v7-0001-Extensible-rmgr.patch

From 7f6acaf93d279baa464ae3ba2be173a01d969402 Mon Sep 17 00:00:00 2001
From: Jeff Davis <jeff@j-davis.com>
Date: Sat, 6 Nov 2021 13:01:38 -0700
Subject: [PATCH] Extensible rmgr.

Allow extensions to specify a new custom rmgr, which allows
specialized WAL. This is meant to be used by a custom Table Access
Method, which would not otherwise be able to offer logical
decoding/replication. It may also be used by new Index Access Methods.

Prior to this commit, only Generic WAL was available, which offers
support for recovery and physical replication but not logical
replication.

Reviewed-by: Julien Rouhaud, Andres Freund
Discussion: https://postgr.es/m/ed1fb2e22d15d3563ae0eb610f7b61bb15999c0a.camel%40j-davis.com
---
 doc/src/sgml/config.sgml                  |  9 +++
 doc/src/sgml/ref/pg_waldump.sgml          |  8 ++
 src/backend/access/transam/rmgr.c         | 91 ++++++++++++++++++++++-
 src/backend/access/transam/xlogreader.c   |  2 +-
 src/backend/access/transam/xlogrecovery.c | 33 +++-----
 src/backend/replication/logical/decode.c  |  8 +-
 src/backend/utils/misc/guc.c              | 26 +++++--
 src/bin/pg_rewind/parsexlog.c             | 11 ++-
 src/bin/pg_waldump/pg_waldump.c           | 66 +++++++++++-----
 src/bin/pg_waldump/rmgrdesc.c             | 67 ++++++++++++++++-
 src/bin/pg_waldump/rmgrdesc.h             |  2 +-
 src/include/access/rmgr.h                 | 15 +++-
 src/include/access/xlog_internal.h        | 23 +++++-
 13 files changed, 300 insertions(+), 61 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 43e4ade83e0..c80327af67a 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -11148,6 +11148,15 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
         <literal>spgist</literal>, <literal>brin</literal>, and <literal>generic</literal>. Only
         superusers can change this setting.
        </para>
+
+       <para>
+        Extensions may define additional resource managers with modules loaded
+        in <xref linkend="guc-shared-preload-libraries"/>. However, the names
+        of the custom resource managers are not available at the time the
+        configuration file is processed, so they must be referred to
+        numerically with the form <literal>custom###</literal>, where
+        "<literal>###</literal>" is the three-digit resource manager ID.
+       </para>
       </listitem>
      </varlistentry>
 
diff --git a/doc/src/sgml/ref/pg_waldump.sgml b/doc/src/sgml/ref/pg_waldump.sgml
index 1a05af5d972..57746d9421f 100644
--- a/doc/src/sgml/ref/pg_waldump.sgml
+++ b/doc/src/sgml/ref/pg_waldump.sgml
@@ -173,6 +173,14 @@ PostgreSQL documentation
         If <literal>list</literal> is passed as name, print a list of valid resource manager
         names, and exit.
        </para>
+       <para>
+        Extensions may define custom resource managers, but pg_waldump does
+        not load the extension module and therefore does not recognize custom
+        resource managers by name. Instead, you can specify the custom
+        resource managers as <literal>custom###</literal> where
+        "<literal>###</literal>" is the three-digit resource manager ID. Names
+        of this form will always be considered valid.
+       </para>
       </listitem>
      </varlistentry>
 
diff --git a/src/backend/access/transam/rmgr.c b/src/backend/access/transam/rmgr.c
index f8847d5aebf..d215116a912 100644
--- a/src/backend/access/transam/rmgr.c
+++ b/src/backend/access/transam/rmgr.c
@@ -24,6 +24,7 @@
 #include "commands/dbcommands_xlog.h"
 #include "commands/sequence.h"
 #include "commands/tablespace.h"
+#include "miscadmin.h"
 #include "replication/decode.h"
 #include "replication/message.h"
 #include "replication/origin.h"
@@ -34,6 +35,94 @@
 #define PG_RMGR(symname,name,redo,desc,identify,startup,cleanup,mask,decode) \
 	{ name, redo, desc, identify, startup, cleanup, mask, decode },
 
