Custom WAL Resource Managers.

Allow extensions to specify a new custom resource manager (rmgr),
which allows specialized WAL. This is meant to be used by a Table
Access Method or Index Access Method.

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, Bharath Rupireddy, Andres Freund
Discussion: https://postgr.es/m/ed1fb2e22d15d3563ae0eb610f7b61bb15999c0a.camel%40j-davis.com

doc/src/sgml/config.sgml

@@ -11189,8 +11189,8 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
         <literal>heap2</literal>, <literal>btree</literal>, <literal>hash</literal>,
         <literal>gin</literal>, <literal>gist</literal>, <literal>sequence</literal>,
         <literal>spgist</literal>, <literal>brin</literal>, and <literal>generic</literal>.
-        Only superusers and users with the appropriate <literal>SET</literal>
+        Extensions may define additional resource managers. Only superusers and users with
-        privilege can change this setting.
+        the appropriate <literal>SET</literal> privilege can change this setting.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/custom-rmgr.sgml

@@ -0,0 +1,98 @@
+<!-- doc/src/sgml/custom-rmgr.sgml -->
+
+<chapter id="custom-rmgr">
+ <title>Custom WAL Resource Managers</title>
+
+ <para>
+  This chapter explains the interface between the core
+  <productname>PostgreSQL</productname> system and custom WAL resource
+  managers, which enable extensions to integrate directly with the <link
+  linkend="wal"><acronym>WAL</acronym></link>.
+ </para>
+ <para>
+  An extension, especially a <link linkend="tableam">Table Access
+  Method</link> or <link linkend="indexam">Index Access Method</link>, may
+  need to use WAL for recovery, replication, and/or <link
+  linkend="logicaldecoding">Logical Decoding</link>. Custom resource managers
+  are a more flexible alternative to <link linkend="generic-wal">Generic
+  WAL</link> (which does not support logical decoding), but more complex for
+  an extension to implement.
+ </para>
+ <para>
+  To create a new custom WAL resouce manager, first define an
+  <structname>RmgrData</structname> structure with implementations for the
+  resource manager methods. Refer to
+  <filename>src/backend/access/transam/README</filename> and
+  <filename>src/include/access/xlog_internal.h</filename> in the
+  <productname>PostgreSQL</productname> source.
+<programlisting>
+/*
+ * Method table for resource managers.
+ *
+ * This struct must be kept in sync with the PG_RMGR definition in
+ * rmgr.c.
+ *
+ * rm_identify must return a name for the record based on xl_info (without
+ * reference to the rmid). For example, XLOG_BTREE_VACUUM would be named
+ * "VACUUM". rm_desc can then be called to obtain additional detail for the
+ * record, if available (e.g. the last block).
+ *
+ * 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). If rm_name is
+ * NULL, the corresponding RmgrTable entry is considered invalid.
+ */
+typedef struct RmgrData
+{
+    const char *rm_name;
+    void        (*rm_redo) (XLogReaderState *record);
+    void        (*rm_desc) (StringInfo buf, XLogReaderState *record);
+    const char *(*rm_identify) (uint8 info);
+    void        (*rm_startup) (void);
+    void        (*rm_cleanup) (void);
+    void        (*rm_mask) (char *pagedata, BlockNumber blkno);
+    void        (*rm_decode) (struct LogicalDecodingContext *ctx,
+                              struct XLogRecordBuffer *buf);
+} RmgrData;
+</programlisting>  
+ </para>
+ <para>
+  Then, register your new resource
+  manager.
+
+<programlisting>
+/*
+ * Register a new custom WAL resource manager.
+ *
+ * Resource manager IDs must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomWALResourceManager to reserve a
+ * unique RmgrId for your extension, to avoid conflicts with other extension
+ * developers. During development, use RM_EXPERIMENTAL_ID to avoid needlessly
+ * reserving a new ID.
+ */
+extern void RegisterCustomRmgr(RmgrId rmid, RmgrData *rmgr);
+</programlisting>
+  <function>RegisterCustomRmgr</function> must be called from the
+  extension module's <link linkend="xfunc-c-dynload">_PG_init</link> function.
+  While developing a new extension, use <literal>RM_EXPERIMENTAL_ID</literal>
+  for <parameter>rmid</parameter>. When you ready to release the extension to
+  users, reserve a new resource manager ID at the <ulink
+  url="https://wiki.postgresql.org/wiki/CustomWALResourceManagers">Custom WAL
+  Resource Manager</ulink> page.
+ </para>
+
+ <para>
+  Place the extension module implementing the custom resource manager in <xref
+  linkend="guc-shared-preload-libraries"/> so that it will be loaded early
+  during <productname>PostgreSQL</productname> startup.
+ </para>
+ <note>
+   <para>    
+    The extension must remain in shared_preload_libraries as long as any
+    custom WAL records may exist in the system. Otherwise
+    <productname>PostgreSQL</productname> will not be able to apply or decode
+    the custom WAL records, which may prevent the server from starting.
+   </para>
+ </note>
+</chapter>

