Allow dynamic allocation of shared memory segments.

Patch by myself and Amit Kapila.  Design help from Noah Misch.  Review
by Andres Freund.

src/backend/utils/misc/guc.c

@@ -61,6 +61,7 @@
 #include "replication/walreceiver.h"
 #include "replication/walsender.h"
 #include "storage/bufmgr.h"
+#include "storage/dsm_impl.h"
 #include "storage/standby.h"
 #include "storage/fd.h"
 #include "storage/proc.h"
@@ -385,6 +386,7 @@ static const struct config_enum_entry synchronous_commit_options[] = {
  */
 extern const struct config_enum_entry wal_level_options[];
 extern const struct config_enum_entry sync_method_options[];
+extern const struct config_enum_entry dynamic_shared_memory_options[];
 
 /*
  * GUC option variables that are exported from this module
@@ -3335,6 +3337,16 @@ static struct config_enum ConfigureNamesEnum[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"dynamic_shared_memory_type", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Selects the dynamic shared memory implementation used."),
+			NULL
+		},
+		&dynamic_shared_memory_type,
+		DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE, dynamic_shared_memory_options,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"wal_sync_method", PGC_SIGHUP, WAL_SETTINGS,
 			gettext_noop("Selects the method used for forcing WAL updates to disk."),

src/backend/utils/misc/postgresql.conf.sample

@@ -123,6 +123,13 @@
 #work_mem = 1MB				# min 64kB
 #maintenance_work_mem = 16MB		# min 1MB
 #max_stack_depth = 2MB			# min 100kB
+#dynamic_shared_memory_type = posix # the default is the first option
+					# supported by the operating system:
+					#   posix
+					#   sysv
+					#   windows
+					#   mmap
+					# use none to disable dynamic shared memory
 
 # - Disk -