-const RmgrData RmgrTable[RM_MAX_ID + 1] = {
+RmgrData RmgrTable[RM_MAX_ID + 1] = {
 #include "access/rmgrlist.h"
 };
+
+/*
+ * Start up all resource managers.
+ */
+void
+RmgrStartup(void)
+{
+	for (int rmid = 0; rmid <= RM_MAX_ID; rmid++)
+	{
+		if (!RmgrExists(rmid))
+			continue;
+
+		if (RmgrTable[rmid].rm_startup != NULL)
+			RmgrTable[rmid].rm_startup();
+	}
+}
+
+/*
+ * Clean up all resource managers.
+ */
+void
+RmgrCleanup(void)
+{
+	for (int rmid = 0; rmid <= RM_MAX_ID; rmid++)
+	{
+		if (!RmgrExists(rmid))
+			continue;
+
+		if (RmgrTable[rmid].rm_cleanup != NULL)
+			RmgrTable[rmid].rm_cleanup();
+	}
+}
+
+/*
+ * Emit PANIC message when we encounter a record with an RmgrId we don't recognize.
+ */
+void
+RmgrNotFound(RmgrId rmid)
+{
+	ereport(PANIC, (errmsg("resource manager with ID %d not registered", rmid),
+					errhint("Include the extension module that implements this resource manager in shared_preload_libraries.")));
+}
+
+/*
+ * Register a new custom rmgr.
+ *
+ * Refer to https://wiki.postgresql.org/wiki/ExtensibleRmgr to reserve a
+ * unique RmgrId for your extension, to avoid conflicts. During development,
+ * use RM_EXPERIMENTAL_ID.
+ */
+void
+RegisterCustomRmgr(RmgrId rmid, RmgrData *rmgr)
+{
+	if (rmgr->rm_name == NULL)
+		ereport(PANIC, errmsg("custom resource manager is invalid"));
+
+	if (rmid < RM_MIN_CUSTOM_ID)
+		ereport(PANIC, errmsg("custom resource manager ID %d is out of range", rmid));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(PANIC,
+				(errmsg("failed to register custom resource manager \"%s\" with ID %d", rmgr->rm_name, rmid),
+				 errdetail("Custom resource manager must be registered while initializing modules in shared_preload_libraries.")));
+
+	if (RmgrTable[rmid].rm_name != NULL)
+		ereport(PANIC,
+				(errmsg("failed to register custom resource manager \"%s\" with ID %d", rmgr->rm_name, rmid),
+				 errdetail("Custom resource manager \"%s\" already registered with the same ID.",
+						   RmgrTable[rmid].rm_name)));
+
+	/* check for existing rmgr with the same name */
+	for (int i = 0; i <= RM_MAX_ID; i++)
+	{
+		if (!RmgrExists(i))
+			continue;
+
+		if (!strcmp(RmgrTable[i].rm_name, rmgr->rm_name))
+			ereport(PANIC,
+				(errmsg("failed to register custom resource manager \"%s\" with ID %d", rmgr->rm_name, rmid),
+				 errdetail("Existing resource manager with ID %d has the same name.", i)));
+	}
+
+	/* register it */
+	RmgrTable[rmid] = *rmgr;
+	ereport(LOG,
+			(errmsg("registered custom resource manager \"%s\" with ID %d",
+					rmgr->rm_name, rmid)));
+}
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index e437c429920..161cf13fed2 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1102,7 +1102,7 @@ ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
 							  (uint32) SizeOfXLogRecord, record->xl_tot_len);
 		return false;
 	}
-	if (record->xl_rmid > RM_MAX_ID)
+	if (!RMID_IS_VALID(record->xl_rmid))
 	{
 		report_invalid_record(state,
 							  "invalid resource manager ID %u at %X/%X",
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 8d2395dae25..999e24fcf2c 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -1531,7 +1531,6 @@ ShutdownWalRecovery(void)
 void
 PerformWalRecovery(void)
 {
-	int			rmid;
 	XLogRecord *record;
 	bool		reachedRecoveryTarget = false;
 	TimeLineID	replayTLI;
@@ -1604,12 +1603,7 @@ PerformWalRecovery(void)
 
