From 9fd2f87d7364dfc1e989717e0b66b207adf6b285 Mon Sep 17 00:00:00 2001
From: Konstantin Knizhnik <knizhnik@neon.tech>
Date: Wed, 19 Mar 2025 08:06:11 +0200
Subject: [PATCH] Add neon.ancestor_lsn GUC

---
 pageserver/src/basebackup.rs                  | 10 +++-
 pgxn/neon/neon.c                              |  9 ++++
 .../regress/test_logical_replication.py       | 52 ++++++++++++++++++-
 3 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/pageserver/src/basebackup.rs b/pageserver/src/basebackup.rs
index de527e307b..d3ca4ec479 100644
--- a/pageserver/src/basebackup.rs
+++ b/pageserver/src/basebackup.rs
@@ -331,7 +331,15 @@ where
                     .append(&header, data)
                     .await
                     .map_err(|e| BasebackupError::Client(e, "send_tarball,pg_hba.conf"))?;
-            } else {
+            } else if *filepath == "postgresql.auto.conf" {
+			    let ancestor_lsn = format!("neon.ancestor_lsn='{}'", self.timeline.get_ancestor_lsn());
+                let data = ancestor_lsn.as_bytes();
+                let header = new_tar_header(filepath, data.len() as u64)?;
+                self.ar
+                    .append(&header, data)
+                    .await
+                    .map_err(|e| BasebackupError::Client(e, "send_tarball,pg_hba.conf"))?;
+             } else {
                 let header = new_tar_header(filepath, 0)?;
                 self.ar
                     .append(&header, io::empty())
diff --git a/pgxn/neon/neon.c b/pgxn/neon/neon.c
index 0f226cc9e2..b14f6c034b 100644
--- a/pgxn/neon/neon.c
+++ b/pgxn/neon/neon.c
@@ -497,6 +497,15 @@ _PG_init(void)
 							GUC_UNIT_KB,
 							NULL, NULL, NULL);
 
+	DefineCustomStringVariable("neon.ancestor_lsn",
+							"LSN of the ancestor timeline",
+							"Zero if there is no arent branch",
+							&neon_ancestor_lsn,
+						   "0/0",
+							PGC_SIGHUP,
+							0,
+							NULL, NULL, NULL);
+
 	/*
 	 * Important: This must happen after other parts of the extension are
 	 * loaded, otherwise any settings to GUCs that were set before the
diff --git a/test_runner/regress/test_logical_replication.py b/test_runner/regress/test_logical_replication.py
index 3a92f0d1d1..bcb74962ee 100644
--- a/test_runner/regress/test_logical_replication.py
+++ b/test_runner/regress/test_logical_replication.py
@@ -433,7 +433,7 @@ def test_large_records(neon_simple_env: NeonEnv, vanilla_pg: VanillaPostgres):
 
 
 #
-# Check that slots are not inherited in brnach
+# Check that slots are not inherited in branch
 #
 def test_slots_and_branching(neon_simple_env: NeonEnv):
     env = neon_simple_env
@@ -624,3 +624,53 @@ def test_subscriber_synchronous_commit(neon_simple_env: NeonEnv, vanilla_pg: Van
     log.info("waiting for sync after restart")
     logical_replication_wait_flush_lsn_sync(vanilla_pg)
     assert sub.safe_psql_scalar("SELECT count(*) FROM t") == 1000
+
+
+
+def test_logical_replication_ancestor_lsn(neon_simple_env: NeonEnv, vanilla_pg: VanillaPostgres):
+    env = neon_simple_env
+    tenant, timeline = env.create_tenant()
+
+    main = env.endpoints.create_start("main", tenant_id=tenant)
+    client = env.pageserver.http_client()
+
+    main_conn = main.connect()
+    main_cur = main_conn.cursor()
+
+    main_cur.execute("create table t(pk integer primary key, payload integer)")
+    main_cur.execute("create publication pub1 for table t")
+
+    # insert some data...
+    main_cur.execute("insert into t values (generate_series(1,1000), 0)")
+
+    main_cur.execute("select pg_create_logical_replication_slot('myslot', 'pgoutput')")
+    wait_for_last_flush_lsn(env, main, tenant, timeline)
+
+    #remember WAL position 
+    main_cur.execute("SELECT pg_current_wal_flush_lsn()")
+    main_lsn = Lsn(main_cur.fetchall()[0][0])
+
+    # insert more data...
+    main_cur.execute("insert into t values (generate_series(1001,2000), 0)")
+
+    # create branch
+    branch_timeline = env.create_branch("branch", ancestor_branch_name="main", tenant_id=tenant, ancestor_start_lsn=main_lsn)
+    #all_reparented = client.detach_ancestor(tenant, branch_timeline, detach_behavior="v1")
+    all_reparented = client.detach_ancestor(tenant, branch_timeline)
+    assert all_reparented == set()
+
+    branch = env.endpoints.create_start("branch", tenant_id=tenant)
+    branch_conn = branch.connect()
+    branch_cur = branch_conn.cursor()
+    connstr = branch.connstr().replace("'", "''")
+    log.info(f"ep connstr is {branch.connstr()}, subscriber connstr {vanilla_pg.connstr()}")
+
+    # now start subscriber
+    vanilla_pg.start()
+    vanilla_pg.safe_psql("create table t(pk integer primary key, payload integer)")
+    vanilla_pg.safe_psql(f"create subscription sub1 connection '{connstr}' publication pub1 with (create_slot=false, slot_name='myslot')")
+    #vanilla_pg.safe_psql(f"create subscription sub1 connection '{connstr}' publication pub1")
+
+    logical_replication_sync(vanilla_pg, branch, "sub1")
+
+    assert vanilla_pg.safe_psql("select count(*) from t")[0][0] == 1000