Add on-demand WAL download to slot funcs (#8705)

## Problem
Currently we can have an issue where if someone does
`pg_logical_slot_advance`, it could fail because it doesn't have the WAL
locally.

## Summary of changes
Adds on-demand WAL download and a test to these slot funcs. Before
adding these, the test fails with
```
requested WAL segment pg_wal/000000010000000000000001 has already been removed
```
After the changes, the test passes


Relies on:
- https://github.com/neondatabase/postgres/pull/466
- https://github.com/neondatabase/postgres/pull/467
- https://github.com/neondatabase/postgres/pull/468

pgxn/neon/neon.c

@@ -599,6 +599,7 @@ _PG_init(void)
 	pg_init_walproposer();
 	WalSender_Custom_XLogReaderRoutines = NeonOnDemandXLogReaderRoutines;
 	LogicalFuncs_Custom_XLogReaderRoutines = NeonOnDemandXLogReaderRoutines;
+	SlotFuncs_Custom_XLogReaderRoutines = NeonOnDemandXLogReaderRoutines;
 
 	InitLogicalReplicationMonitor();
 

test_runner/regress/test_logical_replication.py

@@ -255,6 +255,21 @@ FROM generate_series(1, 16384) AS seq; -- Inserts enough rows to exceed 16MB of
         cur.execute(
             "SELECT * FROM pg_logical_slot_peek_binary_changes('slotty_mcslotface', NULL, NULL, 'include-xids', '0')"
         )
+        cur.execute(
+            """
+INSERT INTO wal_generator (data)
+SELECT repeat('A', 1024) -- Generates a kilobyte of data per row
+FROM generate_series(1, 16384) AS seq; -- Inserts enough rows to exceed 16MB of data
+"""
+        )
+
+    endpoint.stop_and_destroy()
+    endpoint = env.endpoints.create_start("init")
+    with endpoint.connect().cursor() as cur:
+        log.info("advance slot")
+        cur.execute(
+            "SELECT * from pg_replication_slot_advance('slotty_mcslotface', pg_current_wal_lsn())"
+        )
 
 
 # Tests that walsender correctly blocks until WAL is downloaded from safekeepers

vendor/revisions.json

@@ -1,14 +1,14 @@
 {
   "v16": [
     "16.3",
-    "6434b1499b11ed97dccea5618a055034b83b8e2f"
+    "5ea106b2583285849784e774b39d62eb2615bd5d"
   ],
   "v15": [
     "15.7",
-    "47c8d462d169367c8979ce628a523be2d94b46be"
+    "39c51c33b383239c78b86afe561679f980e44842"
   ],
   "v14": [
     "14.12",
-    "ae07734e0ff72759ab425fc8f625d4c1ecb15a50"
+    "a48faca1d9aef59649dd1bf34bc1b6303fa3489e"
   ]
 }