 		InRedo = true;
 
-		/* Initialize resource managers */
-		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
-		{
-			if (RmgrTable[rmid].rm_startup != NULL)
-				RmgrTable[rmid].rm_startup();
-		}
+		RmgrStartup();
 
 		ereport(LOG,
 				(errmsg("redo starts at %X/%X",
@@ -1746,12 +1740,7 @@ PerformWalRecovery(void)
 			}
 		}
 
-		/* Allow resource managers to do any required cleanup. */
-		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
-		{
-			if (RmgrTable[rmid].rm_cleanup != NULL)
-				RmgrTable[rmid].rm_cleanup();
-		}
+		RmgrCleanup();
 
 		ereport(LOG,
 				(errmsg("redo done at %X/%X system usage: %s",
@@ -1871,7 +1860,7 @@ ApplyWalRecord(XLogReaderState *xlogreader, XLogRecord *record, TimeLineID *repl
 		xlogrecovery_redo(xlogreader, *replayTLI);
 
 	/* Now apply the WAL record itself */
-	RmgrTable[record->xl_rmid].rm_redo(xlogreader);
+	GetRmgr(record->xl_rmid).rm_redo(xlogreader);
 
 	/*
 	 * After redo, check whether the backup pages associated with the WAL
@@ -2101,20 +2090,20 @@ rm_redo_error_callback(void *arg)
 void
 xlog_outdesc(StringInfo buf, XLogReaderState *record)
 {
-	RmgrId		rmid = XLogRecGetRmid(record);
+	RmgrData	rmgr = GetRmgr(XLogRecGetRmid(record));
 	uint8		info = XLogRecGetInfo(record);
 	const char *id;
 
-	appendStringInfoString(buf, RmgrTable[rmid].rm_name);
+	appendStringInfoString(buf, rmgr.rm_name);
 	appendStringInfoChar(buf, '/');
 
-	id = RmgrTable[rmid].rm_identify(info);
+	id = rmgr.rm_identify(info);
 	if (id == NULL)
 		appendStringInfo(buf, "UNKNOWN (%X): ", info & ~XLR_INFO_MASK);
 	else
 		appendStringInfo(buf, "%s: ", id);
 
-	RmgrTable[rmid].rm_desc(buf, record);
+	rmgr.rm_desc(buf, record);
 }
 
 #ifdef WAL_DEBUG
@@ -2263,7 +2252,7 @@ getRecordTimestamp(XLogReaderState *record, TimestampTz *recordXtime)
 static void
 verifyBackupPageConsistency(XLogReaderState *record)
 {
-	RmgrId		rmid = XLogRecGetRmid(record);
+	RmgrData	rmgr = GetRmgr(XLogRecGetRmid(record));
 	RelFileNode rnode;
 	ForkNumber	forknum;
 	BlockNumber blkno;
@@ -2343,10 +2332,10 @@ verifyBackupPageConsistency(XLogReaderState *record)
 		 * If masking function is defined, mask both the primary and replay
 		 * images
 		 */
-		if (RmgrTable[rmid].rm_mask != NULL)
+		if (rmgr.rm_mask != NULL)
 		{
-			RmgrTable[rmid].rm_mask(replay_image_masked, blkno);
-			RmgrTable[rmid].rm_mask(primary_image_masked, blkno);
+			rmgr.rm_mask(replay_image_masked, blkno);
+			rmgr.rm_mask(primary_image_masked, blkno);
 		}
 
 		/* Time to compare the primary and replay images. */
diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 77bc7aea7a0..c6ea7c98e15 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -94,7 +94,7 @@ LogicalDecodingProcessRecord(LogicalDecodingContext *ctx, XLogReaderState *recor
 {
 	XLogRecordBuffer buf;
 	TransactionId txid;
-	RmgrId rmid;
+	RmgrData rmgr;
 
 	buf.origptr = ctx->reader->ReadRecPtr;
 	buf.endptr = ctx->reader->EndRecPtr;
@@ -115,10 +115,10 @@ LogicalDecodingProcessRecord(LogicalDecodingContext *ctx, XLogReaderState *recor
 								 buf.origptr);
 	}
 
