v16_fix_snapbuild_bug_v3.patch

diff --git a/contrib/test_decoding/Makefile b/contrib/test_decoding/Makefile
index c7ce603706..a4ba1a509a 100644
--- a/contrib/test_decoding/Makefile
+++ b/contrib/test_decoding/Makefile
@@ -8,7 +8,8 @@ REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
 	spill slot truncate stream stats twophase twophase_stream
 ISOLATION = mxact delayed_startup ondisk_startup concurrent_ddl_dml \
 	oldest_xmin snapshot_transfer subxact_without_top concurrent_stream \
-	twophase_snapshot slot_creation_error catalog_change_snapshot
+	twophase_snapshot slot_creation_error catalog_change_snapshot \
+	skip_snapshot_restore
 
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
 ISOLATION_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
diff --git a/contrib/test_decoding/meson.build b/contrib/test_decoding/meson.build
index 7b05cc25a3..2dd3ede41b 100644
--- a/contrib/test_decoding/meson.build
+++ b/contrib/test_decoding/meson.build
@@ -62,6 +62,7 @@ tests += {
       'concurrent_stream',
       'twophase_snapshot',
       'slot_creation_error',
+      'skip_snapshot_restore',
     ],
     'regress_args': [
       '--temp-config', files('logical.conf'),
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 41243d0187..b57fc33cbb 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -637,6 +637,12 @@ DecodingContextFindStartpoint(LogicalDecodingContext *ctx)
 	elog(DEBUG1, "searching for logical decoding starting point, starting at %X/%X",
 		 LSN_FORMAT_ARGS(slot->data.restart_lsn));
 
+	/*
+	 * Tell snapshot builder to start finding the start point, preventing
+	 * it from restoring snapshots.
+	 */
+	SnapBuildFindingStartPoint(ctx->snapshot_builder, true);
+
 	/* Wait for a consistent starting point */
 	for (;;)
 	{
@@ -659,6 +665,8 @@ DecodingContextFindStartpoint(LogicalDecodingContext *ctx)
 		CHECK_FOR_INTERRUPTS();
 	}
 
+	SnapBuildFindingStartPoint(ctx->snapshot_builder, false);
+
 	SpinLockAcquire(&slot->mutex);
 	slot->data.confirmed_flush = ctx->reader->EndRecPtr;
 	if (slot->data.two_phase)
diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 7a7aba33e1..e8dc7acbf3 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -278,6 +278,15 @@ struct SnapBuild
 static ResourceOwner SavedResourceOwnerDuringExport = NULL;
 static bool ExportInProgress = false;
 
+/*
+ * True if we are finding the logical decoding start point. While finding
+ * the start point, we need to prevent the snapshot being restored
+ * from disk and require all in-progress transactions to finish, in order
+ * to become the consistent state. Otherwise the slot's restart and
+ * confirmed flush LSNs could be initialized to unsafe values.
+ */
+static bool	FindingStartPoint = false;
+
 /* ->committed and ->catchange manipulation */
 static void SnapBuildPurgeOlderTxn(SnapBuild *builder);
 
@@ -346,6 +355,8 @@ AllocateSnapshotBuilder(ReorderBuffer *reorder,
 	builder->building_full_snapshot = need_full_snapshot;
 	builder->two_phase_at = two_phase_at;
 
+	FindingStartPoint = false;
+
 	MemoryContextSwitchTo(oldcontext);
 
 	return builder;
@@ -422,6 +433,15 @@ SnapBuildSetTwoPhaseAt(SnapBuild *builder, XLogRecPtr ptr)
 	builder->two_phase_at = ptr;
 }
 
+/*
+ * Set we are finding the logical decoding start point.
+ */
+void
+SnapBuildFindingStartPoint(SnapBuild *builder, bool finding)
+{
+   FindingStartPoint = finding;
+}
+
 /*
  * Should the contents of transaction ending at 'ptr' be decoded?
  */
@@ -1326,7 +1346,9 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
 	 *	  snapshot to disk that we can use.  Can't use this method for the
 	 *	  initial snapshot when slot is being created and needs full snapshot
 	 *	  for export or direct use, as that snapshot will only contain catalog
-	 *	  modifying transactions.
+	 *    modifying transactions. We cannot use this method also when finding
+	 *    the logical decoding start point, as it requires all in-progress
+	 *    transaction to finish.
 	 *
 	 * c) First incrementally build a snapshot for catalog tuples
 	 *	  (BUILDING_SNAPSHOT), that requires all, already in-progress,
@@ -1391,8 +1413,12 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
 
 		return false;
 	}
-	/* b) valid on disk state and not building full snapshot */
+	/*
+	 * b) valid on disk state and neither building full snapshot nor
+	 * finding the start point.
+	 */
 	else if (!builder->building_full_snapshot &&
+			 !FindingStartPoint &&
 			 SnapBuildRestore(builder, lsn))
 	{
 		/* there won't be any state to cleanup */
diff --git a/src/include/replication/snapbuild.h b/src/include/replication/snapbuild.h
index f49b941b53..8239d6dbb3 100644
--- a/src/include/replication/snapbuild.h
+++ b/src/include/replication/snapbuild.h
@@ -90,5 +90,6 @@ extern void SnapBuildProcessNewCid(SnapBuild *builder, TransactionId xid,
 extern void SnapBuildProcessRunningXacts(SnapBuild *builder, XLogRecPtr lsn,
 										 struct xl_running_xacts *running);
 extern void SnapBuildSerializationPoint(SnapBuild *builder, XLogRecPtr lsn);
+extern void SnapBuildFindingStartPoint(SnapBuild *builder, bool finding);
 
 #endif							/* SNAPBUILD_H */