doc/src/sgml/filelist.sgml

@@ -105,6 +105,7 @@
 <!ENTITY storage    SYSTEM "storage.sgml">
 <!ENTITY tablesample-method SYSTEM "tablesample-method.sgml">
 <!ENTITY generic-wal SYSTEM "generic-wal.sgml">
+<!ENTITY custom-rmgr SYSTEM "custom-rmgr.sgml">
 <!ENTITY backup-manifest SYSTEM "backup-manifest.sgml">
 
 <!-- contrib information -->

doc/src/sgml/func.sgml

@@ -26001,6 +26001,25 @@ postgres=# SELECT * FROM pg_walfile_name_offset((pg_backup_stop()).lsn);
         without recovery, the function returns <literal>NULL</literal>.
        </para></entry>
       </row>
+
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_get_wal_resource_managers</primary>
+        </indexterm>
+        <function>pg_get_wal_resource_managers</function> ()
+        <returnvalue>setof record</returnvalue>
+        ( <parameter>rm_id</parameter> <type>integer</type>,
+        <parameter>rm_name</parameter> <type>text</type>,
+        <parameter>rm_builtin</parameter> <type>boolean</type> )
+       </para>
+       <para>
+        Returns the currently-loaded WAL resource managers in the system. The
+        column <parameter>rm_builtin</parameter> indicates whether it's a
+        built-in resource manager, or a custom resource manager loaded by an
+        extension.
+       </para></entry>
+      </row>
      </tbody>
     </tgroup>
    </table>

doc/src/sgml/generic-wal.sgml

@@ -7,10 +7,24 @@
    Although all built-in WAL-logged modules have their own types of WAL
    records, there is also a generic WAL record type, which describes changes
    to pages in a generic way. This is useful for extensions that provide
-   custom access methods, because they cannot register their own WAL redo
+   custom access methods.
-   routines.
   </para>
 
+  <para>
+   In comparison with <link linkend="custom-rmgr">Custom WAL Resource
+   Managers</link>, Generic WAL is simpler for an extension to implement and
+   does not require the extension library to be loaded in order to apply the
+   records.
+  </para>
+
+  <note>
+   <para>
+    Generic WAL records are ignored during <link
+    linkend="logicaldecoding">Logical Decoding</link>. If logical decoding is
+    required for your extension, consider a Custom WAL Resource Manager.
+   </para>
+  </note>
+
   <para>
    The API for constructing generic WAL records is defined in
    <filename>access/generic_xlog.h</filename> and implemented

doc/src/sgml/postgres.sgml

@@ -262,6 +262,7 @@ break is not needed in a wider output rendering.
   &tableam;
   &indexam;
   &generic-wal;
+  &custom-rmgr;
   &btree;
   &gist;
   &spgist;

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>
 

src/backend/access/transam/rmgr.c

@@ -24,16 +24,138 @@
 #include "commands/dbcommands_xlog.h"
 #include "commands/sequence.h"
 #include "commands/tablespace.h"
+#include "fmgr.h"
+#include "funcapi.h"
+#include "miscadmin.h"
 #include "replication/decode.h"
 #include "replication/message.h"
 #include "replication/origin.h"
 #include "storage/standby.h"
+#include "utils/builtins.h"
 #include "utils/relmapper.h"
 