-	rmid = XLogRecGetRmid(record);
+	rmgr = GetRmgr(XLogRecGetRmid(record));
 
-	if (RmgrTable[rmid].rm_decode != NULL)
-		RmgrTable[rmid].rm_decode(ctx, &buf);
+	if (rmgr.rm_decode != NULL)
+		rmgr.rm_decode(ctx, &buf);
 	else
 	{
 		/* just deal with xid, and done */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 9e8ab1420d9..5a08769e497 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -11769,26 +11769,40 @@ check_wal_consistency_checking(char **newval, void **extra, GucSource source)
 	{
 		char	   *tok = (char *) lfirst(l);
 		bool		found = false;
-		RmgrId		rmid;
+		int			rmid;
 
 		/* Check for 'all'. */
 		if (pg_strcasecmp(tok, "all") == 0)
 		{
-			for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
-				if (RmgrTable[rmid].rm_mask != NULL)
+			for (rmid = 0; rmid <= RM_MAX_BUILTIN_ID; rmid++)
+				if (GetRmgr(rmid).rm_mask != NULL)
 					newwalconsistency[rmid] = true;
 			found = true;
 		}
+		else if (sscanf(tok, "custom%03d", &rmid) == 1)
+		{
+			/*
+			 * The server hasn't loaded the shared_preload_libraries yet, so
+			 * we don't have access to the custom resource manager
+			 * names. Allow the form "custom###", where "###" is the 3-digit
+			 * resource manager ID.
+			 */
+			if (RMID_IS_CUSTOM(rmid))
+			{
+				newwalconsistency[rmid] = true;
+				found = true;
+			}
+		}
 		else
 		{
 			/*
 			 * Check if the token matches with any individual resource
 			 * manager.
 			 */
-			for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
+			for (rmid = 0; rmid <= RM_MAX_BUILTIN_ID; rmid++)
 			{
-				if (pg_strcasecmp(tok, RmgrTable[rmid].rm_name) == 0 &&
-					RmgrTable[rmid].rm_mask != NULL)
+				if (GetRmgr(rmid).rm_mask != NULL &&
+					pg_strcasecmp(tok, GetRmgr(rmid).rm_name) == 0)
 				{
 					newwalconsistency[rmid] = true;
 					found = true;
diff --git a/src/bin/pg_rewind/parsexlog.c b/src/bin/pg_rewind/parsexlog.c
index 49966e7b7fd..dfa836d1561 100644
--- a/src/bin/pg_rewind/parsexlog.c
+++ b/src/bin/pg_rewind/parsexlog.c
@@ -25,8 +25,8 @@
 #include "pg_rewind.h"
 
 /*
- * RmgrNames is an array of resource manager names, to make error messages
- * a bit nicer.
+ * RmgrNames is an array of the built-in resource manager names, to make error
+ * messages a bit nicer.
  */
 #define PG_RMGR(symname,name,redo,desc,identify,startup,cleanup,mask,decode) \
   name,
@@ -35,6 +35,9 @@ static const char *RmgrNames[RM_MAX_ID + 1] = {
 #include "access/rmgrlist.h"
 };
 
+#define RmgrName(rmid) (((rmid) <= RM_MAX_BUILTIN_ID) ? \
+						RmgrNames[rmid] : "custom")
+
 static void extractPageInfo(XLogReaderState *record);
 
 static int	xlogreadfd = -1;
@@ -436,9 +439,9 @@ extractPageInfo(XLogReaderState *record)
 		 * track that change.
 		 */
 		pg_fatal("WAL record modifies a relation, but record type is not recognized: "
-				 "lsn: %X/%X, rmgr: %s, info: %02X",
+				 "lsn: %X/%X, rmid: %d, rmgr: %s, info: %02X",
 				 LSN_FORMAT_ARGS(record->ReadRecPtr),
-				 RmgrNames[rmid], info);
+				 rmid, RmgrName(rmid), info);
 	}
 
 	for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index 4cb40d068a9..4f47449a6cb 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -80,8 +80,8 @@ typedef struct XLogDumpStats
 	uint64		count;
 	XLogRecPtr	startptr;
 	XLogRecPtr	endptr;
-	Stats		rmgr_stats[RM_NEXT_ID];
-	Stats		record_stats[RM_NEXT_ID][MAX_XLINFO_TYPES];
+	Stats		rmgr_stats[RM_MAX_ID + 1];
+	Stats		record_stats[RM_MAX_ID + 1][MAX_XLINFO_TYPES];
 } XLogDumpStats;
 
