Subject: [PATCH v21 1/2] Add WAL relief vent for replication slots

From: Kyotaro Horiguchi <[email-masked]>
Date: Wed, 19 Dec 2018 12:43:57 +0900

Replication slot is useful to maintain replication connection in the
configurations where replication is so delayed that connection is
broken. On the other hand so many WAL files can fill up disk that the
master downs by a long delay. This feature, which is activated by a
GUC "max_slot_wal_keep_size", protects master servers from suffering
disk full by limiting the number of WAL files reserved by replication
slots.
---
 doc/src/sgml/catalogs.sgml                    |  48 +++
 doc/src/sgml/config.sgml                      |  23 ++
 doc/src/sgml/high-availability.sgml           |   8 +-
 src/backend/access/transam/xlog.c             | 322 ++++++++++++++++--
 src/backend/catalog/system_views.sql          |   4 +-
 src/backend/replication/slot.c                |   1 +
 src/backend/replication/slotfuncs.c           |  39 ++-
 src/backend/utils/misc/guc.c                  |  13 +
 src/backend/utils/misc/postgresql.conf.sample |   1 +
 src/include/access/xlog.h                     |  19 ++
 src/include/catalog/pg_proc.dat               |   6 +-
 src/test/recovery/t/018_replslot_limit.pl     | 203 +++++++++++
 src/test/regress/expected/rules.out           |   6 +-
 13 files changed, 657 insertions(+), 36 deletions(-)
 create mode 100644 src/test/recovery/t/018_replslot_limit.pl

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 64614b569c..01a7802ed4 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -9907,6 +9907,54 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       </entry>
      </row>
 
+     <row>
+      <entry><structfield>wal_status</structfield></entry>
+      <entry><type>text</type></entry>
+      <entry></entry>
+
+      <entry>Availability of WAL files claimed by this slot.
+      Valid values are:
+       <simplelist>
+        <member>
+         <literal>normal</literal> means that the claimed files
+         are within <varname>max_wal_size</varname>
+        </member>
+        <member>
+         <literal>keeping</literal> means that <varname>max_wal_size</varname>
+         is exceeded but still held by replication slots or
+         <varname>wal_keep_segments</varname>
+        </member>
+        <member>
+         <literal>losing</literal> means that some of the files are on the verge
+         of deletion, but can still be accessed by a session that's currently
+         reading it
+        </member>
+        <member>
+         <literal>lost</literal> means that some of them are definitely lost
+         and the session using this slot cannot continue replication.
+         This state also implies that the session using this slot has been
+         stopped.
+        </member>
+       </simplelist>
+      The last two states are seen only when
+      <xref linkend="guc-max-slot-wal-keep-size"/> is
+      non-negative. If <structfield>restart_lsn</structfield> is NULL, this
+      field is null.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>remain</structfield></entry>
+      <entry><type>bigint</type></entry>
+      <entry></entry>
+      <entry>The amount in bytes of WAL that can be written before this slot
+        loses required WAL files.
+        If <structfield>restart_lsn</structfield> is null or
+        <structfield>wal_status</structfield> is <literal>losing</literal>
+        or <literal>lost</literal>, this field is null.
+      </entry>
+     </row>
+
     </tbody>
    </tgroup>
   </table>
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2de21903a1..dc99c6868a 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3758,6 +3758,29 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
       </listitem>
      </varlistentry>
 