 /* must be kept in sync with RmgrData definition in xlog_internal.h */
 #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 (!RmgrIdExists(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 (!RmgrIdExists(rmid))
+			continue;
+
+		if (RmgrTable[rmid].rm_cleanup != NULL)
+			RmgrTable[rmid].rm_cleanup();
+	}
+}
+
+/*
+ * Emit ERROR when we encounter a record with an RmgrId we don't
+ * recognize.
+ */
+void
+RmgrNotFound(RmgrId rmid)
+{
+	ereport(ERROR, (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 WAL resource manager.
+ *
+ * Resource manager IDs must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomWALResourceManager to reserve a
+ * unique RmgrId for your extension, to avoid conflicts with other extension
+ * developers. During development, use RM_EXPERIMENTAL_ID to avoid needlessly
+ * reserving a new ID.
+ */
+void
+RegisterCustomRmgr(RmgrId rmid, RmgrData *rmgr)
+{
+	if (rmgr->rm_name == NULL || strlen(rmgr->rm_name) == 0)
+		ereport(ERROR, (errmsg("custom resource manager name is invalid"),
+						errhint("Provide a non-empty name for the custom resource manager.")));
+
+	if (!RMID_IS_CUSTOM(rmid))
+		ereport(ERROR, (errmsg("custom resource manager ID %d is out of range", rmid),
+						errhint("Provide a custom resource manager ID between %d and %d.",
+								RM_MIN_CUSTOM_ID, RM_MAX_CUSTOM_ID)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(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(ERROR,
+				(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 existing_rmid = 0; existing_rmid <= RM_MAX_ID; existing_rmid++)
+	{
+		if (!RmgrIdExists(existing_rmid))
+			continue;
+
+		if (!pg_strcasecmp(RmgrTable[existing_rmid].rm_name, rmgr->rm_name))
+			ereport(ERROR,
+				(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.", existing_rmid)));
+	}
+
+	/* register it */
+	RmgrTable[rmid] = *rmgr;
+	ereport(LOG,
+			(errmsg("registered custom resource manager \"%s\" with ID %d",
+					rmgr->rm_name, rmid)));
+}
+
+/* SQL SRF showing loaded resource managers */
+Datum
+pg_get_wal_resource_managers(PG_FUNCTION_ARGS)
+{
+#define PG_GET_RESOURCE_MANAGERS_COLS 3
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	Datum		values[PG_GET_RESOURCE_MANAGERS_COLS];
+	bool		nulls[PG_GET_RESOURCE_MANAGERS_COLS] = {0};
+
+	SetSingleFuncCall(fcinfo, 0);
+
+	for (int rmid = 0; rmid <= RM_MAX_ID; rmid++)
+	{
+		if (!RmgrIdExists(rmid))
+			continue;
+		values[0] = Int32GetDatum(rmid);
+		values[1] = CStringGetTextDatum(GetRmgr(rmid).rm_name);
+		values[2] = BoolGetDatum(RMID_IS_BUILTIN(rmid));
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
+	}
+
+	return (Datum) 0;
+}

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",

src/backend/access/transam/xlogrecovery.c

@@ -1541,7 +1541,6 @@ ShutdownWalRecovery(void)
 void
 PerformWalRecovery(void)
 {
-	int			rmid;
 	XLogRecord *record;
 	bool		reachedRecoveryTarget = false;
 	TimeLineID	replayTLI;
@@ -1614,12 +1613,7 @@ PerformWalRecovery(void)
 
 		InRedo = true;
 
-		/* Initialize resource managers */
+		RmgrStartup();
-		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
-		{
-			if (RmgrTable[rmid].rm_startup != NULL)
-				RmgrTable[rmid].rm_startup();
-		}
 
 		ereport(LOG,
 				(errmsg("redo starts at %X/%X",
@@ -1756,12 +1750,7 @@ PerformWalRecovery(void)
 			}
 		}
 
-		/* Allow resource managers to do any required cleanup. */
+		RmgrCleanup();
-		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
-		{
-			if (RmgrTable[rmid].rm_cleanup != NULL)
-				RmgrTable[rmid].rm_cleanup();
-		}
 
 		ereport(LOG,
 				(errmsg("redo done at %X/%X system usage: %s",
@@ -1881,7 +1870,7 @@ 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
@@ -2111,20 +2100,20 @@ 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
@@ -2273,7 +2262,7 @@ 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;
@@ -2353,10 +2342,10 @@ 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);
+			rmgr.rm_mask(replay_image_masked, blkno);
-			RmgrTable[rmid].rm_mask(primary_image_masked, blkno);
+			rmgr.rm_mask(primary_image_masked, blkno);
 		}
 
 		/* Time to compare the primary and replay images. */

src/backend/postmaster/postmaster.c

@@ -1039,6 +1039,12 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeShmemGUCs();
 
+	/*
+	 * Now that modules have been loaded, we can process any custom resource
+	 * managers specified in the wal_consistency_checking GUC.
+	 */
+	InitializeWalConsistencyChecking();
+
 	/*
 	 * If -C was specified with a runtime-computed GUC, we held off printing
 	 * the value earlier, as the GUC was not yet initialized.  We handle -C