 #define fatal_error(...) do { pg_log_fatal(__VA_ARGS__); exit(EXIT_FAILURE); } while(0)
@@ -104,9 +104,9 @@ print_rmgr_list(void)
 {
 	int			i;
 
-	for (i = 0; i <= RM_MAX_ID; i++)
+	for (i = 0; i <= RM_MAX_BUILTIN_ID; i++)
 	{
-		printf("%s\n", RmgrDescTable[i].rm_name);
+		printf("%s\n", GetRmgrDesc(i)->rm_name);
 	}
 }
 
@@ -535,7 +535,7 @@ static void
 XLogDumpDisplayRecord(XLogDumpConfig *config, XLogReaderState *record)
 {
 	const char *id;
-	const RmgrDescData *desc = &RmgrDescTable[XLogRecGetRmid(record)];
+	const RmgrDescData *desc = GetRmgrDesc(XLogRecGetRmid(record));
 	uint32		rec_len;
 	uint32		fpi_len;
 	RelFileNode rnode;
@@ -720,7 +720,7 @@ XLogDumpDisplayStats(XLogDumpConfig *config, XLogDumpStats *stats)
 	 * calculate column totals.
 	 */
 
-	for (ri = 0; ri < RM_NEXT_ID; ri++)
+	for (ri = 0; ri < RM_MAX_ID; ri++)
 	{
 		total_count += stats->rmgr_stats[ri].count;
 		total_rec_len += stats->rmgr_stats[ri].rec_len;
@@ -741,13 +741,18 @@ XLogDumpDisplayStats(XLogDumpConfig *config, XLogDumpStats *stats)
 		   "Type", "N", "(%)", "Record size", "(%)", "FPI size", "(%)", "Combined size", "(%)",
 		   "----", "-", "---", "-----------", "---", "--------", "---", "-------------", "---");
 
-	for (ri = 0; ri < RM_NEXT_ID; ri++)
+	for (ri = 0; ri <= RM_MAX_ID; ri++)
 	{
 		uint64		count,
 					rec_len,
 					fpi_len,
 					tot_len;
-		const RmgrDescData *desc = &RmgrDescTable[ri];
+		const RmgrDescData *desc;
+
+		if (!RMID_IS_VALID(ri))
+			continue;
+
+		desc = GetRmgrDesc(ri);
 
 		if (!config->stats_per_record)
 		{
@@ -756,6 +761,9 @@ XLogDumpDisplayStats(XLogDumpConfig *config, XLogDumpStats *stats)
 			fpi_len = stats->rmgr_stats[ri].fpi_len;
 			tot_len = rec_len + fpi_len;
 
+			if (RMID_IS_CUSTOM(ri) && count == 0)
+				continue;
+
 			XLogDumpStatsRow(desc->rm_name,
 							 count, total_count, rec_len, total_rec_len,
 							 fpi_len, total_fpi_len, tot_len, total_len);
@@ -1000,7 +1008,7 @@ main(int argc, char **argv)
 				break;
 			case 'r':
 				{
-					int			i;
+					int			rmid;
 
 					if (pg_strcasecmp(optarg, "list") == 0)
 					{
@@ -1008,20 +1016,42 @@ main(int argc, char **argv)
 						exit(EXIT_SUCCESS);
 					}
 
