypmod = NextRecordTypmod++;
-	entDesc->tdtypmod = newtypmod;
-	RecordCacheArray[newtypmod] = entDesc;
 
-	/* report to caller as well */
-	tupDesc->tdtypmod = newtypmod;
+	/* Update the caller's tuple descriptor. */
+	tupDesc->tdtypmod = entDesc->tdtypmod;
 
 	MemoryContextSwitchTo(oldcxt);
 }
 
+/*
+ * Return the amout of shmem required to hold a SharedRecordTypmodRegistry.
+ * This exists only to avoid exposing private innards of
+ * SharedRecordTypmodRegistry in a header.
+ */
+size_t
+SharedRecordTypmodRegistryEstimate(void)
+{
+	return sizeof(SharedRecordTypmodRegistry);
+}
+
+/*
+ * Initialize 'registry' in a pre-existing shared memory region, which must be
+ * maximally aligned and have space for SharedRecordTypmodRegistryEstimate()
+ * bytes.
+ *
+ * 'area' will be used to allocate shared memory space as required for the
+ * typemod registration.  The current process, expected to be a leader process
+ * in a parallel query, will be attached automatically and its current record
+ * types will be loaded into *registry.  While attached, all calls to
+ * assign_record_type_typmod will use the shared registry.  Worker backends
+ * will need to attach explicitly.
+ *
+ * Note that this function takes 'area' and 'segment' as arguments rather than
+ * accessing them via CurrentSession, because they aren't installed there
+ * until after this function runs.
+ */
+void
+SharedRecordTypmodRegistryInit(SharedRecordTypmodRegistry *registry,
+							   dsm_segment *segment,
+							   dsa_area *area)
+{
+	MemoryContext old_context;
+	dshash_table *record_table;
+	dshash_table *typmod_table;
+	int32		typmod;
+
+	Assert(!IsParallelWorker());
+
+	/* We can't already be attached to a shared registry. */
+	Assert(CurrentSession->shared_typmod_registry == NULL);
+	Assert(CurrentSession->shared_record_table == NULL);
+	Assert(CurrentSession->shared_typmod_table == NULL);
+
+	old_context = MemoryContextSwitchTo(TopMemoryContext);
+
+	/* Create the hash table of tuple descriptors indexed by themselves. */
+	record_table = dshash_create(area, &srtr_record_table_params, area);
+
+	/* Create the hash table of tuple descriptors indexed by typmod. */
+	typmod_table = dshash_create(area, &srtr_typmod_table_params, NULL);
+
+	MemoryContextSwitchTo(old_context);
+
+	/* Initialize the SharedRecordTypmodRegistry. */
+	registry->record_table_handle = dshash_get_hash_table_handle(record_table);
+	registry->typmod_table_handle = dshash_get_hash_table_handle(typmod_table);
+	pg_atomic_init_u32(&registry->next_typmod, NextRecordTypmod);
+
+	/*
+	 * Copy all entries from this backend's private registry into the shared
+	 * registry.
+	 */
+	for (typmod = 0; typmod < NextRecordTypmod; ++typmod)
+	{
+		SharedTypmodTableEntry *typmod_table_entry;
+		SharedRecordTableEntry *record_table_entry;
+		SharedRecordTableKey record_table_key;
+		dsa_pointer shared_dp;
+		TupleDesc	tupdesc;
+		bool		found;
+
+		tupdesc = RecordCacheArray[typmod];
+		if (tupdesc == NULL)
+			continue;
+
+		/* Copy the TupleDesc into shared memory. */
+		shared_dp = share_tupledesc(area, tupdesc, typmod);
+
+		/* Insert into the typmod table. */
+		typmod_table_entry = dshash_find_or_insert(typmod_table,
+												   &tupdesc->tdtypmod,
+												   &found);
+		if (found)
+			elog(ERROR, "cannot create duplicate shared record typmod");
+		typmod_table_entry->typmod = tupdesc->tdtypmod;
+		typmod_table_entry->shared_tupdesc = shared_dp;
+		dshash_release_lock(typmod_table, typmod_table_entry);
+
+		/* Insert into the record table. */
+		record_table_key.shared = false;
+		record_table_key.local_tupdesc = tupdesc;
+		record_table_entry = dshash_find_or_insert(record_table,
+												   &record_table_key,
+												   &found);
+		if (!found)
+		{
+			record_table_entry->key.shared = true;
+			record_table_entry->key.shared_tupdesc = shared_dp;
+		}
+		dshash_release_lock(record_table, record_table_entry);
+	}
+
+	/*
+	 * Set up the global state that will tell assign_record_type_typmod and
+	 * lookup_rowtype_tupdesc_internal about the shared registry.
