All the code to ensure the WAL record lands at a page boundary was
unnecessary for reproducing the original problem. In fact, it's a pretty
basic test that checks that outbound replication (= neon as publisher)
still works after restarting the endpoint. It just used to be very
broken before commit 5ceccdc7de, which also added this test.
To verify that:
1. Check out commit f3af5f4660 (because the next commit, 7dd58e1449,
fixed the same bug in a different way, making it infeasible to revert
the bug fix in an easy way)
2. Revert the bug fix from commit 5ceccdc7de with this:
```
diff --git a/pgxn/neon/walproposer_pg.c b/pgxn/neon/walproposer_pg.c
index 7debb6325..9f03bbd99 100644
--- a/pgxn/neon/walproposer_pg.c
+++ b/pgxn/neon/walproposer_pg.c
@@ -1437,8 +1437,10 @@ XLogWalPropWrite(WalProposer *wp, char *buf, Size nbytes, XLogRecPtr recptr)
*
* https://github.com/neondatabase/neon/issues/5749
*/
+#if 0
if (!wp->config->syncSafekeepers)
XLogUpdateWalBuffers(buf, recptr, nbytes);
+#endif
while (nbytes > 0)
{
```
3. Run the test_wal_page_boundary_start regression test. It fails, as
expected
4. Apply this commit to the test, and run it again. It still fails, with
the same error mentioned in issue #5749:
```
PG:2024-06-30 20:49:08.805 GMT [1248196] STATEMENT: START_REPLICATION SLOT "sub1" LOGICAL 0/0 (proto_version '4', origin 'any', publication_names '"pub1"')
PG:2024-06-30 21:37:52.567 GMT [1467972] LOG: starting logical decoding for slot "sub1"
PG:2024-06-30 21:37:52.567 GMT [1467972] DETAIL: Streaming transactions committing after 0/1532330, reading WAL from 0/1531C78.
PG:2024-06-30 21:37:52.567 GMT [1467972] STATEMENT: START_REPLICATION SLOT "sub1" LOGICAL 0/0 (proto_version '4', origin 'any', publication_names '"pub1"')
PG:2024-06-30 21:37:52.567 GMT [1467972] LOG: logical decoding found consistent point at 0/1531C78
PG:2024-06-30 21:37:52.567 GMT [1467972] DETAIL: There are no running transactions.
PG:2024-06-30 21:37:52.567 GMT [1467972] STATEMENT: START_REPLICATION SLOT "sub1" LOGICAL 0/0 (proto_version '4', origin 'any', publication_names '"pub1"')
PG:2024-06-30 21:37:52.568 GMT [1467972] ERROR: could not find record while sending logically-decoded data: invalid contrecord length 312 (expected 6) at 0/1533FD8
```
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