+      <varlistentry id="guc-max-slot-wal-keep-size" xreflabel="max_slot_wal_keep_size">
+       <term><varname>max_slot_wal_keep_size</varname> (<type>integer</type>)
+       <indexterm>
+        <primary><varname>max_slot_wal_keep_size</varname> configuration parameter</primary>
+       </indexterm>
+       </term>
+       <listitem>
+       <para>
+        Specify the maximum size of WAL files
+        that <link linkend="streaming-replication-slots">replication
+        slots</link> are allowed to retain in the <filename>pg_wal</filename>
+        directory at checkpoint time.
+        If <varname>max_slot_wal_keep_size</varname> is -1 (the default),
+        replication slots retain unlimited amount of WAL files.  If
+        restart_lsn of a replication slot gets behind more than that megabytes
+        from the current LSN, the standby using the slot may no longer be able
+        to continue replication due to removal of required WAL files. You
+        can see the WAL availability of replication slots
+        in <link linkend="view-pg-replication-slots">pg_replication_slots</link>.
+       </para>
+       </listitem>
+      </varlistentry>
+
      <varlistentry id="guc-wal-sender-timeout" xreflabel="wal_sender_timeout">
       <term><varname>wal_sender_timeout</varname> (<type>integer</type>)
       <indexterm>
diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index b5d32bb720..624e5f94ad 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -925,9 +925,11 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
     <xref linkend="guc-archive-command"/>.
     However, these methods often result in retaining more WAL segments than
     required, whereas replication slots retain only the number of segments
-    known to be needed.  An advantage of these methods is that they bound
-    the space requirement for <literal>pg_wal</literal>; there is currently no way
-    to do this using replication slots.
+    known to be needed.  On the other hand, replication slots can retain so
+    many WAL segments that they fill up the space allocated
+    for <literal>pg_wal</literal>;
+    <xref linkend="guc-max-slot-wal-keep-size"/> limits the size of WAL files
+    retained by replication slots.
    </para>
    <para>
     Similarly, <xref linkend="guc-hot-standby-feedback"/>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 977d448f50..db7494e741 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -106,6 +106,7 @@ int			wal_level = WAL_LEVEL_MINIMAL;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 int			wal_retrieve_retry_interval = 5000;
+int			max_slot_wal_keep_size_mb = -1;
 
 #ifdef WAL_DEBUG
 bool		XLOG_DEBUG = false;
@@ -759,7 +760,7 @@ static ControlFileData *ControlFile = NULL;
  */
 #define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)
 
-/* Convert min_wal_size_mb and max_wal_size_mb to equivalent segment count */
+/* Convert values of GUCs measured in megabytes to equiv. segment count */
 #define ConvertToXSegs(x, segsize)	\
 	(x / ((segsize) / (1024 * 1024)))
 
@@ -896,6 +897,7 @@ static void checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI,
 static void LocalSetXLogInsertAllowed(void);
 static void CreateEndOfRecoveryRecord(void);
 static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
+static XLogSegNo GetOldestKeepSegment(XLogRecPtr currpos, XLogRecPtr minSlotPtr);
 static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);
 static XLogRecPtr XLogGetReplicationSlotMinimumLSN(void);
 
@@ -3930,8 +3932,21 @@ XLogGetLastRemovedSegno(void)
 }
 
 /*
- * Update the last removed segno pointer in shared memory, to reflect
- * that the given XLOG file has been removed.
+ * Return the oldest extant WAL segment file.
+ *
+ * When XLogGetLastRemovedSegno returns a non-zero value, then the oldest
+ * valid segment is one further from that; and if it returns zero, then
+ * it is the segment numbered on.
+ */
+XLogSegNo
+FindOldestXLogFileSegNo(void)
+{
+	return XLogGetLastRemovedSegno() + 1;
+}
+
+/*
+ * Update the last removed segno pointer in shared memory, to reflect that the
+ * given XLOG file has been removed.
  */
 static void
 UpdateLastRemovedPtr(char *filename)
@@ -9451,6 +9466,197 @@ CreateRestartPoint(int flags)
 	return true;
 }
 