-					for (i = 0; i <= RM_MAX_ID; i++)
+					/*
+					 * First look for the generated name of a custom rmgr, of
+					 * the form "custom###". We accept this form, because the
+					 * custom rmgr module is not loaded, so there's no way to
+					 * know the real name. This convention should be
+					 * consistent with that in rmgrdesc.c.
+					 */
+					if (sscanf(optarg, "custom%03d", &rmid) == 1)
 					{
-						if (pg_strcasecmp(optarg, RmgrDescTable[i].rm_name) == 0)
+						if (!RMID_IS_CUSTOM(rmid))
 						{
-							config.filter_by_rmgr[i] = true;
-							config.filter_by_rmgr_enabled = true;
-							break;
+							pg_log_error("custom resource manager \"%s\" does not exist",
+										 optarg);
+							goto bad_argument;
 						}
+						config.filter_by_rmgr[rmid] = true;
+						config.filter_by_rmgr_enabled = true;
 					}
-					if (i > RM_MAX_ID)
+					else
 					{
-						pg_log_error("resource manager \"%s\" does not exist",
-									 optarg);
-						goto bad_argument;
+						/* then look for builtin rmgrs */
+						for (rmid = 0; rmid <= RM_MAX_BUILTIN_ID; rmid++)
+						{
+							if (pg_strcasecmp(optarg, GetRmgrDesc(rmid)->rm_name) == 0)
+							{
+								config.filter_by_rmgr[rmid] = true;
+								config.filter_by_rmgr_enabled = true;
+								break;
+							}
+						}
+						if (rmid > RM_MAX_BUILTIN_ID)
+						{
+							pg_log_error("resource manager \"%s\" does not exist",
+										 optarg);
+							goto bad_argument;
+						}
 					}
 				}
 				break;
diff --git a/src/bin/pg_waldump/rmgrdesc.c b/src/bin/pg_waldump/rmgrdesc.c
index 6a4ebd1310b..7871a16b08a 100644
--- a/src/bin/pg_waldump/rmgrdesc.c
+++ b/src/bin/pg_waldump/rmgrdesc.c
@@ -35,6 +35,71 @@
 #define PG_RMGR(symname,name,redo,desc,identify,startup,cleanup,mask,decode) \
 	{ name, desc, identify},
 
-const RmgrDescData RmgrDescTable[RM_MAX_ID + 1] = {
+static const RmgrDescData RmgrDescTable[RM_MAX_BUILTIN_ID + 1] = {
 #include "access/rmgrlist.h"
 };
+
+/*
+ * We are unable to get the real name of a custom rmgr because the module is
+ * not loaded. Generate a table of numeric names of the form "custom###" where
+ * "###" is the 3-digit resource manager ID.
+ */
+#define CUSTOM_NUMERIC_NAME_LEN sizeof("custom###")
+
+static char CustomNumericNames[RM_N_CUSTOM_IDS][CUSTOM_NUMERIC_NAME_LEN] = {0};
+static RmgrDescData CustomRmgrDesc[RM_N_CUSTOM_IDS] = {0};
+static bool CustomRmgrDescInitialized = false;
+
+/*
+ * No information on custom resource managers; just print the ID.
+ */
+static void
+default_desc(StringInfo buf, XLogReaderState *record)
+{
+	appendStringInfo(buf, "rmid: %d", XLogRecGetRmid(record));
+}
+
+/*
+ * No information on custom resource managers; just return NULL and let the
+ * caller handle it.
+ */
+static const char *
+default_identify(uint8 info)
+{
+	return NULL;
+}
+
+/*
+ * pg_waldump does not load the modules that register the resource managers;
+ * therefore we are missing the RmgrDesc information. Generate phony data
+ * where the name follows the convention "custom###" where ### is the 3-digit
+ * rmgr ID.
+ */
+static void
+initialize_custom_rmgrs(void)
+{
+	for (int i = 0; i < RM_N_CUSTOM_IDS; i++)
+	{
+		snprintf(CustomNumericNames[i], CUSTOM_NUMERIC_NAME_LEN,
+				 "custom%03d", i + RM_MIN_CUSTOM_ID);
+		CustomRmgrDesc[i].rm_name = CustomNumericNames[i];
+		CustomRmgrDesc[i].rm_desc = default_desc;
+		CustomRmgrDesc[i].rm_identify = default_identify;
+	}
+	CustomRmgrDescInitialized = true;
+}
+
+const RmgrDescData *
+GetRmgrDesc(RmgrId rmid)
+{
+	Assert(RMID_IS_VALID(rmid));
+
+	if (RMID_IS_BUILTIN(rmid))
+		return &RmgrDescTable[rmid];
+	else
+	{
+		if (!CustomRmgrDescInitialized)
+			initialize_custom_rmgrs();
+		return &CustomRmgrDesc[rmid - RM_MIN_CUSTOM_ID];
+	}
+}
diff --git a/src/bin/pg_waldump/rmgrdesc.h b/src/bin/pg_waldump/rmgrdesc.h
index 42f8483b482..f733cd467d5 100644
--- a/src/bin/pg_waldump/rmgrdesc.h
+++ b/src/bin/pg_waldump/rmgrdesc.h
@@ -18,6 +18,6 @@ typedef struct RmgrDescData
 	const char *(*rm_identify) (uint8 info);
 } RmgrDescData;
 
