postgres_ffi/wal_generate: add last_wal_record_xlog_switch and use it in tests

Fix #1190: WalDecoder did not return correct LSN of the next record after
processing a XLOG_SWITCH record

libs/postgres_ffi/src/waldecoder.rs

@@ -226,10 +226,10 @@ impl WalStreamDecoder {
             self.padlen = self.lsn.calc_padding(8u32) as u32;
         }
 
-        // Always align resulting LSN on 0x8 boundary -- that is important for getPage()
-        // and WalReceiver integration. Since this code is used both for WalReceiver and
-        // initial WAL import let's force alignment right here.
-        let result = (self.lsn.align(), recordbuf);
+        // We should return LSN of the next record, not the last byte of this record or
+        // the byte immediately after. Note that this handles both XLOG_SWITCH and usual
+        // records, the former "spans" until the next WAL segment (see test_xlog_switch).
+        let result = (self.lsn + self.padlen as u64, recordbuf);
         Ok(Some(result))
     }
 }

libs/postgres_ffi/wal_generate/src/bin/wal_generate.rs

@@ -13,6 +13,7 @@ fn main() -> Result<()> {
         .help("Type of WAL to generate")
         .possible_values([
             "simple",
+            "last_wal_record_xlog_switch",
             "last_wal_record_crossing_segment",
             "wal_record_crossing_segment_followed_by_small_one",
         ])
@@ -57,6 +58,7 @@ fn main() -> Result<()> {
     let wal_generate = |arg_matches: &ArgMatches, client| {
         let lsn = match arg_matches.value_of("type").unwrap() {
             "simple" => generate_simple(client)?,
+            "last_wal_record_xlog_switch" => generate_last_wal_record_xlog_switch(client)?,
             "last_wal_record_crossing_segment" => {
                 generate_last_wal_record_crossing_segment(client)?
             }

libs/postgres_ffi/wal_generate/src/lib.rs

@@ -250,6 +250,25 @@ pub fn generate_simple(client: &mut impl postgres::GenericClient) -> Result<PgLs
     })
 }
 
+pub fn generate_last_wal_record_xlog_switch(
+    client: &mut impl postgres::GenericClient,
+) -> Result<PgLsn> {
+    // Do not use generate_internal because here we end up with flush_lsn exactly on
+    // the segment boundary and insert_lsn after the initial page header, which is unusual.
+    ensure_server_config(client)?;
+
+    client.execute("CREATE table t(x int)", &[])?;
+    let after_xlog_switch: PgLsn = client.query_one("SELECT pg_switch_wal()", &[])?.get(0);
+    let next_segment = PgLsn::from(0x0200_0000);
+    ensure!(
+        after_xlog_switch <= next_segment,
+        "XLOG_SWITCH message ended after the expected segment boundary: {} > {}",
+        after_xlog_switch,
+        next_segment
+    );
+    Ok(next_segment)
+}
+
 fn generate_single_logical_message(
     client: &mut impl postgres::GenericClient,
     transactional: bool,

test_runner/batch_others/test_crafted_wal_end.py

@@ -9,6 +9,7 @@ import pytest
 @pytest.mark.parametrize('wal_type',
                          [
                              'simple',
+                             'last_wal_record_xlog_switch',
                              'last_wal_record_crossing_segment',
                              'wal_record_crossing_segment_followed_by_small_one',
                          ])