Move SQL-callable code related to multixacts into its own file

A patch is under discussion to add more SQL capabilities related to
multixacts, and this move avoids bloating the file more than necessary.
This affects pg_get_multixact_members().  A side effect of this move is
the requirement to add mxstatus_to_string() to multixact.h.

Extracted from a larger patch by the same author, tweaked by me.

Author: Naga Appani <nagnrik@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CA+QeY+AAsYK6WvBW4qYzHz4bahHycDAY_q5ECmHkEV_eB9ckzg@mail.gmail.com

src/backend/access/transam/multixact.c

@@ -398,8 +398,6 @@ static int	mXactCacheGetById(MultiXactId multi, MultiXactMember **members);
 static void mXactCachePut(MultiXactId multi, int nmembers,
 						  MultiXactMember *members);
 
-static char *mxstatus_to_string(MultiXactStatus status);
-
 /* management of SLRU infrastructure */
 static bool MultiXactOffsetPagePrecedes(int64 page1, int64 page2);
 static bool MultiXactMemberPagePrecedes(int64 page1, int64 page2);
@@ -1747,7 +1745,7 @@ mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
 	}
 }
 
-static char *
+char *
 mxstatus_to_string(MultiXactStatus status)
 {
 	switch (status)
@@ -3414,68 +3412,6 @@ multixact_redo(XLogReaderState *record)
 		elog(PANIC, "multixact_redo: unknown op code %u", info);
 }
 
-Datum
-pg_get_multixact_members(PG_FUNCTION_ARGS)
-{
-	typedef struct
-	{
-		MultiXactMember *members;
-		int			nmembers;
-		int			iter;
-	} mxact;
-	MultiXactId mxid = PG_GETARG_TRANSACTIONID(0);
-	mxact	   *multi;
-	FuncCallContext *funccxt;
-
-	if (mxid < FirstMultiXactId)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("invalid MultiXactId: %u", mxid)));
-
-	if (SRF_IS_FIRSTCALL())
-	{
-		MemoryContext oldcxt;
-		TupleDesc	tupdesc;
-
-		funccxt = SRF_FIRSTCALL_INIT();
-		oldcxt = MemoryContextSwitchTo(funccxt->multi_call_memory_ctx);
-
-		multi = palloc(sizeof(mxact));
-		/* no need to allow for old values here */
-		multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
-												false);
-		multi->iter = 0;
-
-		if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
-			elog(ERROR, "return type must be a row type");
-		funccxt->tuple_desc = tupdesc;
-		funccxt->attinmeta = TupleDescGetAttInMetadata(tupdesc);
-		funccxt->user_fctx = multi;
-
-		MemoryContextSwitchTo(oldcxt);
-	}
-
-	funccxt = SRF_PERCALL_SETUP();
-	multi = (mxact *) funccxt->user_fctx;
-
-	while (multi->iter < multi->nmembers)
-	{
-		HeapTuple	tuple;
-		char	   *values[2];
-
-		values[0] = psprintf("%u", multi->members[multi->iter].xid);
-		values[1] = mxstatus_to_string(multi->members[multi->iter].status);
-
-		tuple = BuildTupleFromCStrings(funccxt->attinmeta, values);
-
-		multi->iter++;
-		pfree(values[0]);
-		SRF_RETURN_NEXT(funccxt, HeapTupleGetDatum(tuple));
-	}
-
-	SRF_RETURN_DONE(funccxt);
-}
-
 /*
  * Entrypoint for sync.c to sync offsets files.
  */

src/backend/utils/adt/Makefile

@@ -68,6 +68,7 @@ OBJS = \
 	misc.o \
 	multirangetypes.o \
 	multirangetypes_selfuncs.o \
+	multixactfuncs.o \
 	name.o \
 	network.o \
 	network_gist.o \

src/backend/utils/adt/meson.build

@@ -55,6 +55,7 @@ backend_sources += files(
   'misc.c',
   'multirangetypes.c',
   'multirangetypes_selfuncs.c',
+  'multixactfuncs.c',
   'name.c',
   'network.c',
   'network_gist.c',

src/backend/utils/adt/multixactfuncs.c

@@ -0,0 +1,87 @@
+/*-------------------------------------------------------------------------
+ *
+ * multixactfuncs.c
+ *	  Functions for accessing multixact-related data.
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/adt/multixactfuncs.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/multixact.h"
+#include "funcapi.h"
+#include "utils/builtins.h"
+
+/*
+ * pg_get_multixact_members
+ *
+ * Returns information about the MultiXactMembers of the specified
+ * MultiXactId.
+ */
+Datum
+pg_get_multixact_members(PG_FUNCTION_ARGS)
+{
+	typedef struct
+	{
+		MultiXactMember *members;
+		int			nmembers;
+		int			iter;
+	} mxact;
+	MultiXactId mxid = PG_GETARG_TRANSACTIONID(0);
+	mxact	   *multi;
+	FuncCallContext *funccxt;
+
+	if (mxid < FirstMultiXactId)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid MultiXactId: %u", mxid)));
+
+	if (SRF_IS_FIRSTCALL())
+	{
+		MemoryContext oldcxt;
+		TupleDesc	tupdesc;
+
+		funccxt = SRF_FIRSTCALL_INIT();
+		oldcxt = MemoryContextSwitchTo(funccxt->multi_call_memory_ctx);
+
+		multi = palloc(sizeof(mxact));
+		/* no need to allow for old values here */
+		multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
+												false);
+		multi->iter = 0;
+
+		if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+			elog(ERROR, "return type must be a row type");
+		funccxt->tuple_desc = tupdesc;
+		funccxt->attinmeta = TupleDescGetAttInMetadata(tupdesc);
+		funccxt->user_fctx = multi;
+
+		MemoryContextSwitchTo(oldcxt);
+	}
+
+	funccxt = SRF_PERCALL_SETUP();
+	multi = (mxact *) funccxt->user_fctx;
+
+	while (multi->iter < multi->nmembers)
+	{
+		HeapTuple	tuple;
+		char	   *values[2];
+
+		values[0] = psprintf("%u", multi->members[multi->iter].xid);
+		values[1] = mxstatus_to_string(multi->members[multi->iter].status);
+
+		tuple = BuildTupleFromCStrings(funccxt->attinmeta, values);
+
+		multi->iter++;
+		pfree(values[0]);
+		SRF_RETURN_NEXT(funccxt, HeapTupleGetDatum(tuple));
+	}
+
+	SRF_RETURN_DONE(funccxt);
+}

src/include/access/multixact.h

@@ -158,5 +158,6 @@ extern void multixact_desc(StringInfo buf, XLogReaderState *record);
 extern const char *multixact_identify(uint8 info);
 extern char *mxid_to_string(MultiXactId multi, int nmembers,
 							MultiXactMember *members);
+extern char *mxstatus_to_string(MultiXactStatus status);
 
 #endif							/* MULTIXACT_H */