[CF 6043] v6 - Checkpointer write combining

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/6043

The branch will be overwritten each time a new patch version is posted to
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Patch(es): https://www.postgresql.org/message-id/CAAKRu_atZuU3gYY6hVVuvqDW+mfXM+fCWCyYODbZPesxyr=y6g@mail.gmail.com
Author(s): Melanie Plageman

Author: Commitfest Bot

src/backend/access/transam/xlog.c

@@ -3102,21 +3102,26 @@ XLogBackgroundFlush(void)
 }
 
 /*
- * Test whether XLOG data has been flushed up to (at least) the given position.
+ * Test whether XLOG data has been flushed up to (at least) the given position
+ * or whether the minimum recovery point is updated past the given position.
  *
- * Returns true if a flush is still needed.  (It may be that someone else
- * is already in process of flushing that far, however.)
+ * Returns true if a flush is still needed or if the minimum recovery point
+ * must be updated.
+ *
+ * It is possible that someone else is already in the process of flushing that
+ * far or updating the minimum recovery point that far.
  */
 bool
 XLogNeedsFlush(XLogRecPtr record)
 {
 	/*
-	 * During recovery, we don't flush WAL but update minRecoveryPoint
-	 * instead. So "needs flush" is taken to mean whether minRecoveryPoint
-	 * would need to be updated.
+	 * During recovery, when WAL inserts are forbidden, "needs flush" is taken
+	 * to mean whether minRecoveryPoint would need to be updated.
 	 */
-	if (RecoveryInProgress())
+	if (!XLogInsertAllowed())
 	{
+		Assert(RecoveryInProgress());
+
 		/*
 		 * An invalid minRecoveryPoint means that we need to recover all the
 		 * WAL, i.e., we're doing crash recovery.  We never modify the control