Fix error handling of XLogReaderAllocate in case of OOM

Similarly to previous fix 9b8d478, commit 2c03216 has switched
XLogReaderAllocate() to use a set of palloc calls instead of malloc,
causing any callers of this function to fail with an error instead of
receiving a NULL pointer in case of out-of-memory error. Fix this by
using palloc_extended with MCXT_ALLOC_NO_OOM that will safely return
NULL in case of an OOM.

Michael Paquier, slightly modified by me.

src/backend/access/transam/xlog.c

@@ -1062,8 +1062,11 @@ XLogInsertRecord(XLogRecData *rdata, XLogRecPtr fpw_lsn)
 		if (!debug_reader)
 			debug_reader = XLogReaderAllocate(NULL, NULL);
 
-		if (!debug_reader ||
-			!DecodeXLogRecord(debug_reader, (XLogRecord *) recordBuf.data,
+		if (!debug_reader)
+		{
+			appendStringInfo(&buf, "error decoding record: out of memory");
+		}
+		else if	(!DecodeXLogRecord(debug_reader, (XLogRecord *) recordBuf.data,
 							  &errormsg))
 		{
 			appendStringInfo(&buf, "error decoding record: %s",

src/backend/access/transam/xlogreader.c

@@ -58,15 +58,18 @@ report_invalid_record(XLogReaderState *state, const char *fmt,...)
 /*
  * Allocate and initialize a new XLogReader.
  *
- * The returned XLogReader is palloc'd. (In FRONTEND code, that means that
- * running out-of-memory causes an immediate exit(1).
+ * Returns NULL if the xlogreader couldn't be allocated.
  */
 XLogReaderState *
 XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
 {
 	XLogReaderState *state;
 
-	state = (XLogReaderState *) palloc0(sizeof(XLogReaderState));
+	state = (XLogReaderState *)
+		palloc_extended(sizeof(XLogReaderState),
+						MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO);
+	if (!state)
+		return NULL;
 
 	state->max_block_id = -1;
 
@@ -74,17 +77,30 @@ XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
 	 * Permanently allocate readBuf.  We do it this way, rather than just
 	 * making a static array, for two reasons: (1) no need to waste the
 	 * storage in most instantiations of the backend; (2) a static char array
-	 * isn't guaranteed to have any particular alignment, whereas palloc()
-	 * will provide MAXALIGN'd storage.
+	 * isn't guaranteed to have any particular alignment, whereas
+	 * palloc_extended() will provide MAXALIGN'd storage.
 	 */
-	state->readBuf = (char *) palloc(XLOG_BLCKSZ);
+	state->readBuf = (char *) palloc_extended(XLOG_BLCKSZ,
+											  MCXT_ALLOC_NO_OOM);
+	if (!state->readBuf)
+	{
+		pfree(state);
+		return NULL;
+	}
 
 	state->read_page = pagereadfunc;
 	/* system_identifier initialized to zeroes above */
 	state->private_data = private_data;
 	/* ReadRecPtr and EndRecPtr initialized to zeroes above */
 	/* readSegNo, readOff, readLen, readPageTLI initialized to zeroes above */
-	state->errormsg_buf = palloc(MAX_ERRORMSG_LEN + 1);
+	state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1,
+										  MCXT_ALLOC_NO_OOM);
+	if (!state->errormsg_buf)
+	{
+		pfree(state->readBuf);
+		pfree(state);
+		return NULL;
+	}
 	state->errormsg_buf[0] = '\0';
 
 	/*

src/backend/replication/logical/logical.c

@@ -163,6 +163,11 @@ StartupDecodingContext(List *output_plugin_options,
 	ctx->slot = slot;
 
 	ctx->reader = XLogReaderAllocate(read_page, ctx);
+	if (!ctx->reader)
+		ereport(ERROR,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+
 	ctx->reader->private_data = ctx;
 
 	ctx->reorder = ReorderBufferAllocate();

src/bin/pg_rewind/parsexlog.c

@@ -70,6 +70,8 @@ extractPageMap(const char *datadir, XLogRecPtr startpoint, TimeLineID tli,
 	private.datadir = datadir;
 	private.tli = tli;
 	xlogreader = XLogReaderAllocate(&SimpleXLogPageRead, &private);
+	if (xlogreader == NULL)
+		pg_fatal("out of memory");
 
 	do
 	{
@@ -121,6 +123,8 @@ readOneRecord(const char *datadir, XLogRecPtr ptr, TimeLineID tli)
 	private.datadir = datadir;
 	private.tli = tli;
 	xlogreader = XLogReaderAllocate(&SimpleXLogPageRead, &private);
+	if (xlogreader == NULL)
+		pg_fatal("out of memory");
 
 	record = XLogReadRecord(xlogreader, ptr, &errormsg);
 	if (record == NULL)
@@ -171,6 +175,8 @@ findLastCheckpoint(const char *datadir, XLogRecPtr forkptr, TimeLineID tli,
 	private.datadir = datadir;
 	private.tli = tli;
 	xlogreader = XLogReaderAllocate(&SimpleXLogPageRead, &private);
+	if (xlogreader == NULL)
+		pg_fatal("out of memory");
 
 	searchptr = forkptr;
 	for (;;)