+	 */
+	CurrentSession->shared_record_table = record_table;
+	CurrentSession->shared_typmod_table = typmod_table;
+	CurrentSession->shared_typmod_registry = registry;
+
+	/*
+	 * We install a detach hook in the leader, but only to handle cleanup on
+	 * failure during GetSessionDsmHandle().  Once GetSessionDsmHandle() pins
+	 * the memory, the leader process will use a shared registry until it
+	 * exits.
+	 */
+	on_dsm_detach(segment, shared_record_typmod_registry_detach, (Datum) 0);
+}
+
+/*
+ * Attach to 'registry', which must have been initialized already by another
+ * backend.  Future calls to assign_record_type_typmod and
+ * lookup_rowtype_tupdesc_internal will use the shared registry until the
+ * current session is detached.
+ */
+void
+SharedRecordTypmodRegistryAttach(SharedRecordTypmodRegistry *registry)
+{
+	MemoryContext old_context;
+	dshash_table *record_table;
+	dshash_table *typmod_table;
+
+	Assert(IsParallelWorker());
+
+	/* We can't already be attached to a shared registry. */
+	Assert(CurrentSession != NULL);
+	Assert(CurrentSession->segment != NULL);
+	Assert(CurrentSession->area != NULL);
+	Assert(CurrentSession->shared_typmod_registry == NULL);
+	Assert(CurrentSession->shared_record_table == NULL);
+	Assert(CurrentSession->shared_typmod_table == NULL);
+
+	/*
+	 * We can't already have typmods in our local cache, because they'd clash
+	 * with those imported by SharedRecordTypmodRegistryInit.  This should be
+	 * a freshly started parallel worker.  If we ever support worker
+	 * recycling, a worker would need to zap its local cache in between
+	 * servicing different queries, in order to be able to call this and
+	 * synchronize typmods with a new leader; see
+	 * shared_record_typmod_registry_detach().
+	 */
+	Assert(NextRecordTypmod == 0);
+
+	old_context = MemoryContextSwitchTo(TopMemoryContext);
+
+	/* Attach to the two hash tables. */
+	record_table = dshash_attach(CurrentSession->area,
+								 &srtr_record_table_params,
+								 registry->record_table_handle,
+								 CurrentSession->area);
+	typmod_table = dshash_attach(CurrentSession->area,
+								 &srtr_typmod_table_params,
+								 registry->typmod_table_handle,
+								 NULL);
+
+	MemoryContextSwitchTo(old_context);
+
+	/*
+	 * We install a different detach callback that performs a more complete
+	 * reset of backend local state.
+	 */
+	on_dsm_detach(CurrentSession->segment,
+				  shared_record_typmod_registry_worker_detach,
+				  PointerGetDatum(registry));
+
+	/*
+	 * Set up the session state that will tell assign_record_type_typmod and
+	 * lookup_rowtype_tupdesc_internal about the shared registry.
