Disallow setting MAX_PARTITION_BUFFERS to less than 2

david-rowley · david-rowley · commit c19615fe391c · 2024-07-30T20:19:59.000+12:00
Add some comments to mention that this value must be at least 2 and also add a StaticAssertDecl to cause compilation failure if anyone tries to build with an invalid value. The multiInsertBuffers list must have at least two elements due to how the code in CopyMultiInsertInfoFlush() pushes the current ResultRelInfo's CopyMultiInsertBuffer to the end of the list. If the first element is also the last element, bad things will happen. Author: Zhang Mingli <avamingli@gmail.com> Discussion: https://postgr.es/m/CAApHDvpQ6t9ROcqbD-OgqR04Kfq4vQKw79Vo6r5j%2BciHwsSfkA%40mail.gmail.com
diff --git a/src/backend/commands/copyfrom.c b/src/backend/commands/copyfrom.c
@@ -68,7 +68,10 @@
  */
 #define MAX_BUFFERED_BYTES		65535
 
-/* Trim the list of buffers back down to this number after flushing */
+/*
+ * Trim the list of buffers back down to this number after flushing.  This
+ * must be >= 2.
+ */
 #define MAX_PARTITION_BUFFERS	32
 
 /* Stores multi-insert data related to a single relation in CopyFrom. */
@@ -550,6 +553,13 @@ CopyMultiInsertInfoFlush(CopyMultiInsertInfo *miinfo, ResultRelInfo *curr_rri,
 		 */
 		if (buffer->resultRelInfo == curr_rri)
 		{
+			/*
+			 * The code below would misbehave if we were trying to reduce the
+			 * list to less than two items.
+			 */
+			StaticAssertDecl(MAX_PARTITION_BUFFERS >= 2,
+							 "MAX_PARTITION_BUFFERS must be >= 2");
+
 			miinfo->multiInsertBuffers = list_delete_first(miinfo->multiInsertBuffers);
 			miinfo->multiInsertBuffers = lappend(miinfo->multiInsertBuffers, buffer);
 			buffer = (CopyMultiInsertBuffer *) linitial(miinfo->multiInsertBuffers);