+/*
+ * Detect availability of the record at given targetLSN.
+ *
+ * targetLSN is restart_lsn of a slot.
+ * walsender_pid is the slot's walsender PID.
+ *
+ * Returns one of the following enum values.
+ *
+ * WALAVAIL_NORMAL means targetLSN is available because it is in the range of
+ * max_wal_size.  If max_slot_wal_keep_size is smaller than max_wal_size, this
+ * state is not returned.
+ *
+ * WALAVAIL_PRESERVED means it is still available by preserving extra segments
+ * beyond max_wal_size.
+ *
+ * WALAVAIL_BEING_REMOVED means it is being removed or already removed but the
+ * replication stream on the given slot is live yet. The state may transit to
+ * WALAVAIL_PRESERVED or WALAVAIL_NORMAL state if the walsender advances
+ * restart_lsn.
+ *
+ * WALAVAIL_REMOVED means it is definitely lost. The replication stream on the
+ * slot cannot continue.
+ *
+ * returns WALAVAIL_NULL if restart_lsn is invalid.
+ */
+WalAvailability
+GetWalAvailability(XLogRecPtr restart_lsn, pid_t walsender_pid)
+{
+	XLogRecPtr currpos;
+	XLogRecPtr slotPtr;
+	XLogSegNo currSeg;		/* segid of currpos */
+	XLogSegNo restartSeg;	/* segid of restart_lsn */
+	XLogSegNo oldestSeg;	/* actual oldest segid */
+	XLogSegNo oldestSegMaxWalSize;	/* oldest segid kept by max_wal_size */
+	XLogSegNo oldestSlotSeg;/* oldest segid kept by slot */
+	uint64	  keepSegs;
+
+	/* the case where the slot has never been activated */
+	if (XLogRecPtrIsInvalid(restart_lsn))
+		return WALAVAIL_INVALID_LSN;
+
+	currpos = GetXLogWriteRecPtr();
+
+	/* calculate oldest segment currently needed by slots */
+	XLByteToSeg(restart_lsn, restartSeg, wal_segment_size);
+	slotPtr = XLogGetReplicationSlotMinimumLSN();
+	oldestSlotSeg = GetOldestKeepSegment(currpos, slotPtr);
+
+	/* find the oldest extant segment file */
+	oldestSeg = FindOldestXLogFileSegNo();
+
+	/* calculate oldest segment by max_wal_size */
+	XLByteToSeg(currpos, currSeg, wal_segment_size);
+	keepSegs = ConvertToXSegs(max_wal_size_mb, wal_segment_size) + 1;
+
+	if (currSeg > keepSegs)
+		oldestSegMaxWalSize = currSeg - keepSegs;
+	else
+		oldestSegMaxWalSize = 1;
+
+	/*
+	 * If max_slot_wal_keep_size has changed after the last call, the segment
+	 * that would been kept by the current setting might have been lost by the
+	 * previous setting. No point in showing normal or keeping status values if
+	 * the restartSeg is known to be lost.
+	 */
+	if (restartSeg >= oldestSeg)
+	{
+		/*
+		 * show "normal" when restartSeg is within max_wal_size. If
+		 * max_slot_wal_keep_size is smaller than max_wal_size, there's no
+		 * point in showing the status.
+		 */
+		if ((max_slot_wal_keep_size_mb <= 0 ||
+			 max_slot_wal_keep_size_mb >= max_wal_size_mb) &&
+			oldestSegMaxWalSize <= restartSeg)
+			return WALAVAIL_NORMAL;
+
+		/* being retained by slots */
+		if (oldestSlotSeg <= restartSeg)
+			return WALAVAIL_PRESERVED;
+	}
+	
+	/*
+	 * The segment is already lost or being lost. If the oldest segment is just
+	 * after the restartSeg, running walsender may be reading the just removed
+	 * segment. The walsender may safely move to the oldest existing segment in
+	 * that case.
+	 */
+	if (oldestSeg == restartSeg + 1 && walsender_pid != 0)
+		return WALAVAIL_BEING_REMOVED;
+
+	/* definitely lost. the walsender can no longer restart */