+	 */
+	CurrentSession->shared_typmod_registry = registry;
+	CurrentSession->shared_record_table = record_table;
+	CurrentSession->shared_typmod_table = typmod_table;
+}
+
 /*
  * TypeCacheRelCallback
  *		Relcache inval callback function
@@ -1858,3 +2218,213 @@ enum_oid_cmp(const void *left, const void *right)
 	else
 		return 0;
 }
+
+/*
+ * Copy 'tupdesc' into newly allocated shared memory in 'area', set its typmod
+ * to the given value and return a dsa_pointer.
+ */
+static dsa_pointer
+share_tupledesc(dsa_area *area, TupleDesc tupdesc, uint32 typmod)
+{
+	dsa_pointer shared_dp;
+	TupleDesc	shared;
+
+	shared_dp = dsa_allocate(area, TupleDescSize(tupdesc));
+	shared = (TupleDesc) dsa_get_address(area, shared_dp);
+	TupleDescCopy(shared, tupdesc);
+	shared->tdtypmod = typmod;
+
+	return shared_dp;
+}
+
+/*
+ * If we are attached to a SharedRecordTypmodRegistry, use it to find or
+ * create a shared TupleDesc that matches 'tupdesc'.  Otherwise return NULL.
+ * Tuple descriptors returned by this function are not reference counted, and
+ * will exist at least as long as the current backend remained attached to the
+ * current session.
+ */
+static TupleDesc
+find_or_make_matching_shared_tupledesc(TupleDesc tupdesc)
+{
+	TupleDesc	result;
+	SharedRecordTableKey key;
+	SharedRecordTableEntry *record_table_entry;
+	SharedTypmodTableEntry *typmod_table_entry;
+	dsa_pointer shared_dp;
+	bool		found;
+	uint32		typmod;
+
+	/* If not even attached, nothing to do. */
+	if (CurrentSession->shared_typmod_registry == NULL)
+		return NULL;
+
+	/* Try to find a matching tuple descriptor in the record table. */
+	key.shared = false;
+	key.local_tupdesc = tupdesc;
+	record_table_entry = (SharedRecordTableEntry *)
+		dshash_find(CurrentSession->shared_record_table, &key, false);
+	if (record_table_entry)
+	{
+		Assert(record_table_entry->key.shared);
+		dshash_release_lock(CurrentSession->shared_record_table,
+							record_table_entry);
+		result = (TupleDesc)
+			dsa_get_address(CurrentSession->area,
+							record_table_entry->key.shared_tupdesc);
+		Assert(result->tdrefcount == -1);
+
+		return result;
+	}
+
+	/* Allocate a new typmod number.  This will be wasted if we error out. */
+	typmod = (int)
+		pg_atomic_fetch_add_u32(&CurrentSession->shared_typmod_registry->next_typmod,
+								1);
+
+	/* Copy the TupleDesc into shared memory. */
+	shared_dp = share_tupledesc(CurrentSession->area, tupdesc, typmod);
+
+	/*
+	 * Create an entry in the typmod table so that others will understand this
+	 * typmod number.
+	 */
+	PG_TRY();
+	{
+		typmod_table_entry = (SharedTypmodTableEntry *)
+			dshash_find_or_insert(CurrentSession->shared_typmod_table,
+								  &typmod, &found);
+		if (found)
+			elog(ERROR, "cannot create duplicate shared record typmod");
+	}
+	PG_CATCH();
+	{
+		dsa_free(CurrentSession->area, shared_dp);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+	typmod_table_entry->typmod = typmod;
+	typmod_table_entry->shared_tupdesc = shared_dp;
+	dshash_release_lock(CurrentSession->shared_typmod_table,
+						typmod_table_entry);
+
+	/*
+	 * Finally create an entry in the record table so others with matching
+	 * tuple descriptors can reuse the typmod.
+	 */
+	record_table_entry = (SharedRecordTableEntry *)
+		dshash_find_or_insert(CurrentSession->shared_record_table, &key,
+							  &found);
+	if (found)
+	{
+		/*
+		 * Someone concurrently inserted a matching tuple descriptor since the
+		 * first time we checked.  Use that one instead.
