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diff --git a/src/test/modules/injection_points/regress_injection.c b/src/test/modules/injection_points/regress_injection.c
new file mode 100644
index 00000000000..422f4168935
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+++ b/src/test/modules/injection_points/regress_injection.c
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+/*--------------------------------------------------------------------------
+ *
+ * regress_injection.c
+ *		Functions supporting test-specific subject matter.
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/regress_injection.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/table.h"
+#include "fmgr.h"
+#include "miscadmin.h"
+#include "storage/procarray.h"
+#include "utils/xid8.h"
+
+/*
+ * removable_cutoff - for syscache-update-pruned.spec
+ *
+ * Wrapper around GetOldestNonRemovableTransactionId().  In general, this can
+ * move backward.  runningcheck=false isolation tests can reasonably prevent
+ * that.  For the causes of backward movement, see
+ * postgr.es/m/CAEze2Wj%2BV0kTx86xB_YbyaqTr5hnE_igdWAwuhSyjXBYscf5-Q%40mail.gmail.com
+ * and the header comment for ComputeXidHorizons().  One can assume this
+ * doesn't move backward if one arranges for concurrent activity not to reach
+ * AbortTransaction() and not to allocate an XID while connected to another
+ * database.  Non-runningcheck tests can control most concurrent activity,
+ * except autovacuum and the isolationtester control connection.  Neither
+ * allocates XIDs, and AbortTransaction() in those would justify test failure.
+ */
+PG_FUNCTION_INFO_V1(removable_cutoff);
+Datum
+removable_cutoff(PG_FUNCTION_ARGS)
+{
+	Relation	rel = NULL;
+	TransactionId xid;
+	FullTransactionId next_fxid_before,
+				next_fxid;
+
+	/* could take other relkinds callee takes, but we've not yet needed it */
+	if (!PG_ARGISNULL(0))
+		rel = table_open(PG_GETARG_OID(0), AccessShareLock);
+
+	/*
+	 * No lock or snapshot necessarily prevents oldestXid from advancing past
+	 * "xid" while this function runs.  That concerns us only in that we must
+	 * not ascribe "xid" to the wrong epoch.  (That may never arise in
+	 * isolation testing, but let's set a good example.)  As a crude solution,
+	 * retry until nextXid doesn't change.
+	 */
+	next_fxid = ReadNextFullTransactionId();
+	do
+	{
+		CHECK_FOR_INTERRUPTS();
+		next_fxid_before = next_fxid;
+		xid = GetOldestNonRemovableTransactionId(rel);
+		next_fxid = ReadNextFullTransactionId();
+	} while (!FullTransactionIdEquals(next_fxid, next_fxid_before));
+
+	if (rel)
+		table_close(rel, AccessShareLock);
+
+	PG_RETURN_FULLTRANSACTIONID(FullTransactionIdFromAllowableAt(next_fxid,
+																 xid));
+}