Re: Attempt to consolidate reading of XLOG page

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Antonin Houska <ah@cybertec.at>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Attempt to consolidate reading of XLOG page
Date: Fri, 12 Apr 2019 12:46:56 -0400
Message-ID: <20190412164656.GA21726@alvherre.pgsql> (raw)
In-Reply-To: <14984.1554998742@spoje.net>

On 2019-Apr-11, Antonin Houska wrote:

> While working on the instance encryption I found it annoying to apply
> decyption of XLOG page to three different functions. Attached is a patch that
> tries to merge them all into one function, XLogRead(). The existing
> implementations differ in the way new segment is opened. So I added a pointer
> to callback function as a new argument. This callback handles the specific
> ways to determine segment file name and to open the file.
> 
> I can split the patch into multiple diffs to make detailed review easier, but
> first I'd like to hear if anything is seriously wrong about this
> design. Thanks.

I agree that xlog reading is pretty messy.

I think ifdef'ing the way XLogRead reports errors is not great.  Maybe
we can pass a function pointer that is to be called in case of errors?
Not sure about the walsize; maybe it can be a member in XLogReadPos, and
given to XLogReadInitPos()?  (Maybe rename XLogReadPos as
XLogReadContext or something like that, indicating it's not just the
read position.)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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