Re: Timeline switching with partial WAL records can break replica recovery

I've done more research and identified that replicas enter
an indefinite loop in the 'XLogReadPage' function.
The loop works as follows:
0. timeline N contains a partially written record with LSN = targetRecPtr;
1. In 'XLogReadPage' we attempt to read the next page, which has to
   contain the rest of the unfinished record;
2. In 'WaitForWALToBecomeAvailable' walrcv is requested to fetch
   records starting from LSN = targetRecPtr on timeline N + 1;
3. Walrcv retrieves data up to the end of page containing the end of
   timeline N + 1;
4. Then, in 'WaitForWALToBecomeAvailable', replica switches to
   XLOG_FROM_ARCHIVE state, and the function returns true;
5. Execution continues in 'XLogReadPage';
6. The page at addr = targetPagePtr is checked for validity, but we
   get an 'invalid magic number' error because walrcv hasn't retrieved
   this page;
7. Execution jumps to 'next_record_is_invalid' label;
8. Since we are in StandBy mode, the process retries from the beginning.

See the attachments for more colorful illustration this time =)

From my point of view, the first solution which I described in my
previous message still seems like a good choice.

I've also found the current solution in commit [1]. With all due
respect, but it seems to treat the symptom rather than the underlying
issue.

[1]
https://github.com/postgres/postgres/commit/6cf1647d87e7cd423d71525a8759b75c4e4a47ec