+	return WALAVAIL_REMOVED;
+}
+
+/*
+ * Returns minimum segment number that the next checkpoint must leave
+ * considering wal_keep_segments, replication slots and
+ * max_slot_wal_keep_size.
+ *
+ * currLSN is the current insert location.
+ * minSlotLSN is the minimum restart_lsn of all active slots.
+ */
+static XLogSegNo
+GetOldestKeepSegment(XLogRecPtr currLSN, XLogRecPtr minSlotLSN)
+{
+	XLogSegNo	currSeg;
+	XLogSegNo	minSlotSeg;
+	uint64		keepSegs = 0;	/* # of segments actually kept */
+
+	XLByteToSeg(currLSN, currSeg, wal_segment_size);
+	XLByteToSeg(minSlotLSN, minSlotSeg, wal_segment_size);
+
+	/*
+	 * Calculate how many segments are kept by slots first. The second
+	 * term of the condition is just a sanity check.
+	 */
+	if (minSlotLSN != InvalidXLogRecPtr && minSlotSeg <= currSeg)
+		keepSegs = currSeg - minSlotSeg;
+
+	/* Cap keepSegs by max_slot_wal_keep_size */
+	if (max_slot_wal_keep_size_mb >= 0)
+	{
+		uint64 limitSegs;
+
+		limitSegs = ConvertToXSegs(max_slot_wal_keep_size_mb, wal_segment_size);
+
+		/* Reduce it if slots already reserves too many. */
+		if (limitSegs < keepSegs)
+			keepSegs = limitSegs;
+	}
+
+	/* but, keep at least wal_keep_segments segments if any */
+	if (wal_keep_segments > 0 && keepSegs < wal_keep_segments)
+		keepSegs = wal_keep_segments;
+
+	/* avoid underflow, don't go below 1 */
+	if (currSeg <= keepSegs)
+		return 1;
+
+	return currSeg - keepSegs;
+}
+
+/*
+ * Calculate remaining bytes until WAL segment for targetLSN will be removed.
+ */
+int64
+DistanceToWalRemoval(XLogRecPtr currLSN, XLogRecPtr targetLSN)
+{
+	XLogSegNo	currSeg;
+	uint64		limitSegs = 0;
+	int64 		restbytes;
+	uint64		fragbytes;
+	XLogSegNo	targetSeg;
+
+	XLByteToSeg(currLSN, currSeg, wal_segment_size);
+
+	/* Calculate how far back WAL segments are preserved */
+	if (max_slot_wal_keep_size_mb >= 0)
+		limitSegs = ConvertToXSegs(max_slot_wal_keep_size_mb, wal_segment_size);
+
+	if (wal_keep_segments > 0 && limitSegs < wal_keep_segments)
+		limitSegs = wal_keep_segments;
+
+	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+
+	/* avoid underflow */
+	if (targetSeg + limitSegs < currSeg)
+		return 0;
+
+	/*
+	 * This slot still has all required segments. Calculate how
+	 * many LSN bytes the slot has until it loses targetLSN.
+	 */
+	fragbytes = wal_segment_size - (currLSN % wal_segment_size);
+	XLogSegNoOffsetToRecPtr(targetSeg + limitSegs - currSeg,
+							fragbytes, wal_segment_size,
+							restbytes);
+
+	/*
+	 * Not realistic, but make sure that it is not out of the
+	 * range of int64. No problem to do so since such large values
+	 * have no significant difference.
+	 */
+	if (restbytes > PG_INT64_MAX)
+		restbytes = PG_INT64_MAX;
+
+	return restbytes;
+}
+
 /*
  * Retreat *logSegNo to the last segment that we need to retain because of
  * either wal_keep_segments or replication slots.
@@ -9462,38 +9668,102 @@ CreateRestartPoint(int flags)
 static void
 KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 {
-	XLogSegNo	segno;
-	XLogRecPtr	keep;
+	static XLogSegNo last_lost_segs = 0;
+	static int last_nslots = 0;
+	static char *last_slot_name = NULL;
+	XLogRecPtr	slotminptr = InvalidXLogRecPtr;
+	XLogSegNo	minSegNo;
+	XLogSegNo	minSlotSegNo;
+	int			nslots_affected = 0;
 
