Do not append snipped plans when out memory

Do not try to snip it if there is not enough memory to append the whole
plan, that is error prone and is not worth it. Instead, spit out a
warning when this occurs.

pg_show_plans.c

@@ -280,24 +280,23 @@ append_query_plan(ExplainState *es)
 
 	offset = 0;
 	for (i = 0; i < nest_level; i++)
-		offset += pgsp_cache->plan_len[i] + 1;
+		offset += pgsp_cache->plan_len[i] + 1; /* +1 for '\0'. */
 	space_left = max_plan_length - offset;
 
 	if (pgsp->plan_format == EXPLAIN_FORMAT_TEXT)
 		new_plan->len--; /* Discard '\n'. */
 
-	if (new_plan->len < space_left) { /* Enough space for a new plan. */
-		memcpy(pgsp_cache->plan + offset,
-		       new_plan->data, new_plan->len);
-		pgsp_cache->plan[offset + new_plan->len] = '\0';
-		pgsp_cache->plan_len[nest_level] = new_plan->len;
-	} else { /* No space left to hold a new plan, snip it. */
-		memcpy(pgsp_cache->plan + offset,
-		       new_plan->data, space_left);
-		pgsp_cache->plan[max_plan_length-1] = '\0';
-		pgsp_cache->plan[max_plan_length-2] = '|'; /* Snip indicator. */
-		pgsp_cache->plan_len[nest_level] = space_left;
+	if (space_left < new_plan->len+1) {
+		ereport(WARNING,
+		        errcode(ERRCODE_OUT_OF_MEMORY),
+		        errmsg("not enough memory to append new query plans"));
+		return;
 	}
+
+	memcpy(pgsp_cache->plan + offset,
+		   new_plan->data, new_plan->len);
+	pgsp_cache->plan[offset + new_plan->len] = '\0';
+	pgsp_cache->plan_len[nest_level] = new_plan->len;
 	pgsp_cache->db_id = MyDatabaseId;
 	pgsp_cache->n_plans = nest_level+1;
 }