Allow dynamic shared memory segments to be kept until shutdown.

Amit Kapila, reviewed by Kyotaro Horiguchi, with some further
changes by me.

src/backend/storage/ipc/dsm.c

@@ -885,6 +885,33 @@ dsm_keep_mapping(dsm_segment *seg)
 	}
 }
 
+/*
+ * Keep a dynamic shared memory segment until postmaster shutdown.
+ *
+ * This function should not be called more than once per segment;
+ * on Windows, doing so will create unnecessary handles which will
+ * consume system resources to no benefit.
+ *
+ * Note that this function does not arrange for the current process to
+ * keep the segment mapped indefinitely; if that behavior is desired,
+ * dsm_keep_mapping() should be used from each process that needs to
+ * retain the mapping.
+ */
+void
+dsm_keep_segment(dsm_segment *seg)
+{
+	/*
+	 * Bump reference count for this segment in shared memory. This will
+	 * ensure that even if there is no session which is attached to this
+	 * segment, it will remain until postmaster shutdown.
+	 */
+	LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+	dsm_control->item[seg->control_slot].refcnt++;
+	LWLockRelease(DynamicSharedMemoryControlLock);
+
+	dsm_impl_keep_segment(seg->handle, seg->impl_private);
+}
+
 /*
  * Find an existing mapping for a shared memory segment, if there is one.
  */