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

1 files changed, 44 insertions, 0 deletions

diff --git a/src/include/access/session.h b/src/include/access/session.h
new file mode 100644
index 00000000000..8376dc53127
--- /dev/null
+++ b/src/include/access/session.h
@@ -0,0 +1,44 @@
+/*-------------------------------------------------------------------------
+ *
+ * session.h
+ *	  Encapsulation of user session.
+ *
+ * Copyright (c) 2017, PostgreSQL Global Development Group
+ *
+ * src/include/access/session.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SESSION_H
+#define SESSION_H
+
+#include "lib/dshash.h"
+
+/* Defined in typcache.c */
+typedef struct SharedRecordTypmodRegistry SharedRecordTypmodRegistry;
+
+/*
+ * A struct encapsulating some elements of a user's session.  For now this
+ * manages state that applies to parallel query, but it principle it could
+ * include other things that are currently global variables.
+ */
+typedef struct Session
+{
+	dsm_segment *segment;		/* The session-scoped DSM segment. */
+	dsa_area   *area;			/* The session-scoped DSA area. */
+
+	/* State managed by typcache.c. */
+	SharedRecordTypmodRegistry *shared_typmod_registry;
+	dshash_table *shared_record_table;
+	dshash_table *shared_typmod_table;
+} Session;
+
+extern void InitializeSession(void);
+extern dsm_handle GetSessionDsmHandle(void);
+extern void AttachSession(dsm_handle handle);
+extern void DetachSession(void);
+
+/* The current session, or NULL for none. */
+extern Session *CurrentSession;
+
+#endif							/* SESSION_H */