Teach DSM registry to ERROR if attaching to an uninitialized entry.

If DSM entry initialization fails, backends could try to use an
uninitialized DSM segment, DSA, or dshash table (since the entry is
still added to the registry).  To fix, keep track of whether
initialization completed, and ERROR if a backend tries to attach to
an uninitialized entry.  We could instead retry initialization as
needed, but that seemed complicated, error prone, and unlikely to
help most cases.  Furthermore, such problems probably indicate a
coding error.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/dd36d384-55df-4fc2-825c-5bc56c950fa9%40gmail.com
Backpatch-through: 17

src/backend/storage/ipc/dsm_registry.c

@@ -45,6 +45,7 @@ typedef struct DSMRegistryEntry
 	char		name[64];
 	dsm_handle	handle;
 	size_t		size;
+	bool		initialized;
 } DSMRegistryEntry;
 
 static const dshash_parameters dsh_params = {
@@ -158,8 +159,12 @@ GetNamedDSMSegment(const char *name, size_t size,
 	entry = dshash_find_or_insert(dsm_registry_table, name, found);
 	if (!(*found))
 	{
+		dsm_segment *seg;
+
+		entry->initialized = false;
+
 		/* Initialize the segment. */
-		dsm_segment *seg = dsm_create(size, 0);
+		seg = dsm_create(size, 0);
 
 		dsm_pin_segment(seg);
 		dsm_pin_mapping(seg);
@@ -169,13 +174,17 @@ GetNamedDSMSegment(const char *name, size_t size,
 
 		if (init_callback)
 			(*init_callback) (ret);
+
+		entry->initialized = true;
 	}
+	else if (!entry->initialized)
+		ereport(ERROR,
+				(errmsg("requested DSM segment \"%s\" failed initialization",
+						name)));
 	else if (entry->size != size)
-	{
 		ereport(ERROR,
-				(errmsg("requested DSM segment size does not match size of "
-						"existing segment")));
-	}
+				(errmsg("requested DSM segment \"%s\" does not match size of existing entry",
+						name)));
 	else
 	{
 		dsm_segment *seg = dsm_find_mapping(entry->handle);