dsm: allow use in single user mode.

It might seem pointless to allow use of dsm in single user mode, but otherwise
subsystems might need dedicated single user mode code paths.

Besides changing the assert, all that's needed is to make some windows code
assuming the presence of postmaster conditional.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGL9hY_VY=+oUK+Gc1iSRx-Ls5qeYJ6q=dQVZnT3R63Taw@mail.gmail.com

src/backend/storage/ipc/dsm.c

@@ -397,6 +397,7 @@ static void
 dsm_backend_startup(void)
 {
 #ifdef EXEC_BACKEND
+	if (IsUnderPostmaster)
 	{
 		void	   *control_address = NULL;
 
@@ -496,8 +497,12 @@ dsm_create(Size size, int flags)
 	FreePageManager *dsm_main_space_fpm = dsm_main_space_begin;
 	bool		using_main_dsm_region = false;
 
-	/* Unsafe in postmaster (and pointless in a stand-alone backend). */
+	/*
-	Assert(IsUnderPostmaster);
+	 * Unsafe in postmaster. It might seem pointless to allow use of dsm in
+	 * single user mode, but otherwise some subsystems will need dedicated
+	 * single user mode code paths.
+	 */
+	Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
 
 	if (!dsm_init_done)
 		dsm_backend_startup();

src/backend/storage/ipc/dsm_impl.c

@@ -959,6 +959,7 @@ dsm_impl_pin_segment(dsm_handle handle, void *impl_private,
 	{
 #ifdef USE_DSM_WINDOWS
 		case DSM_IMPL_WINDOWS:
+			if (IsUnderPostmaster)
 			{
 				HANDLE		hmap;
 
@@ -984,8 +985,8 @@ dsm_impl_pin_segment(dsm_handle handle, void *impl_private,
 				 * is unpinned, dsm_impl_unpin_segment can close it.
 				 */
 				*impl_private_pm_handle = hmap;
-				break;
 			}
+			break;
 #endif
 		default:
 			break;
@@ -1008,6 +1009,7 @@ dsm_impl_unpin_segment(dsm_handle handle, void **impl_private)
 	{
 #ifdef USE_DSM_WINDOWS
 		case DSM_IMPL_WINDOWS:
+			if (IsUnderPostmaster)
 			{
 				if (*impl_private &&
 					!DuplicateHandle(PostmasterHandle, *impl_private,
@@ -1025,8 +1027,8 @@ dsm_impl_unpin_segment(dsm_handle handle, void **impl_private)
 				}
 
 				*impl_private = NULL;
-				break;
 			}
+			break;
 #endif
 		default:
 			break;