0004-Add-an-explicit-valid-flag-to-MVCCSnapshotData.patch

From 6f49e70cd926f0a5906636e8e82253598bff561f Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Mon, 31 Mar 2025 23:47:48 +0300
Subject: [PATCH 4/9] Add an explicit 'valid' flag to MVCCSnapshotData

The lifetime of the "static" snapshots returned by
GetTransactionSnapshot(), GetLatestSnapshot() and GetCatalogSnapshot()
is a bit vague. By adding an explicit 'valid' flag, we can make it
more clear when a function call updates a static snapshot, making it
valid, and when another function makes it invalid again. It's
currently only used in assertions, and can also be handy when
debugging.
---
 src/backend/storage/ipc/procarray.c |  2 ++
 src/backend/utils/time/snapmgr.c    | 15 +++++++++++++++
 src/include/utils/snapshot.h        |  1 +
 3 files changed, 18 insertions(+)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 2292d8a09af..29a743ee49c 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2083,6 +2083,7 @@ GetSnapshotDataReuse(MVCCSnapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
+	snapshot->valid = true;
 
 	return true;
 }
@@ -2462,6 +2463,7 @@ GetSnapshotData(MVCCSnapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
+	snapshot->valid = true;
 
 	return snapshot;
 }
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 85395f0a8df..455afc9bf98 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -459,6 +459,7 @@ InvalidateCatalogSnapshot(void)
 	{
 		pairingheap_remove(&RegisteredSnapshots, &CatalogSnapshot->ph_node);
 		CatalogSnapshot = NULL;
+		CatalogSnapshotData.valid = false;
 		SnapshotResetXmin();
 		INJECTION_POINT("invalidate-catalog-snapshot-end", NULL);
 	}
@@ -624,6 +625,7 @@ CopyMVCCSnapshot(MVCCSnapshot snapshot)
 	newsnap->regd_count = 0;
 	newsnap->active_count = 0;
 	newsnap->copied = true;
+	newsnap->valid = true;
 	newsnap->snapXactCompletionCount = 0;
 
 	/* setup XID array */
@@ -665,6 +667,7 @@ FreeMVCCSnapshot(MVCCSnapshot snapshot)
 	Assert(snapshot->regd_count == 0);
 	Assert(snapshot->active_count == 0);
 	Assert(snapshot->copied);
+	Assert(snapshot->valid);
 
 	pfree(snapshot);
 }
@@ -701,6 +704,7 @@ PushActiveSnapshotWithLevel(Snapshot snapshot, int snap_level)
 
 	Assert(snapshot->snapshot_type == SNAPSHOT_MVCC);
 	origsnap = &snapshot->mvcc;
+	Assert(origsnap->valid);
 
 	Assert(ActiveSnapshot == NULL || snap_level >= ActiveSnapshot->as_level);
 
@@ -860,6 +864,7 @@ RegisterSnapshotOnOwner(Snapshot orig_snapshot, ResourceOwner owner)
 
 	Assert(orig_snapshot->snapshot_type == SNAPSHOT_MVCC);
 	snapshot = &orig_snapshot->mvcc;
+	Assert(snapshot->valid);
 
 	/* Static snapshot?  Create a persistent copy */
 	snapshot = snapshot->copied ? snapshot : CopyMVCCSnapshot(snapshot);
@@ -981,6 +986,15 @@ SnapshotResetXmin(void)
 {
 	MVCCSnapshot minSnapshot;
 
+	/*
+	 * These static snapshots are not in the RegisteredSnapshots list, so we
+	 * might advance MyProc->xmin past their xmin. (Note that in case of
+	 * IsolationUsesXactSnapshot() == true, CurrentSnapshot points to the copy
+	 * in FirstSnapshot rather than CurrentSnapshotData.)
+	 */
+	CurrentSnapshotData.valid = false;
+	SecondarySnapshotData.valid = false;
+
 	if (ActiveSnapshot != NULL)
 		return;
 
@@ -1884,6 +1898,7 @@ RestoreSnapshot(char *start_address)
 	snapshot->regd_count = 0;
 	snapshot->active_count = 0;
 	snapshot->copied = true;
+	snapshot->valid = true;
 
 	return snapshot;
 }
diff --git a/src/include/utils/snapshot.h b/src/include/utils/snapshot.h
index bca0ad16e68..1697c6df856 100644
--- a/src/include/utils/snapshot.h
+++ b/src/include/utils/snapshot.h
@@ -161,6 +161,7 @@ typedef struct MVCCSnapshotData
 
 	bool		takenDuringRecovery;	/* recovery-shaped snapshot? */
 	bool		copied;			/* false if it's a static snapshot */
+	bool		valid;			/* is this snapshot valid? */
 
 	CommandId	curcid;			/* in my xact, CID < curcid are visible */
 
-- 
2.47.3