Make XLog storage extensible

and allow extensions to override it
For now, it extends on `pread` and `pwrite` from/into segment files.
This is the minimum we need for full XLog encryption with pg_de.

src/backend/access/transam/xlog.c

@@ -61,6 +61,7 @@
 #include "access/xloginsert.h"
 #include "access/xlogreader.h"
 #include "access/xlogrecovery.h"
+#include "access/xlog_smgr.h"
 #include "access/xlogutils.h"
 #include "backup/basebackup.h"
 #include "catalog/catversion.h"
@@ -2442,7 +2443,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 					INSTR_TIME_SET_ZERO(start);
 
 				pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
-				written = pg_pwrite(openLogFile, from, nleft, startoffset);
+				written = xlog_smgr->seg_write(openLogFile, from, nleft, startoffset);
 				pgstat_report_wait_end();
 
 				/*

src/backend/access/transam/xlogreader.c

@@ -29,6 +29,7 @@
 #include "access/xlog_internal.h"
 #include "access/xlogreader.h"
 #include "access/xlogrecord.h"
+#include "access/xlog_smgr.h"
 #include "catalog/pg_control.h"
 #include "common/pg_lzcompress.h"
 #include "replication/origin.h"
@@ -63,6 +64,21 @@ static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
  */
 #define DEFAULT_DECODE_BUFFER_SIZE (64 * 1024)
 
+/* 
+ * XLog storage manager
+ *
+ * TODO: should be in xlog.c or new xlog_smgr.c ?
+ * Now it's here because pg_rewind and other tools compile only
+ * w/ xlogreader.c  
+ */
+XLogSmgr *xlog_smgr = &xlog_smgr_standard;
+
+void 
+SetXLogSmgr(XLogSmgr *xlsmgr)
+{
+	xlog_smgr = xlsmgr;
+}
+
 /*
  * Construct a string in state->errormsg_buf explaining what's wrong with
  * the current record being read.
@@ -1557,7 +1573,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+		readbytes = xlog_smgr->seg_read(state->seg.ws_file, p, segbytes, (off_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();

src/backend/access/transam/xlogrecovery.c

@@ -39,6 +39,7 @@
 #include "access/xlogprefetcher.h"
 #include "access/xlogreader.h"
 #include "access/xlogrecovery.h"
+#include "access/xlog_smgr.h"
 #include "access/xlogutils.h"
 #include "backup/basebackup.h"
 #include "catalog/pg_control.h"
@@ -3397,7 +3398,7 @@ retry:
 	readOff = targetPageOff;
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
+	r = xlog_smgr->seg_read(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
 	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];

src/backend/replication/walreceiver.c

@@ -57,6 +57,7 @@
 #include "access/xlog_internal.h"
 #include "access/xlogarchive.h"
 #include "access/xlogrecovery.h"
+#include "access/xlog_smgr.h"
 #include "catalog/pg_authid.h"
 #include "funcapi.h"
 #include "libpq/pqformat.h"
@@ -941,7 +942,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		/* OK to write the logs */
 		errno = 0;
 
-		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		byteswritten = xlog_smgr->seg_write(recvFile, buf, segbytes, (off_t) startoff);
 		if (byteswritten <= 0)
 		{
 			char		xlogfname[MAXFNAMELEN];

src/include/access/xlog_internal.h

@@ -78,8 +78,10 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
 #define XLP_BKP_REMOVABLE			0x0004
 /* Replaces a missing contrecord; see CreateOverwriteContrecordRecord */
 #define XLP_FIRST_IS_OVERWRITE_CONTRECORD 0x0008
+/* The page is encrypted */
+#define XLP_ENCRYPTED 0x0010
 /* All defined flag bits in xlp_info (used for validity checking of header) */
-#define XLP_ALL_FLAGS				0x000F
+#define XLP_ALL_FLAGS				0x001F
 
 #define XLogPageHeaderSize(hdr)		\
 	(((hdr)->xlp_info & XLP_LONG_HEADER) ? SizeOfXLogLongPHD : SizeOfXLogShortPHD)

src/include/access/xlog_smgr.h

@@ -0,0 +1,24 @@
+#ifndef XLOG_SMGR_H
+#define XLOG_SMGR_H
+
+#include "postgres.h"
+
+#include <unistd.h>
+
+/* XLog storage manager interface */
+typedef struct XLogSmgr {
+	ssize_t (*seg_read) (int fd, void *buf, size_t count, off_t offset);
+
+ 	ssize_t (*seg_write) (int fd, const void *buf, size_t count, off_t offset);
+} XLogSmgr;
+
+/* Default (standard) XLog storage manager */
+static const XLogSmgr xlog_smgr_standard = {
+	.seg_read = pg_pread,
+	.seg_write = pg_pwrite,
+};
+
+extern XLogSmgr *xlog_smgr;
+extern void SetXLogSmgr(XLogSmgr *xlsmgr);
+
+#endif							/* XLOG_SMGR_H */