Don't get confused on request of latest page version with very old LSN.

If the 'latest' flag in the client request is true, the client wants the
latest page version regardless of the LSN in the request. The LSN is just
a hint in that case, indicating that the page hasn't been modified since
since that LSN. The LSN can be very old, so it's possible that the page
server has already garbage collected away the layer at that LSN. We tried
to fetch the old layer and errored out if that happened. To fix, always
fetch the data as of last-record-LSN, if 'latest' is set in the client
request. We now only use the LSN to wait if the requested LSN hasn't been
received and processed yet.

Fixes https://github.com/zenithdb/zenith/issues/567

test_runner/batch_others/test_old_request_lsn.py

@@ -0,0 +1,65 @@
+from contextlib import closing
+
+from fixtures.zenith_fixtures import PostgresFactory, ZenithPageserver
+
+pytest_plugins = ("fixtures.zenith_fixtures")
+
+#
+# Test where Postgres generates a lot of WAL, and it's garbage collected away, but
+# no pages are evicted so that Postgres uses an old LSN in a GetPage request.
+# We had a bug where the page server failed to find the page version because it
+# thought it was already garbage collected away, because the LSN in the GetPage
+# request was very old and the WAL from that time was indeed already removed.
+# In reality, the LSN on a GetPage request coming from a primary server is
+# just a hint that the page hasn't been modified since that LSN, and the page
+# server should return the latest page version regardless of the LSN.
+#
+def test_old_request_lsn(zenith_cli, pageserver: ZenithPageserver, postgres: PostgresFactory, pg_bin):
+    # Create a branch for us
+    zenith_cli.run(["branch", "test_old_request_lsn", "empty"])
+    pg = postgres.create_start('test_old_request_lsn')
+    print('postgres is running on test_old_request_lsn branch')
+
+    pg_conn = pg.connect()
+    cur = pg_conn.cursor()
+
+    # Get the timeline ID of our branch. We need it for the 'do_gc' command
+    cur.execute("SHOW zenith.zenith_timeline")
+    timeline = cur.fetchone()[0]
+
+    psconn = pageserver.connect()
+    pscur = psconn.cursor()
+
+    # Create table, and insert some rows. Make it big enough that it doesn't fit in
+    # shared_buffers.
+    cur.execute('CREATE TABLE foo (id int4 PRIMARY KEY, val int, t text)')
+    cur.execute('''
+        INSERT INTO foo
+            SELECT g, 1, 'long string to consume some space' || g
+            FROM generate_series(1, 100000) g
+    ''')
+
+    # Verify that the table is larger than shared_buffers, so that the SELECT below
+    # will cause GetPage requests.
+    cur.execute('''
+        select setting::int * pg_size_bytes(unit) as shared_buffers, pg_relation_size('foo') as tbl_ize
+        from pg_settings where name = 'shared_buffers'
+    ''')
+    row = cur.fetchone()
+    print("shared_buffers is {}, table size {}", row[0], row[1]);
+    assert int(row[0]) < int(row[1])
+
+    cur.execute('VACUUM foo');
+
+    # Make a lot of updates on a single row, generating a lot of WAL. Trigger
+    # garbage collections so that the page server will remove old page versions.
+    for i in range(10):
+        pscur.execute(f"do_gc {pageserver.initial_tenant} {timeline} 0")
+        for j in range(100):
+            cur.execute('UPDATE foo SET val = val + 1 WHERE id = 1;');
+
+    # All (or at least most of) the updates should've been on the same page, so
+    # that we haven't had to evict any dirty pages for a long time. Now run
+    # a query that sends GetPage@LSN requests with the old LSN.
+    cur.execute("SELECT COUNT(*), SUM(val) FROM foo");
+    assert cur.fetchone() == (100000, 101000)