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/access/common/Makefile

@@ -13,6 +13,6 @@ top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
 OBJS = bufmask.o heaptuple.o indextuple.o printsimple.o printtup.o \
-	reloptions.o scankey.o tupconvert.o tupdesc.o
+	reloptions.o scankey.o session.o tupconvert.o tupdesc.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/access/common/session.c

@@ -0,0 +1,208 @@
+/*-------------------------------------------------------------------------
+ *
+ * session.c
+ *		Encapsulation of user session.
+ *
+ * This is intended to contain data that needs to be shared between backends
+ * performing work for a client session.  In particular such a session is
+ * shared between the leader and worker processes for parallel queries.  At
+ * some later point it might also become useful infrastructure for separating
+ * backends from client connections, e.g. for the purpose of pooling.
+ *
+ * Currently this infrastructure is used to share:
+ * - typemod registry for ephemeral row-types, i.e. BlessTupleDesc etc.
+ *
+ * Portions Copyright (c) 2017, PostgreSQL Global Development Group
+ *
+ * src/backend/access/common/session.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/session.h"
+#include "storage/lwlock.h"
+#include "storage/shm_toc.h"
+#include "utils/memutils.h"
+#include "utils/typcache.h"
+
+/* Magic number for per-session DSM TOC. */
+#define SESSION_MAGIC						0xabb0fbc9
+
+/*
+ * We want to create a DSA area to store shared state that has the same
+ * lifetime as a session.  So far, it's only used to hold the shared record
+ * type registry.  We don't want it to have to create any DSM segments just
+ * yet in common cases, so we'll give it enough space to hold a very small
+ * SharedRecordTypmodRegistry.
+ */
+#define SESSION_DSA_SIZE					0x30000
+
+/*
+ * Magic numbers for state sharing in the per-session DSM area.
+ */
+#define SESSION_KEY_DSA						UINT64CONST(0xFFFFFFFFFFFF0001)
+#define SESSION_KEY_RECORD_TYPMOD_REGISTRY	UINT64CONST(0xFFFFFFFFFFFF0002)
+
+/* This backend's current session. */
+Session    *CurrentSession = NULL;
+
+/*
+ * Set up CurrentSession to point to an empty Session object.
+ */
+void
+InitializeSession(void)
+{
+	CurrentSession = MemoryContextAllocZero(TopMemoryContext, sizeof(Session));
+}
+
+/*
+ * Initialize the per-session DSM segment if it isn't already initialized, and
+ * return its handle so that worker processes can attach to it.
+ *
+ * Unlike the per-context DSM segment, this segement and its contents are
+ * reused for future parallel queries.
+ *
+ * Return DSM_HANDLE_INVALID if a segment can't be allocated due to lack of
+ * resources.
+ */
+dsm_handle
+GetSessionDsmHandle(void)
+{
+	shm_toc_estimator estimator;
+	shm_toc    *toc;
+	dsm_segment *seg;
+	size_t		typmod_registry_size;
+	size_t		size;
+	void	   *dsa_space;
+	void	   *typmod_registry_space;
+	dsa_area   *dsa;
+	MemoryContext old_context;
+
+	/*
+	 * If we have already created a session-scope DSM segment in this backend,
+	 * return its handle.  The same segment will be used for the rest of this
+	 * backend's lifetime.
+	 */
+	if (CurrentSession->segment != NULL)
+		return dsm_segment_handle(CurrentSession->segment);
+
+	/* Otherwise, prepare to set one up. */
+	old_context = MemoryContextSwitchTo(TopMemoryContext);
+	shm_toc_initialize_estimator(&estimator);
+
+	/* Estimate space for the per-session DSA area. */
+	shm_toc_estimate_keys(&estimator, 1);
+	shm_toc_estimate_chunk(&estimator, SESSION_DSA_SIZE);
+
+	/* Estimate space for the per-session record typmod registry. */
+	typmod_registry_size = SharedRecordTypmodRegistryEstimate();
+	shm_toc_estimate_keys(&estimator, 1);
+	shm_toc_estimate_chunk(&estimator, typmod_registry_size);
+
+	/* Set up segment and TOC. */
+	size = shm_toc_estimate(&estimator);