-	XLByteToSeg(recptr, segno, wal_segment_size);
-	keep = XLogGetReplicationSlotMinimumLSN();
+	if (max_replication_slots > 0)
+		slotminptr = XLogGetReplicationSlotMinimumLSN();
 
-	/* compute limit for wal_keep_segments first */
-	if (wal_keep_segments > 0)
+	/*
+	 * We should keep certain number of WAL segments after this checkpoint.
+	 */
+	minSegNo = GetOldestKeepSegment(recptr, slotminptr);
+
+	/*
+	 * Warn the checkpoint is going to flush the segments required by
+	 * replication slots.
+	 */
+	if (!XLogRecPtrIsInvalid(slotminptr))
 	{
-		/* avoid underflow, don't go below 1 */
-		if (segno <= wal_keep_segments)
-			segno = 1;
-		else
-			segno = segno - wal_keep_segments;
+		Assert (max_replication_slots > 0);
+
+		XLByteToSeg(slotminptr, minSlotSegNo, wal_segment_size);
+
+		if (minSlotSegNo < minSegNo)
+		{
+			/* Some slots has lost required segments */
+			XLogSegNo	lost_segs = minSegNo - minSlotSegNo;
+			ReplicationSlot *earliest = NULL;
+			char	   *earliest_name = NULL;
+			int			i;
+
+			/* Find the most affected slot */
+			LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
+			for (i = 0 ; i < max_replication_slots ; i++)
+			{
+				ReplicationSlot *s =
+					&ReplicationSlotCtl->replication_slots[i];
+				XLogSegNo slotSegNo;
+
+				XLByteToSeg(s->data.restart_lsn, slotSegNo, wal_segment_size);
+
+				if (s->in_use && s->active_pid == 0 && slotSegNo < minSegNo)
+				{
+					nslots_affected++;
+
+					if (earliest == NULL ||
+						s->data.restart_lsn < earliest->data.restart_lsn)
+						earliest = s;
+				}
+			}
+
+			if (earliest)
+			{
+				MemoryContext oldcxt = MemoryContextSwitchTo(TopMemoryContext);
+				earliest_name = pstrdup(NameStr(earliest->data.name));
+				MemoryContextSwitchTo(oldcxt);
+			}
+
+			LWLockRelease(ReplicationSlotControlLock);
+
+			/* Emit WARNING if something has changed */
+			if (earliest_name &&
+				(last_lost_segs != lost_segs || last_nslots != nslots_affected))
+			{
+				ereport(WARNING,
+						(errmsg_plural ("%d replication slot has lost required WAL segments by %lu segments",
+										"%d replication slots have lost required WAL segments by %lu segments",
+										nslots_affected, nslots_affected,
+										lost_segs),
+						 errdetail("Most affected slot is %s.",
+								   earliest_name)));
+
+				if (last_slot_name)
+					pfree(last_slot_name);
+				last_slot_name = earliest_name;
+				last_lost_segs = lost_segs;
+				last_nslots = nslots_affected;
+			}
+		}
 	}
 
-	/* then check whether slots limit removal further */
-	if (max_replication_slots > 0 && keep != InvalidXLogRecPtr)
+	/* Reset the state if no affected slots remain. */
+	if (nslots_affected == 0 && last_slot_name)
 	{
-		XLogSegNo	slotSegNo;
-
-		XLByteToSeg(keep, slotSegNo, wal_segment_size);
-
-		if (slotSegNo <= 0)
-			segno = 1;
-		else if (slotSegNo < segno)
-			segno = slotSegNo;
+		pfree(last_slot_name);
+		last_slot_name = NULL;
+		last_lost_segs = 0;
+		last_nslots = 0;
 	}
 
 	/* don't delete WAL segments newer than the calculated segment */
-	if (segno < *logSegNo)
-		*logSegNo = segno;
+	if (minSegNo < *logSegNo)
+		*logSegNo = minSegNo;
 }
 
 /*
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 83d00c6cde..775b8b7f20 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -863,7 +863,9 @@ CREATE VIEW pg_replication_slots AS
             L.xmin,
             L.catalog_xmin,
             L.restart_lsn,
-            L.confirmed_flush_lsn
+            L.confirmed_flush_lsn,
+            L.wal_status,
+            L.remain
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index d90c7235e9..a26f7999aa 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -49,6 +49,7 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/builtins.h"
+#include "utils/memutils.h"
 
 /*
  * Replication slot on-disk data structure.
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index ce0c9127bc..47cd4375a1 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -234,7 +234,7 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 Datum
 pg_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_GET_REPLICATION_SLOTS_COLS 11
+#define PG_GET_REPLICATION_SLOTS_COLS 13
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	TupleDesc	tupdesc;
 	Tuplestorestate *tupstore;
@@ -288,6 +288,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		Oid			database;
 		NameData	slot_name;
 		NameData	plugin;
+		WalAvailability walstate;
 		int			i;
 
 		if (!slot->in_use)
@@ -355,6 +356,42 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		else
 			nulls[i++] = true;
 