-extern const RmgrDescData RmgrDescTable[];
+extern const RmgrDescData *GetRmgrDesc(RmgrId rmid);
 
 #endif							/* RMGRDESC_H */
diff --git a/src/include/access/rmgr.h b/src/include/access/rmgr.h
index d9b512630ca..dfa8521997c 100644
--- a/src/include/access/rmgr.h
+++ b/src/include/access/rmgr.h
@@ -30,6 +30,19 @@ typedef enum RmgrIds
 
 #undef PG_RMGR
 
-#define RM_MAX_ID				(RM_NEXT_ID - 1)
+#define RM_MAX_ID				UINT8_MAX
+#define RM_MAX_BUILTIN_ID		(RM_NEXT_ID - 1)
+#define RM_MIN_CUSTOM_ID		128
+#define RM_N_CUSTOM_IDS			(RM_MAX_ID - RM_MIN_CUSTOM_ID + 1)
+#define RMID_IS_BUILTIN(rmid)	((rmid) <= RM_MAX_BUILTIN_ID)
+#define RMID_IS_CUSTOM(rmid)	((rmid) >= RM_MIN_CUSTOM_ID && (rmid) < RM_MAX_ID)
+#define RMID_IS_VALID(rmid)		(RMID_IS_BUILTIN((rmid)) || RMID_IS_CUSTOM((rmid)))
+
+/*
+ * RmgrId to use for extensions that require an RmgrId, but are still in
+ * development and have not reserved their own unique RmgrId yet. See:
+ * https://wiki.postgresql.org/wiki/ExtensibleRmgr
+ */
+#define RM_EXPERIMENTAL_ID		128
 
 #endif							/* RMGR_H */
diff --git a/src/include/access/xlog_internal.h b/src/include/access/xlog_internal.h
index 0e94833129a..9370f346c83 100644
--- a/src/include/access/xlog_internal.h
+++ b/src/include/access/xlog_internal.h
@@ -304,7 +304,8 @@ struct XLogRecordBuffer;
  * rm_mask takes as input a page modified by the resource manager and masks
  * out bits that shouldn't be flagged by wal_consistency_checking.
  *
- * RmgrTable[] is indexed by RmgrId values (see rmgrlist.h).
+ * RmgrTable[] is indexed by RmgrId values (see rmgrlist.h). If rm_name is
+ * NULL, the structure is considered invalid.
  */
 typedef struct RmgrData
 {
@@ -319,7 +320,25 @@ typedef struct RmgrData
 							  struct XLogRecordBuffer *buf);
 } RmgrData;
 
-extern const RmgrData RmgrTable[];
+extern RmgrData RmgrTable[];
+extern void RmgrStartup(void);
+extern void RmgrCleanup(void);
+extern void RmgrNotFound(RmgrId rmid);
+extern void RegisterCustomRmgr(RmgrId rmid, RmgrData *rmgr);
+
+static inline bool
+RmgrExists(RmgrId rmid)
+{
+	return RmgrTable[rmid].rm_name != NULL;
+}
+
+static inline RmgrData
+GetRmgr(RmgrId rmid)
+{
+	if (unlikely(!RmgrExists(rmid)))
+		RmgrNotFound(rmid);
+	return RmgrTable[rmid];
+}
 
 /*
  * Exported to support xlog switching from checkpointer
-- 
2.17.1