+		 */
+		dshash_release_lock(CurrentSession->shared_record_table,
+							record_table_entry);
+
+		/* Might as well free up the space used by the one we created. */
+		found = dshash_delete_key(CurrentSession->shared_typmod_table,
+								  &typmod);
+		Assert(found);
+		dsa_free(CurrentSession->area, shared_dp);
+
+		/* Return the one we found. */
+		Assert(record_table_entry->key.shared);
+		result = (TupleDesc)
+			dsa_get_address(CurrentSession->area,
+							record_table_entry->key.shared);
+		Assert(result->tdrefcount == -1);
+
+		return result;
+	}
+
+	/* Store it and return it. */
+	record_table_entry->key.shared = true;
+	record_table_entry->key.shared_tupdesc = shared_dp;
+	dshash_release_lock(CurrentSession->shared_record_table,
+						record_table_entry);
+	result = (TupleDesc)
+		dsa_get_address(CurrentSession->area, shared_dp);
+	Assert(result->tdrefcount == -1);
+
+	return result;
+}
+
+/*
+ * Detach hook to forget about the current shared record typmod
+ * infrastructure.  This is registered directly in leader backends, and
+ * reached only in case of error or shutdown.  It's also reached indirectly
+ * via the worker detach callback below.
+ */
+static void
+shared_record_typmod_registry_detach(dsm_segment *segment, Datum datum)
+{
+	/* Be cautious here: maybe we didn't finish initializing. */
+	if (CurrentSession->shared_record_table != NULL)
+	{
+		dshash_detach(CurrentSession->shared_record_table);
+		CurrentSession->shared_record_table = NULL;
+	}
+	if (CurrentSession->shared_typmod_table != NULL)
+	{
+		dshash_detach(CurrentSession->shared_typmod_table);
+		CurrentSession->shared_typmod_table = NULL;
+	}
+	CurrentSession->shared_typmod_registry = NULL;
+}
+
+/*
+ * Deatch hook allowing workers to disconnect from shared record typmod
+ * registry.  The resulting state should allow a worker to attach to a
+ * different leader, if worker reuse pools are invented.
+ */
+static void
+shared_record_typmod_registry_worker_detach(dsm_segment *segment, Datum datum)
+{
+	/*
+	 * Forget everything we learned about record typmods as part of the
+	 * session we are disconnecting from, and return to the initial state.
+	 */
+	if (RecordCacheArray != NULL)
+	{
+		int32		i;
+
+		for (i = 0; i < RecordCacheArrayLen; ++i)
+		{
+			if (RecordCacheArray[i] != NULL)
+			{
+				TupleDesc	tupdesc = RecordCacheArray[i];
+
+				/*
+				 * Pointers to tuple descriptors in shared memory are not
+				 * reference counted, so we are not responsible for freeing
+				 * them.  They'll survive as long as the shared session
+				 * exists, which should be as long as the owning leader
+				 * backend exists.  In theory we do need to free local
+				 * reference counted tuple descriptors however, and we can't
+				 * do that with DescTupleDescRefCount() because we aren't
+				 * using a resource owner.  In practice we don't expect to
+				 * find any non-shared TupleDesc object in a worker.
+				 */
+				if (tupdesc->tdrefcount != -1)
+				{
+					Assert(tupdesc->tdrefcount > 0);
+					if (--tupdesc->tdrefcount == 0)
+						FreeTupleDesc(tupdesc);
+				}
+			}
+		}
+		pfree(RecordCacheArray);
+		RecordCacheArray = NULL;
+	}
+	if (RecordCacheHash != NULL)
+	{
+		hash_destroy(RecordCacheHash);
+		RecordCacheHash = NULL;
+	}
+	NextRecordTypmod = 0;
+	/* Call the code common to leader and worker detach. */
+	shared_record_typmod_registry_detach(segment, datum);
+}