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)
+	if (rmgr.rm_decode != NULL)
-		RmgrTable[rmid].rm_decode(ctx, &buf);
+		rmgr.rm_decode(ctx, &buf);
 	else
 	{
 		/* just deal with xid, and done */

src/backend/utils/init/miscinit.c

@@ -1610,6 +1610,7 @@ char	   *local_preload_libraries_string = NULL;
 
 /* Flag telling that we are loading shared_preload_libraries */
 bool		process_shared_preload_libraries_in_progress = false;
+bool		process_shared_preload_libraries_done = false;
 
 /*
  * load the shared libraries listed in 'libraries'
@@ -1677,6 +1678,7 @@ process_shared_preload_libraries(void)
 				   "shared_preload_libraries",
 				   false);
 	process_shared_preload_libraries_in_progress = false;
+	process_shared_preload_libraries_done = true;
 }
 
 /*

src/backend/utils/misc/guc.c

@@ -245,6 +245,11 @@ static bool check_default_with_oids(bool *newval, void **extra, GucSource source
 static ConfigVariable *ProcessConfigFileInternal(GucContext context,
 												 bool applySettings, int elevel);
 
+/*
+ * Track whether there were any deferred checks for custom resource managers
+ * specified in wal_consistency_checking.
+ */
+static bool check_wal_consistency_checking_deferred = false;
 
 /*
  * Options for enum values defined in this module.
@@ -5835,6 +5840,36 @@ InitializeGUCOptions(void)
 	InitializeGUCOptionsFromEnvironment();
 }
 
+/*
+ * If any custom resource managers were specified in the
+ * wal_consistency_checking GUC, processing was deferred. Now that
+ * shared_preload_libraries have been loaded, process wal_consistency_checking
+ * again.
+ */
+void
+InitializeWalConsistencyChecking(void)
+{
+	Assert(process_shared_preload_libraries_done);
+
+	if (check_wal_consistency_checking_deferred)
+	{
+		struct config_generic *guc;
+
+		guc = find_option("wal_consistency_checking", false, false, ERROR);
+
+		check_wal_consistency_checking_deferred = false;
+
+		set_config_option("wal_consistency_checking",
+						  wal_consistency_checking_string,
+						  PGC_POSTMASTER, guc->source,
+						  GUC_ACTION_SET, true, ERROR, false);
+
+		/* checking should not be deferred again */
+		Assert(!check_wal_consistency_checking_deferred);
+	}
+
+}
+
 /*
  * Assign any GUC values that can come from the server's environment.
  *
@@ -11882,13 +11917,13 @@ 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)
+				if (RmgrIdExists(rmid) && GetRmgr(rmid).rm_mask != NULL)
 					newwalconsistency[rmid] = true;
 			found = true;
 		}
@@ -11900,8 +11935,8 @@ check_wal_consistency_checking(char **newval, void **extra, GucSource source)
 			 */
 			for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
 			{
-				if (pg_strcasecmp(tok, RmgrTable[rmid].rm_name) == 0 &&
+				if (RmgrIdExists(rmid) && GetRmgr(rmid).rm_mask != NULL &&
-					RmgrTable[rmid].rm_mask != NULL)
+					pg_strcasecmp(tok, GetRmgr(rmid).rm_name) == 0)
 				{
 					newwalconsistency[rmid] = true;
 					found = true;
@@ -11911,6 +11946,16 @@ check_wal_consistency_checking(char **newval, void **extra, GucSource source)
 
 		/* If a valid resource manager is found, check for the next one. */
 		if (!found)
+		{
+			/*
+			 * Perhaps it's a custom resource manager. If so, defer checking
+			 * until InitializeWalConsistencyChecking().
+			 */
+			if (!process_shared_preload_libraries_done)
+			{
+				check_wal_consistency_checking_deferred = true;
+			}
+			else
 			{
 				GUC_check_errdetail("Unrecognized key word: \"%s\".", tok);
 				pfree(rawstring);
@@ -11918,6 +11963,7 @@ check_wal_consistency_checking(char **newval, void **extra, GucSource source)
 				return false;
 			}
 		}
+	}
 
 	pfree(rawstring);
 	list_free(elemlist);
@@ -11931,7 +11977,20 @@ check_wal_consistency_checking(char **newval, void **extra, GucSource source)
 static void
 assign_wal_consistency_checking(const char *newval, void *extra)
 {
-	wal_consistency_checking = (bool *) extra;
+	/*
+	 * If some checks were deferred, it's possible that the checks will fail
+	 * later during InitializeWalConsistencyChecking(). But in that case, the
+	 * postmaster will exit anyway, so it's safe to proceed with the
+	 * assignment.
+	 *
+	 * Any built-in resource managers specified are assigned immediately,
+	 * which affects WAL created before shared_preload_libraries are
+	 * processed. Any custom resource managers specified won't be assigned
+	 * until after shared_preload_libraries are processed, but that's OK
+	 * because WAL for a custom resource manager can't be written before the
+	 * module is loaded anyway.
+	 */
+	wal_consistency_checking = extra;
 }
 
 static bool

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
+ * RmgrNames is an array of the built-in resource manager names, to make error
- * a bit nicer.
+ * 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++)

