From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40363) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1axwiE-00069Q-3m for qemu-devel@nongnu.org; Wed, 04 May 2016 09:19:41 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1axwi1-0008P2-Mg for qemu-devel@nongnu.org; Wed, 04 May 2016 09:19:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47899) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1axwi1-0008Ko-GG for qemu-devel@nongnu.org; Wed, 04 May 2016 09:19:21 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 29BBC61A0E for ; Wed, 4 May 2016 13:19:10 +0000 (UTC) Date: Wed, 4 May 2016 14:19:06 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20160504131905.GH2302@work-vm> References: <1461903820-3092-1-git-send-email-eblake@redhat.com> <1461903820-3092-11-git-send-email-eblake@redhat.com> <20160503094447.GE2242@work-vm> <87inyv5nv7.fsf@dusky.pond.sub.org> <20160503132358.GH2242@work-vm> <87eg9iyypp.fsf@dusky.pond.sub.org> <20160504092249.GC2302@work-vm> <87lh3qrr4a.fsf@dusky.pond.sub.org> <20160504115616.GE2302@work-vm> <87pot2ou5r.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pot2ou5r.fsf@dusky.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH v3 10/18] vmstate: Use new JSON output visitor List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Amit Shah , famz@redhat.com, qemu-devel@nongnu.org, Juan Quintela * Markus Armbruster (armbru@redhat.com) wrote: > "Dr. David Alan Gilbert" writes: > > > * Markus Armbruster (armbru@redhat.com) wrote: > >> "Dr. David Alan Gilbert" writes: > >> > >> > * Markus Armbruster (armbru@redhat.com) wrote: > >> >> "Dr. David Alan Gilbert" writes: > >> > > >> >> "git-grep assert migration" suggests you do kill the source on certain > >> >> programming errors. > >> > > >> > I'm just trying hard to reduce them; I know I'm not there, but I'd rather > >> > we didn't have any - especially on the source side. > >> > > >> >> I reiterate my point that fancy, untestable error recovery is unlikely > >> >> to actually recover. "Fancy" can work, "untestable" might work (but > >> >> color me skeptic), but once you got both, you're a dead man walking. > >> > > >> > Then we should make the error recovery paths easy; at the moment visitor > >> > error paths are just too painful. > >> > >> I've never seen error handling in C that wasn't painful and still > >> correct. Surprise me! > > > > The thing that makes it hard for the visitor code is the need to check > > it after every call and the check is complicated. > > Having to check every call is certainly painful, but there's no general > and safe way around it. Accumulating errors that need to be checked > only at the end of a job can be less painful, but then the job's code > needs to be very carefully written to be safe even in presence of > errors. Most code isn't, and some code can't. Yes; output visitors would seem to be the easiest case though? > The check for failure is simple, but annoyingly verbose when the > function's return value is useless: > > Error *err = NULL; > foo(..., &err); > if (err) { > ... > } > > I'm playing with a update to conventions and usage to permit > > if (!foo(..., &err)) { > ... > } If that became; if (!foo(..., &err) || !foo(..., &err) || !foo(..., &err)) { ... } That would be both readable and not verbose. > Just as simple, but more readable. > > [...] > >> I figure we're unlikely to reach consensus on this, so I'd like to > >> propose we agree to disagree, and do the following: > >> > >> * We shelve the de-duplication of JSON formatting (this patch) > >> indefinitely. > >> > >> * We move qjson.c to migration/, next to its only user, and add a > >> comment explaining why it migration doesn't want to use general > >> infrastructure here (JSON output visitor), but needs its own thing. > >> This gets the file covered in MAINTAINERS, and will help prevent it > >> growing additional users. > >> > >> Deal? > > > > No, sorry; the JSON use in the migration is just a debug thing; > > we don't want to maintain a separate JSON instance for it. > > Well, you already do, except in name. Who else do you think is > maintaining qjson.[ch], created by migration people, for migration's > use? Certainly not me. That came from migration? Really? I didn't think we used JSON at all until last year. > If you can't use the general JSON output code I maintain because of > special requirements, you get to continue maintaining your own. All 109 > SLOC of it. All I'm asking is to make it official, and to deter > accidental use of migration's JSON writer instead of the general one. Yeh; I'd love to share the JSON code; just lets try and avoid anything that can kill the source, however broken the migration. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK