Introduce dynamic shared memory areas.

Programmers discovered decades ago that it was useful to have a simple
interface for allocating and freeing memory, which is why malloc() and
free() were invented.  Unfortunately, those handy tools don't work
with dynamic shared memory segments because those are specific to
PostgreSQL and are not necessarily mapped at the same address in every
cooperating process.  So invent our own allocator instead.  This makes
it possible for processes cooperating as part of parallel query
execution to allocate and free chunks of memory without having to
reserve them prior to the start of execution.  It could also be used
for longer lived objects; for example, we could consider storing data
for pg_stat_statements or the stats collector in shared memory using
these interfaces, rather than writing them to files.  Basically,
anything that needs shared memory but can't predict in advance how
much it's going to need might find this useful.

Thomas Munro and Robert Haas.  The original code (of mine) on which
Thomas based his work was actually designed to be a new backend-local
memory allocator for PostgreSQL, but that hasn't gone anywhere - or
not yet, anyway.  Thomas took that work and performed major
refactoring and extensive modifications to make it work with dynamic
shared memory, including the addition of appropriate locking.

Discussion: CA+TgmobkeWptGwiNa+SGFWsTLzTzD-CeLz0KcE-y6LFgoUus4A@mail.gmail.com
Discussion: CAEepm=1z5WLuNoJ80PaCvz6EtG9dN0j-KuHcHtU6QEfcPP5-qA@mail.gmail.com

src/backend/utils/mmgr/Makefile

@@ -12,6 +12,6 @@ subdir = src/backend/utils/mmgr
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = aset.o freepage.o mcxt.o portalmem.o
+OBJS = aset.o dsa.o freepage.o mcxt.o portalmem.o
 
 include $(top_srcdir)/src/backend/common.mk

src/include/utils/dsa.h

@@ -0,0 +1,108 @@
+/*-------------------------------------------------------------------------
+ *
+ * dsa.h
+ *	  Dynamic shared memory areas.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/include/utils/dsa.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef DSA_H
+#define DSA_H
+
+#include "postgres.h"
+
+#include "port/atomics.h"
+#include "storage/dsm.h"
+
+/* The opaque type used for an area. */
+struct dsa_area;
+typedef struct dsa_area dsa_area;
+
+/*
+ * If this system doesn't support atomic operations on 64 bit values then
+ * we fall back to 32 bit dsa_pointer.  For testing purposes,
+ * USE_SMALL_DSA_POINTER can be defined to force the use of 32 bit
+ * dsa_pointer even on systems that support 64 bit atomics.
+ */
+#ifndef PG_HAVE_ATOMIC_U64_SUPPORT
+#define SIZEOF_DSA_POINTER 4
+#else
+#ifdef USE_SMALL_DSA_POINTER
+#define SIZEOF_DSA_POINTER 4
+#else
+#define SIZEOF_DSA_POINTER 8
+#endif
+#endif
+
+/*
+ * The type of 'relative pointers' to memory allocated by a dynamic shared
+ * area.  dsa_pointer values can be shared with other processes, but must be
+ * converted to backend-local pointers before they can be dereferenced.  See
+ * dsa_get_address.  Also, an atomic version and appropriately sized atomic
+ * operations.
+ */
+#if DSA_POINTER_SIZEOF == 4
+typedef uint32 dsa_pointer;
+typedef pg_atomic_uint32 dsa_pointer_atomic;
+#define dsa_pointer_atomic_init pg_atomic_init_u32
+#define dsa_pointer_atomic_read pg_atomic_read_u32
+#define dsa_pointer_atomic_write pg_atomic_write_u32
+#define dsa_pointer_atomic_fetch_add pg_atomic_fetch_add_u32
+#define dsa_pointer_atomic_compare_exchange pg_atomic_compare_exchange_u32
+#else
+typedef uint64 dsa_pointer;
+typedef pg_atomic_uint64 dsa_pointer_atomic;
+#define dsa_pointer_atomic_init pg_atomic_init_u64
+#define dsa_pointer_atomic_read pg_atomic_read_u64
+#define dsa_pointer_atomic_write pg_atomic_write_u64
+#define dsa_pointer_atomic_fetch_add pg_atomic_fetch_add_u64
+#define dsa_pointer_atomic_compare_exchange pg_atomic_compare_exchange_u64
+#endif
+
+/* A sentinel value for dsa_pointer used to indicate failure to allocate. */
+#define InvalidDsaPointer ((dsa_pointer) 0)
+
+/* Check if a dsa_pointer value is valid. */
+#define DsaPointerIsValid(x) ((x) != InvalidDsaPointer)
+
+/*
+ * The type used for dsa_area handles.  dsa_handle values can be shared with
+ * other processes, so that they can attach to them.  This provides a way to
+ * share allocated storage with other processes.
+ *
+ * The handle for a dsa_area is currently implemented as the dsm_handle
+ * for the first DSM segment backing this dynamic storage area, but client
+ * code shouldn't assume that is true.
+ */
+typedef dsm_handle dsa_handle;
+
+extern void dsa_startup(void);
+
+extern dsa_area *dsa_create(int tranche_id, const char *tranche_name);
+extern dsa_area *dsa_create_in_place(void *place, Size size,
+					int tranche_id, const char *tranche_name,
+					dsm_segment *segment);
+extern dsa_area *dsa_attach(dsa_handle handle);
+extern dsa_area *dsa_attach_in_place(void *place, dsm_segment *segment);
+extern void dsa_release_in_place(void *place);
+extern void dsa_on_dsm_detach_release_in_place(dsm_segment *, Datum);
+extern void dsa_on_shmem_exit_release_in_place(int, Datum);
+extern void dsa_pin_mapping(dsa_area *area);
+extern void dsa_detach(dsa_area *area);
+extern void dsa_pin(dsa_area *area);
+extern void dsa_unpin(dsa_area *area);
+extern void dsa_set_size_limit(dsa_area *area, Size limit);
+extern Size dsa_minimum_size(void);
+extern dsa_handle dsa_get_handle(dsa_area *area);
+extern dsa_pointer dsa_allocate(dsa_area *area, Size size);
+extern void dsa_free(dsa_area *area, dsa_pointer dp);
+extern void *dsa_get_address(dsa_area *area, dsa_pointer dp);
+extern void dsa_trim(dsa_area *area);
+extern void dsa_dump(dsa_area *area);
+
+#endif   /* DSA_H */

src/tools/pgindent/typedefs.list

@@ -2416,6 +2416,12 @@ dlist_iter
 dlist_mutable_iter
 dlist_node
 ds_state
+dsa_area
+dsa_area_control
+dsa_area_pool
+dsa_area_span
+dsa_segment_header
+dsa_segment_map
 dsm_control_header
 dsm_control_item
 dsm_handle