Re: standby recovery fails (tablespace related) (tentative patch and discussion)

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Cc: dilipbalaut(at)gmail(dot)com, robertmhaas(at)gmail(dot)com, michael(at)paquier(dot)xyz, rjuju123(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: standby recovery fails (tablespace related) (tentative patch and discussion)
Date: 2022-07-15 10:58:44
Message-ID: 20220715105844.e5iuer7bav4chdnl@alvherre.pgsql
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On 2022-Jul-15, Kyotaro Horiguchi wrote:

> 0002:
> +is_path_tslink(const char *path)
> What the "ts" of tslink stands for? If it stands for tablespace, the
> function is not specific for table spaces. We already have
> + errmsg("could not stat file \"%s\": %m", path));
> I'm not sure we need such correctness, but what is failing there is
> lstat. I found similar codes in two places in backend and one place
> in frontend. So couldn't it be moved to /common and have a more
> generic name?

I wondered whether it'd be better to check whether get_dirent_type
returns PGFILETYPE_LNK. However, that doesn't deal with junction points
at all, which seems pretty odd ... I mean, isn't it rather useful as an
abstraction if it doesn't abstract away the one platform-dependent point
we have in the area?

However, looking closer I noticed that on Windows we use our own
readdir() implementation, which AFAICT includes everything to handle
reparse points as symlinks correctly in get_dirent_type. Which means
that do_pg_start_backup is wasting its time with the "#ifdef WIN32" bits
to handle junction points separately. We could just do this

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index b809a2152c..4966213fde 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8302,13 +8302,8 @@ do_pg_backup_start(const char *backupidstr, bool fast, TimeLineID *starttli_p,
* we sometimes use allow_in_place_tablespaces to create
* directories directly under pg_tblspc, which would fail below.
-#ifdef WIN32
- if (!pgwin32_is_junction(fullpath))
- continue;
if (get_dirent_type(fullpath, de, false, ERROR) != PGFILETYPE_LNK)

#if defined(HAVE_READLINK) || defined(WIN32)
rllen = readlink(fullpath, linkpath, sizeof(linkpath));

And everything should continue to work.

Álvaro Herrera PostgreSQL Developer —

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