Date: Thu, 28 May 2020 17:14:25 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>,
	David Steele <david@pgmasters.net>,
	Andres Freund <andres@anarazel.de>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
Message-ID: <20200528211425.GA5942@alvherre.pgsql>
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Thomas Munro escribió:

> @@ -1094,8 +1103,16 @@ WALRead(XLogReaderState *state,
>  			XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize);
>  			state->routine.segment_open(state, nextSegNo, &tli);
>  
> -			/* This shouldn't happen -- indicates a bug in segment_open */
> -			Assert(state->seg.ws_file >= 0);
> +			/* callback reported that there was no such file */
> +			if (state->seg.ws_file < 0)
> +			{
> +				errinfo->wre_errno = errno;
> +				errinfo->wre_req = 0;
> +				errinfo->wre_read = 0;
> +				errinfo->wre_off = startoff;
> +				errinfo->wre_seg = state->seg;
> +				return false;
> +			}

Ah, this is what Michael was saying ... we need to fix WALRead so that
it doesn't depend on segment_open alway returning a good FD.  This needs
a fix everywhere, not just here, and improve the error report interface.

Maybe it does make sense to get it fixed in pg13 and avoid a break
later.

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