nbtdesc.c and nbtpage.c are inconsistent with XLOG_BTREE_META_CLEANUP (11~)

Hi all,
(Adding in CC relevant committer and author, Teodor and Alexander)

I have been looking today at a crash of pg_waldump on one of the test
builds keeping running in some internal environment. Luckily, I have
been able to put my hands on a core file with 11.2 running. The
backtrace is not that interesting:
(gdb) bt
#0 btree_desc (buf=0x0, record=0x22ce590) at nbtdesc.c:103
#1 0x000000000040419f in XLogDumpDisplayRecord (config=0x7fff1ccfd360,
record=0x22ce590) at
/build/mts/release/bora-13598999/vpostgres/src/postgres/src/bin/pg_waldump/pg_waldump.c:558
#2 main (argc=<optimized out>, argv=<optimized out>) at
/build/mts/release/bora-13598999/vpostgres/src/postgres/src/bin/pg_waldump/pg_waldump.c:1170
(gdb) down 1
#0 btree_desc (buf=0x0, record=0x22ce590) at nbtdesc.c:103
103 nbtdesc.c: No such file or directory.
(gdb) p record
$1 = (XLogReaderState *) 0x22ce590
(gdb) p *record
$2 = {wal_segment_size = 16777216, read_page = 0x405220
<XLogDumpReadPage>, system_identifier = 0, private_data =
0x7fff1ccfd380, ReadRecPtr = 67109592, EndRecPtr = 67109672,
decoded_record = 0x22cf178, main_data = 0x0, main_data_len = 0,
main_data_bufsz = 0, record_origin = 0, blocks = {{in_use = true,
rnode = {spcNode = 16399, dbNode = 16386, relNode = 19907}, forknum =
MAIN_FORKNUM, blkno = 0, flags = 96 '`', has_image = false,
apply_image = false, bkp_image = 0x0, hole_offset = 0, hole_length =
0, bimg_len = 0, bimg_info = 0 '\000', has_data = true, data =
0x22db2c0 "\003", data_len = 32, data_bufsz = 8192}, {in_use = false,
rnode = {spcNode = 0, dbNode = 0, relNode = 0}, forknum =
MAIN_FORKNUM, blkno = 0, flags = 0 '\000', has_image = false,
apply_image = false, bkp_image = 0x0, hole_offset = 0, hole_length =
0, bimg_len = 0, bimg_info = 0 '\000', has_data = false, data = 0x0,
data_len = 0, data_bufsz = 0} <repeats 32 times>}, max_block_id = 0,
readBuf = 0x22ceea0 "\230\320\a", readLen = 8192, readSegNo = 4,
readOff = 0, readPageTLI = 0, latestPagePtr = 67108864, latestPageTLI
= 1, currRecPtr = 67109592, currTLI = 0, currTLIValidUntil = 0,
nextTLI = 0, readRecordBuf = 0x22d12b0 "L", readRecordBufSize = 40960,
errormsg_buf = 0x22d0eb0 ""}
(gdb) p xlrec
$5 = (xl_btree_metadata *) 0x0

Anyway, after looking at the code relevant to XLOG_BTREE_META_CLEANUP,
I have noticed that the meta-data associated to the first buffer is
registered via XLogRegisterBufData() (this is correct because we want
to associate this data to the metadata buffer). However, nbtdesc.c
assumes that xl_btree_metadata is from the main record data, causing a
crash because we have nothing in this case.

I think that we could just patch nbtpage.c so as we fetch the
metadata using XLogRecGetBlockData(), and log its data. The error
comes from 3d92796, which is one year-old and has been visibly
committed untested. I am surprised that we have not seen that
complain yet. Attached is a patch, which looks right to me and should
be back-patched down to v11. I have not taken the time to actually
test it though.

Thoughts?
--
Michael