Fix leaking of small spilled subtransactions during logical decoding.

When, during logical decoding, a transaction gets too big, it's
contents get spilled to disk. Not just the top-transaction gets
spilled, but *also* all of its subtransactions, even if they're not
that large themselves.  Unfortunately we didn't clean up
such small spilled subtransactions from disk.

Fix that, by keeping better track of whether a transaction has been
spilled to disk.

Author: Andres Freund
Reported-By: Dmitriy Sarafannikov, Fabrízio de Royes Mello
Discussion:
    https://postgr.es/m/1457621358.355011041@f382.i.mail.ru
    https://postgr.es/m/CAFcNs+qNMhNYii4nxpO6gqsndiyxNDYV0S=JNq0v_sEE+9PHXg@mail.gmail.com
Backpatch: 9.4-, where logical decoding was introduced

Author: anarazel

src/backend/replication/logical/reorderbuffer.c

@@ -893,7 +893,7 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 	{
 		ReorderBufferChange *cur_change;
 
-		if (txn->nentries != txn->nentries_mem)
+		if (txn->serialized)
 		{
 			/* serialize remaining changes */
 			ReorderBufferSerializeTXN(rb, txn);
@@ -922,7 +922,7 @@ ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn)
 		{
 			ReorderBufferChange *cur_change;
 
-			if (cur_txn->nentries != cur_txn->nentries_mem)
+			if (cur_txn->serialized)
 			{
 				/* serialize remaining changes */
 				ReorderBufferSerializeTXN(rb, cur_txn);
@@ -1142,7 +1142,7 @@ ReorderBufferCleanupTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
 	Assert(found);
 
 	/* remove entries spilled to disk */
-	if (txn->nentries != txn->nentries_mem)
+	if (txn->serialized)
 		ReorderBufferRestoreCleanup(rb, txn);
 
 	/* deallocate */
@@ -2124,6 +2124,7 @@ ReorderBufferSerializeTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
 	Assert(spilled == txn->nentries_mem);
 	Assert(dlist_is_empty(&txn->changes));
 	txn->nentries_mem = 0;
+	txn->serialized = true;
 
 	if (fd != -1)
 		CloseTransientFile(fd);

src/include/replication/reorderbuffer.h

@@ -212,6 +212,15 @@ typedef struct ReorderBufferTXN
 	 */
 	uint64		nentries_mem;
 
+	/*
+	 * Has this transaction been spilled to disk?  It's not always possible to
+	 * deduce that fact by comparing nentries with nentries_mem, because
+	 * e.g. subtransactions of a large transaction might get serialized
+	 * together with the parent - if they're restored to memory they'd have
+	 * nentries_mem == nentries.
+	 */
+	bool		serialized;
+
 	/*
 	 * List of ReorderBufferChange structs, including new Snapshots and new
 	 * CommandIds