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		rmgr_stats[RM_MAX_ID + 1];
-	Stats		record_stats[RM_NEXT_ID][MAX_XLINFO_TYPES];
+	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)
 					{
@@ -1008,22 +1016,44 @@ 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;
+							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;
+					}
+					else
+					{
+						/* 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 (i > RM_MAX_ID)
+						if (rmid > RM_MAX_BUILTIN_ID)
 						{
 							pg_log_error("resource manager \"%s\" does not exist",
 										 optarg);
 							goto bad_argument;
 						}
 					}
+				}
 				break;
 			case 'R':
 				if (sscanf(optarg, "%u/%u/%u",

src/bin/pg_waldump/rmgrdesc.c

@@ -35,6 +35,65 @@
 #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_N_BUILTIN_IDS] = {
 #include "access/rmgrlist.h"
 };
+
+#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;
+}
+
+/*
+ * We are unable to get the real name of a custom rmgr because the module is
+ * not loaded. Generate a table of rmgrs with numeric names of the form
+ * "custom###", where "###" is the 3-digit resource manager 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];
+	}
+}

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 */

src/include/access/rmgr.h

@@ -30,6 +30,23 @@ 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_MAX_CUSTOM_ID	UINT8_MAX
+#define RM_N_IDS			(UINT8_MAX + 1)
+#define RM_N_BUILTIN_IDS	(RM_MAX_BUILTIN_ID + 1)
+#define RM_N_CUSTOM_IDS		(RM_MAX_CUSTOM_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_CUSTOM_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/CustomWALResourceManagers
+ */
+#define RM_EXPERIMENTAL_ID		128
 
 #endif							/* RMGR_H */

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 corresponding RmgrTable entry 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
+RmgrIdExists(RmgrId rmid)
+{
+	return RmgrTable[rmid].rm_name != NULL;
+}
+
+static inline RmgrData
+GetRmgr(RmgrId rmid)
+{
+	if (unlikely(!RmgrIdExists(rmid)))
+		RmgrNotFound(rmid);
+	return RmgrTable[rmid];
+}
 
 /*
  * Exported to support xlog switching from checkpointer

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202204071
+#define CATALOG_VERSION_NO	202204072
 
 #endif

src/include/catalog/pg_proc.dat

@@ -6356,6 +6356,13 @@
   prorettype => 'text', proargtypes => '',
   prosrc => 'pg_get_wal_replay_pause_state' },
 
+{ oid => '8189', descr => 'get resource managers loaded in system',
+  proname => 'pg_get_wal_resource_managers', prorows => '50', proretset => 't',
+  provolatile => 'v', prorettype => 'record', proargtypes => '',
+  proallargtypes => '{int4,text,bool}', proargmodes => '{o,o,o}',
+  proargnames => '{rm_id, rm_name, rm_builtin}',
+  prosrc => 'pg_get_wal_resource_managers' },
+
 { oid => '2621', descr => 'reload configuration files',
   proname => 'pg_reload_conf', provolatile => 'v', prorettype => 'bool',
   proargtypes => '', prosrc => 'pg_reload_conf' },

src/include/miscadmin.h

@@ -465,6 +465,7 @@ extern void BaseInit(void);
 /* in utils/init/miscinit.c */
 extern bool IgnoreSystemIndexes;
 extern PGDLLIMPORT bool process_shared_preload_libraries_in_progress;
+extern bool process_shared_preload_libraries_done;
 extern char *session_preload_libraries_string;
 extern char *shared_preload_libraries_string;
 extern char *local_preload_libraries_string;

src/include/utils/guc.h

@@ -367,6 +367,7 @@ extern void ProcessConfigFile(GucContext context);
 extern char *convert_GUC_name_for_parameter_acl(const char *name);
 extern bool check_GUC_name_for_parameter_acl(const char *name);
 extern void InitializeGUCOptions(void);
+extern void InitializeWalConsistencyChecking(void);
 extern bool SelectConfigFiles(const char *userDoption, const char *progname);
 extern void ResetAllOptions(void);
 extern void AtStart_GUC(void);