+	seg = dsm_create(size, DSM_CREATE_NULL_IF_MAXSEGMENTS);
+	if (seg == NULL)
+	{
+		MemoryContextSwitchTo(old_context);
+
+		return DSM_HANDLE_INVALID;
+	}
+	toc = shm_toc_create(SESSION_MAGIC,
+						 dsm_segment_address(seg),
+						 size);
+
+	/* Create per-session DSA area. */
+	dsa_space = shm_toc_allocate(toc, SESSION_DSA_SIZE);
+	dsa = dsa_create_in_place(dsa_space,
+							  SESSION_DSA_SIZE,
+							  LWTRANCHE_SESSION_DSA,
+							  seg);
+	shm_toc_insert(toc, SESSION_KEY_DSA, dsa_space);
+
+
+	/* Create session-scoped shared record typmod registry. */
+	typmod_registry_space = shm_toc_allocate(toc, typmod_registry_size);
+	SharedRecordTypmodRegistryInit((SharedRecordTypmodRegistry *)
+								   typmod_registry_space, seg, dsa);
+	shm_toc_insert(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY,
+				   typmod_registry_space);
+
+	/*
+	 * If we got this far, we can pin the shared memory so it stays mapped for
+	 * the rest of this backend's life.  If we don't make it this far, cleanup
+	 * callbacks for anything we installed above (ie currently
+	 * SharedRecordTypemodRegistry) will run when the DSM segment is detached
+	 * by CurrentResourceOwner so we aren't left with a broken CurrentSession.
+	 */
+	dsm_pin_mapping(seg);
+	dsa_pin_mapping(dsa);
+
+	/* Make segment and area available via CurrentSession. */
+	CurrentSession->segment = seg;
+	CurrentSession->area = dsa;
+
+	MemoryContextSwitchTo(old_context);
+
+	return dsm_segment_handle(seg);
+}
+
+/*
+ * Attach to a per-session DSM segment provided by a parallel leader.
+ */
+void
+AttachSession(dsm_handle handle)
+{
+	dsm_segment *seg;
+	shm_toc    *toc;
+	void	   *dsa_space;
+	void	   *typmod_registry_space;
+	dsa_area   *dsa;
+	MemoryContext old_context;
+
+	old_context = MemoryContextSwitchTo(TopMemoryContext);
+
+	/* Attach to the DSM segment. */
+	seg = dsm_attach(handle);
+	if (seg == NULL)
+		elog(ERROR, "could not attach to per-session DSM segment");
+	toc = shm_toc_attach(SESSION_MAGIC, dsm_segment_address(seg));
+
+	/* Attach to the DSA area. */
+	dsa_space = shm_toc_lookup(toc, SESSION_KEY_DSA, false);
+	dsa = dsa_attach_in_place(dsa_space, seg);
+
+	/* Make them available via the current session. */
+	CurrentSession->segment = seg;
+	CurrentSession->area = dsa;
+
+	/* Attach to the shared record typmod registry. */
+	typmod_registry_space =
+		shm_toc_lookup(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY, false);
+	SharedRecordTypmodRegistryAttach((SharedRecordTypmodRegistry *)
+									 typmod_registry_space);
+
+	/* Remain attached until end of backend or DetachSession(). */
+	dsm_pin_mapping(seg);
+	dsa_pin_mapping(dsa);
+
+	MemoryContextSwitchTo(old_context);
+}
+
+/*
+ * Detach from the current session DSM segment.  It's not strictly necessary
+ * to do this explicitly since we'll detach automatically at backend exit, but
+ * if we ever reuse parallel workers it will become important for workers to
+ * detach from one session before attaching to another.  Note that this runs
+ * detach hooks.
+ */
+void
+DetachSession(void)
+{
+	/* Runs detach hooks. */
+	dsm_detach(CurrentSession->segment);
+	CurrentSession->segment = NULL;
+	dsa_detach(CurrentSession->area);
+	CurrentSession->area = NULL;
+}

src/backend/access/common/tupdesc.c

@@ -184,6 +184,22 @@ CreateTupleDescCopyConstr(TupleDesc tupdesc)
 	return desc;
 }
 
+/*
+ * TupleDescCopy
+ *		Copy a tuple descriptor into caller-supplied memory.
+ *		The memory may be shared memory mapped at any address, and must
+ *		be sufficient to hold TupleDescSize(src) bytes.
+ *
+ * !!! Constraints and defaults are not copied !!!
+ */
+void
+TupleDescCopy(TupleDesc dst, TupleDesc src)
+{
+	memcpy(dst, src, TupleDescSize(src));
+	dst->constr = NULL;
+	dst->tdrefcount = -1;
+}
+
 /*
  * TupleDescCopyEntry
  *		This function copies a single attribute structure from one tuple