+		walstate = GetWalAvailability(restart_lsn, active_pid);
+
+		switch (walstate)
+		{
+			case WALAVAIL_INVALID_LSN:
+				nulls[i++] = true;
+				break;
+
+			case WALAVAIL_NORMAL:
+				values[i++] = CStringGetTextDatum("normal");
+				break;
+
+			case WALAVAIL_PRESERVED:
+				values[i++] = CStringGetTextDatum("keeping");
+				break;
+
+			case WALAVAIL_BEING_REMOVED:
+				values[i++] = CStringGetTextDatum("losing");
+				break;
+
+			case WALAVAIL_REMOVED:
+				values[i++] = CStringGetTextDatum("lost");
+				break;
+		}
+
+		if (max_slot_wal_keep_size_mb >= 0 &&
+			(walstate == WALAVAIL_NORMAL ||
+			 walstate == WALAVAIL_PRESERVED))
+		{
+			values[i++] =
+				Int64GetDatum(DistanceToWalRemoval(GetXLogWriteRecPtr(),
+												   restart_lsn));
+		}
+		else
+			nulls[i++] = true;
+
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
 	}
 	LWLockRelease(ReplicationSlotControlLock);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 79bc7ac8ca..a4f0a4e0e3 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2771,6 +2771,19 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"max_slot_wal_keep_size", PGC_SIGHUP, REPLICATION_SENDING,
+			gettext_noop("Sets the maximum WAL size that can be reserved by replication slots."),
+			gettext_noop("Replication slots will be marked as failed, and segments released "
+						 "for deletion or recycling, if this much space is occupied by WAL "
+						 "on disk."),
+			GUC_UNIT_MB
+		},
+		&max_slot_wal_keep_size_mb,
+		-1, -1, MAX_KILOBYTES,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"wal_sender_timeout", PGC_USERSET, REPLICATION_SENDING,
 			gettext_noop("Sets the maximum time to wait for WAL replication."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index e9f8ca775d..0b696e7044 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -287,6 +287,7 @@
 #max_wal_senders = 10		# max number of walsender processes
 				# (change requires restart)
 #wal_keep_segments = 0		# in logfile segments; 0 disables
+#max_slot_wal_keep_size = -1	# measured in bytes; -1 disables
 #wal_sender_timeout = 60s	# in milliseconds; 0 disables
 
 #max_replication_slots = 10	# max number of replication slots
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 9ec7b31cce..9d29d2263f 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -108,6 +108,7 @@ extern int	wal_segment_size;
 extern int	min_wal_size_mb;
 extern int	max_wal_size_mb;
 extern int	wal_keep_segments;
+extern int	max_slot_wal_keep_size_mb;
 extern int	XLOGbuffers;
 extern int	XLogArchiveTimeout;
 extern int	wal_retrieve_retry_interval;
@@ -255,6 +256,20 @@ typedef struct CheckpointStatsData
 
 extern CheckpointStatsData CheckpointStats;
 
+/*
+ * WAL segment availability status
+ *
+ * This is used as the return value of GetWalAvailability.
+ */
+typedef enum WalAvailability
+{
+	WALAVAIL_INVALID_LSN,			/* parameter error */
+	WALAVAIL_NORMAL,				/* WAL segment is within max_wal_size */
+	WALAVAIL_PRESERVED,				/* WAL segment is preserved by repslots */
+	WALAVAIL_BEING_REMOVED,			/* WAL segment is no longer preserved */
+	WALAVAIL_REMOVED				/* WAL segment has been removed */
+} WalAvailability;
+
 struct XLogRecData;
 
 extern XLogRecPtr XLogInsertRecord(struct XLogRecData *rdata,
@@ -268,6 +283,7 @@ extern int	XLogFileOpen(XLogSegNo segno);
 
 extern void CheckXLogRemoved(XLogSegNo segno, TimeLineID tli);
 extern XLogSegNo XLogGetLastRemovedSegno(void);
+extern XLogSegNo FindOldestXLogFileSegNo(void);
 extern void XLogSetAsyncXactLSN(XLogRecPtr record);
 extern void XLogSetReplicationSlotMinimumLSN(XLogRecPtr lsn);
 
