Fix on-demand SLRU download on standby starting at WAL segment bounda…

…ry (#8031)

If a standby is started right after switching to a new WAL segment, the
request in the SLRU download request would point to the beginning of the
segment (e.g. 0/5000000), while the not-modified-since LSN would point
to just after the page header (e.g. 0/5000028). It's effectively the
same position, as there cannot be any WAL records in between, but the
pageserver rightly errors out on any request where the request LSN <
not-modified since LSN.

To fix, round down the not-modified since LSN to the beginning of the
page like the request LSN.

Fixes issue #8030

pgxn/neon/pagestore_smgr.c

@@ -3112,12 +3112,12 @@ neon_read_slru_segment(SMgrRelation reln, const char* path, int segno, void* buf
 		request_lsn = UINT64_MAX;
 
 	/*
-	 * GetRedoStartLsn() returns LSN of basebackup. We know that the SLRU
+	 * GetRedoStartLsn() returns LSN of the basebackup. We know that the SLRU
 	 * segment has not changed since the basebackup, because in order to
 	 * modify it, we would have had to download it already. And once
 	 * downloaded, we never evict SLRU segments from local disk.
 	 */
-	not_modified_since = GetRedoStartLsn();
+	not_modified_since = nm_adjust_lsn(GetRedoStartLsn());
 
 	SlruKind kind;
 

test_runner/regress/test_ondemand_slru_download.py

@@ -129,3 +129,33 @@ def test_ondemand_download_replica(neon_env_builder: NeonEnvBuilder, shard_count
     cur_replica = conn_replica.cursor()
     cur_replica.execute("SELECT * FROM clogtest")
     assert cur_replica.fetchall() == [(1,), (3,)]
+
+
+def test_ondemand_download_after_wal_switch(neon_env_builder: NeonEnvBuilder):
+    """
+    Test on-demand SLRU download on standby, when starting right after
+    WAL segment switch.
+
+    This is a repro for a bug in how the LSN at WAL page/segment
+    boundary was handled (https://github.com/neondatabase/neon/issues/8030)
+    """
+
+    tenant_conf = {
+        "lazy_slru_download": "true",
+    }
+    env = neon_env_builder.init_start(initial_tenant_conf=tenant_conf)
+
+    endpoint = env.endpoints.create_start("main")
+    pg_conn = endpoint.connect()
+    cur = pg_conn.cursor()
+
+    # Create a test table
+    cur.execute("CREATE TABLE clogtest (id integer)")
+    cur.execute("INSERT INTO clogtest VALUES (1)")
+
+    # Start standby at WAL segment boundary
+    cur.execute("SELECT pg_switch_wal()")
+    lsn = Lsn(query_scalar(cur, "SELECT pg_current_wal_insert_lsn()"))
+    _endpoint_at_lsn = env.endpoints.create_start(
+        branch_name="main", endpoint_id="ep-at-lsn", lsn=lsn
+    )