From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.6 Received: from out1.vger.email (out1.vger.email [IPv6:2620:137:e000::1:20]) by dcvr.yhbt.net (Postfix) with ESMTP id 65BD21F626 for ; Wed, 22 Feb 2023 19:34:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231562AbjBVTee (ORCPT ); Wed, 22 Feb 2023 14:34:34 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42036 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229513AbjBVTed (ORCPT ); Wed, 22 Feb 2023 14:34:33 -0500 Received: from cloud.peff.net (cloud.peff.net [104.130.231.41]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C71401259F for ; Wed, 22 Feb 2023 11:34:32 -0800 (PST) Received: (qmail 14761 invoked by uid 109); 22 Feb 2023 19:34:32 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with ESMTP; Wed, 22 Feb 2023 19:34:32 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 10517 invoked by uid 111); 22 Feb 2023 19:34:31 -0000 Received: from coredump.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.2) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Wed, 22 Feb 2023 14:34:31 -0500 Authentication-Results: peff.net; auth=none Date: Wed, 22 Feb 2023 14:34:31 -0500 From: Jeff King To: Emily Shaffer Cc: "brian m. carlson" , Git List , Jonathan Nieder , Jose Lopes , Aleksandr Mikhailov Subject: Re: Proposal/Discussion: Turning parts of Git into libraries Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Fri, Feb 17, 2023 at 02:49:51PM -0800, Emily Shaffer wrote: > > Personally, I'd like to see some sort of standard error type (whether > > integral or not) that would let us do more bubbling up of errors and > > less die(). I don't know if that's in the cards, but I thought I'd > > suggest it in case other folks are interested. > > Yes!!! We have talked about this a lot internally - but this is one > thing that will be difficult to introduce into Git without making > parts of the codebase a little uglier. Since obviously C doesn't have > an intrinsic to do this, we'll have to roll our own, which means that > manipulating it consistently at function exits might end up pretty > ugly. So hearing that there's interest outside of my team to come up > with such a type makes me optimistic that we can figure out a > neat-enough solution. Here are some past discussions on what I thought would be a good approach to error handling. The basic idea is to replace the "pass a strbuf that people shove error messages into" approach with an error context struct that has a callback. And that callback can then stuff them into a strbuf, or report them directly, or even die. This thread sketches out the idea, though sadly I no longer have the more fleshed-out patches I mentioned there: https://lore.kernel.org/git/20160927191955.mympqgylrxhkp24n@sigill.intra.peff.net/ And then the sub-thread starting here discusses a similar approach: https://lore.kernel.org/git/20171103191309.sth4zjokgcupvk2e@sigill.intra.peff.net/ It does mean passing a "struct error_context" just about everywhere. Though since the context doesn't change very much and most calls are just forwarding it along, it would probably also be reasonable to have a thread-local global context, and push/pop from it (sort of a poor man's dynamic scoping). One thing that strategy doesn't help with, though, that your libification might want: it's not very machine-readable. The error reporters would still fundamentally be working with strings. So a libified process can know "OK, writing this ref failed, and I have some error messages in a buffer". But the calling code can't know specifics like "it failed because we tried to open file 'foo' and it got EPERM". We _could_ design an error context that stores individual errno values or codes in a list, but having each caller report those specific errors is a much bigger job (and ongoing maintenance burden as we catalogue and give an identifier to each error). -Peff