@@ -305,6 +321,9 @@ extern void ShutdownXLOG(int code, Datum arg);
 extern void InitXLOGAccess(void);
 extern void CreateCheckPoint(int flags);
 extern bool CreateRestartPoint(int flags);
+extern WalAvailability GetWalAvailability(XLogRecPtr restart_lsn,
+										  pid_t walsender_pid);
+extern int64 DistanceToWalRemoval(XLogRecPtr currLSN, XLogRecPtr targetLSN);
 extern void XLogPutNextOid(Oid nextOid);
 extern XLogRecPtr XLogRestorePoint(const char *rpName);
 extern void UpdateFullPageWrites(void);
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index a6a708cca9..2025f34bfd 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -9971,9 +9971,9 @@
   proname => 'pg_get_replication_slots', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', prorettype => 'record',
   proargtypes => '',
-  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn}',
+  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8}',
+  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,remain}',
   prosrc => 'pg_get_replication_slots' },
 { oid => '3786', descr => 'set up a logical replication slot',
   proname => 'pg_create_logical_replication_slot', provolatile => 'v',
diff --git a/src/test/recovery/t/018_replslot_limit.pl b/src/test/recovery/t/018_replslot_limit.pl
new file mode 100644
index 0000000000..39a8780b0f
--- /dev/null
+++ b/src/test/recovery/t/018_replslot_limit.pl
@@ -0,0 +1,203 @@
+# Test for replication slot limit
+# Ensure that max_slot_wal_keep_size limits the number of WAL files to
+# be kept by replication slots.
+
+use strict;
+use warnings;
+use File::Path qw(rmtree);
+use PostgresNode;
+use TestLib;
+use Test::More tests => 13;
+use Time::HiRes qw(usleep);
+
+$ENV{PGDATABASE} = 'postgres';
+
+# Initialize master node, setting wal-segsize to 1MB
+my $node_master = get_new_node('master');
+$node_master->init(allows_streaming => 1, extra => ['--wal-segsize=1']);
+$node_master->append_conf('postgresql.conf', qq(
+min_wal_size = 2MB
+max_wal_size = 4MB
+log_checkpoints = yes
+));
+$node_master->start;
+$node_master->safe_psql('postgres', "SELECT pg_create_physical_replication_slot('rep1')");
+
+# The slot state and remain should be null before the first connection
+my $result = $node_master->safe_psql('postgres', "SELECT restart_lsn is NULL, wal_status is NULL, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "t|t|t", 'check the state of non-reserved slot is "unknown"');
+
+
+# Take backup
+my $backup_name = 'my_backup';
+$node_master->backup($backup_name);
+
+# Create a standby linking to it using the replication slot
+my $node_standby = get_new_node('standby_1');
+$node_standby->init_from_backup($node_master, $backup_name, has_streaming => 1);
+$node_standby->append_conf('postgresql.conf', "primary_slot_name = 'rep1'");
+
+$node_standby->start;
+
+# Wait until standby has replayed enough data
+my $start_lsn = $node_master->lsn('write');
+$node_master->wait_for_catchup($node_standby, 'replay', $start_lsn);
+
+# Stop standby
+$node_standby->stop;
+
+
+# Preparation done, the slot is the state "normal" now
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|t", 'check the catching-up state');
+
+# Advance WAL by five segments (= 5MB) on master
+advance_wal($node_master, 1);
+$node_master->safe_psql('postgres', "CHECKPOINT;");
+
+# The slot is always "safe" when fitting max_wal_size
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|t", 'check that restart_lsn is in max_wal_size');
+
+advance_wal($node_master, 4);
+$node_master->safe_psql('postgres', "CHECKPOINT;");
+
+# The slot is always "safe" when max_slot_wal_keep_size is not set
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|t", 'check that slot is working');
+
+# The standby can reconnect to master
+$node_standby->start;
+
+$start_lsn = $node_master->lsn('write');
+$node_master->wait_for_catchup($node_standby, 'replay', $start_lsn);
+
+$node_standby->stop;
+
+# Set max_slot_wal_keep_size on master
+my $max_slot_wal_keep_size_mb = 6;
+$node_master->append_conf('postgresql.conf', qq(
+max_slot_wal_keep_size = ${max_slot_wal_keep_size_mb}MB
+));
+$node_master->reload;
+
+# The slot is in safe state. The remaining bytes should be as almost
+# (max_slot_wal_keep_size + 1) times large as the segment size
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, pg_size_pretty(remain) as remain FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|7168 kB", 'check that max_slot_wal_keep_size is working');
