2pc leaks fds

Hi,

Using 2PC with master very quickly leads to:

2020-04-05 19:42:18.368 PDT [2298126][5/2009:0] LOG: out of file descriptors: Too many open files; release and retry
2020-04-05 19:42:18.368 PDT [2298126][5/2009:0] STATEMENT: COMMIT PREPARED 'ptx_2';

This started with:

commit 0dc8ead46363fec6f621a12c7e1f889ba73b55a9 (HEAD -> master)
Author: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Date: 2019-11-25 15:04:54 -0300

Refactor WAL file-reading code into WALRead()

I found this while trying to benchmark the effect of my snapshot changes
on 2pc. I just used the attached pgbench file.

andres(at)awork3:~/build/postgres/dev-assert/vpath$ pgbench -n -s 500 -c 4 -j 4 -T 100000 -P1 -f ~/tmp/pgbench-write-2pc.sql
progress: 1.0 s, 3723.8 tps, lat 1.068 ms stddev 0.305
client 2 script 0 aborted in command 8 query 0: ERROR: could not seek to end of file "base/14036/16396": Too many open files
client 1 script 0 aborted in command 8 query 0: ERROR: could not seek to end of file "base/14036/16396": Too many open files
client 3 script 0 aborted in command 8 query 0: ERROR: could not seek to end of file "base/14036/16396": Too many open files
client 0 script 0 aborted in command 8 query 0: ERROR: could not seek to end of file "base/14036/16396": Too many open files
transaction type: /home/andres/tmp/pgbench-write-2pc.sql

I've not yet reviewed the change sufficiently to pinpoint the issue.

It's a bit sad that nobody has hit this in the last few months :(.

Greetings,

Andres Freund