+
+# Advance WAL again then checkpoint, reducing remain by 2 MB.
+advance_wal($node_master, 2);
+$node_master->safe_psql('postgres', "CHECKPOINT;");
+
+# The slot is still working
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, pg_size_pretty(remain) as remain FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|5120 kB", 'check that remaining byte is calculated correctly');
+
+# wal_keep_segments overrides max_slot_wal_keep_size
+$result = $node_master->safe_psql('postgres', "ALTER SYSTEM SET wal_keep_segments to 8; SELECT pg_reload_conf();");
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, pg_size_pretty(remain) as remain FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|normal|7168 kB", 'check that wal_keep_segments overrides max_slot_wal_keep_size');
+
+# restore wal_keep_segments
+$result = $node_master->safe_psql('postgres', "ALTER SYSTEM SET wal_keep_segments to 0; SELECT pg_reload_conf();");
+
+# Advance WAL again without checkpoint, reducing remain by 4 MB.
+advance_wal($node_master, 4);
+
+# Slot gets into 'keeping' state
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, pg_size_pretty(remain) as remain FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|keeping|1024 kB", 'check that the slot state changes to "keeping"');
+
+# do checkpoint so that the next checkpoint runs too early
+$node_master->safe_psql('postgres', "CHECKPOINT;");
+
+# Advance WAL again without checkpoint; remain goes to 0.
+advance_wal($node_master, 1);
+
+# Slot gets into 'lost' state
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|lost|t", 'check that the slot state changes to "lost"');
+
+# The standby still can connect to master before a checkpoint
+$node_standby->start;
+
+$start_lsn = $node_master->lsn('write');
+$node_master->wait_for_catchup($node_standby, 'replay', $start_lsn);
+
+$node_standby->stop;
+
+ok(!find_in_log($node_standby,
+				"requested WAL segment [0-9A-F]+ has already been removed"),
+   'check that required WAL segments are still available');
+
+# Advance WAL again, the slot loses the oldest segment.
+my $logstart = get_log_size($node_master);
+advance_wal($node_master, 7);
+$node_master->safe_psql('postgres', "CHECKPOINT;");
+
+# WARNING should be issued
+ok(find_in_log($node_master,
+			   "1 replication slot has lost required WAL segments by 1 segments\n".
+			   ".*Most affected slot is rep1.",
+			   $logstart),
+   'check that the warning is logged');
+
+# This slot should be broken
+$result = $node_master->safe_psql('postgres', "SELECT restart_lsn, wal_status, remain is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'");
+is($result, "$start_lsn|lost|t", 'check that the slot state changes to "lost"');
+
+# The standby no longer can connect to the master
+$logstart = get_log_size($node_standby);
+$node_standby->start;
+
+my $failed = 0;
+for (my $i = 0 ; $i < 10000 ; $i++)
+{
+	if (find_in_log($node_standby,
+					"requested WAL segment [0-9A-F]+ has already been removed",
+					$logstart))
+	{
+		$failed = 1;
+		last;
+	}
+	usleep(100_000);
+}
+ok($failed, 'check that replication has been broken');
+
+$node_standby->stop;
+
+#####################################
+# Advance WAL of $node by $n segments
+sub advance_wal
+{
+	my ($node, $n) = @_;
+
+	# Advance by $n segments (= (16 * $n) MB) on master
+	for (my $i = 0 ; $i < $n ; $i++)
+	{
+		$node->safe_psql('postgres', "CREATE TABLE t (); DROP TABLE t; SELECT pg_switch_wal();");
+	}
+}
+
+# return the size of logfile of $node in bytes
+sub get_log_size
+{
+	my ($node) = @_;
+
+	return (stat $node->logfile)[7];
+}
+
+# find $pat in logfile of $node after $off-th byte
+sub find_in_log
+{
+	my ($node, $pat, $off) = @_;
+
+	$off = 0 unless defined $off;
+	my $log = TestLib::slurp_file($node->logfile);
+	return 0 if (length($log) <= $off);
+
+	$log = substr($log, $off);
+
+	return $log =~ m/$pat/;
+}
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 7245b0e13b..8688f7138f 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1462,8 +1462,10 @@ pg_replication_slots| SELECT l.slot_name,
     l.xmin,
     l.catalog_xmin,
     l.restart_lsn,
-    l.confirmed_flush_lsn
-   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn)
+    l.confirmed_flush_lsn,
+    l.wal_status,
+    l.remain
+   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, remain)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
-- 
2.20.1

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