From patchwork Wed Feb 3 18:03:48 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Markus Armbruster X-Patchwork-Id: 578270 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from lists.gnu.org (lists.gnu.org [IPv6:2001:4830:134:3::11]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 84F671409B7 for ; Thu, 4 Feb 2016 05:04:21 +1100 (AEDT) Received: from localhost ([::1]:36790 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aR1mt-0000tv-Le for incoming@patchwork.ozlabs.org; Wed, 03 Feb 2016 13:04:19 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49377) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aR1mU-0000Aa-8w for qemu-devel@nongnu.org; Wed, 03 Feb 2016 13:03:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aR1mR-0003p1-1F for qemu-devel@nongnu.org; Wed, 03 Feb 2016 13:03:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44826) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aR1mQ-0003oj-Pt for qemu-devel@nongnu.org; Wed, 03 Feb 2016 13:03:50 -0500 Received: from int-mx14.intmail.prod.int.phx2.redhat.com (int-mx14.intmail.prod.int.phx2.redhat.com [10.5.11.27]) by mx1.redhat.com (Postfix) with ESMTPS id 4F24B8F4F4; Wed, 3 Feb 2016 18:03:50 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-116-39.ams2.redhat.com [10.36.116.39]) by int-mx14.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id u13I3mpN003516 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Wed, 3 Feb 2016 13:03:49 -0500 Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 33F4C300383C; Wed, 3 Feb 2016 19:03:48 +0100 (CET) From: Markus Armbruster To: qemu-devel@nongnu.org Date: Wed, 3 Feb 2016 19:03:48 +0100 Message-Id: <1454522628-28294-3-git-send-email-armbru@redhat.com> In-Reply-To: <1454522628-28294-1-git-send-email-armbru@redhat.com> References: <1454522628-28294-1-git-send-email-armbru@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.68 on 10.5.11.27 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x X-Received-From: 209.132.183.28 Cc: thuth@redhat.com, vilanova@ac.upc.edu, dgilbert@redhat.com Subject: [Qemu-devel] [PATCH 2/2] HACKING: Add a section on error handling and reporting X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Inspired by an RFC PATCH from Lluís Vilanova. Signed-off-by: Markus Armbruster --- HACKING | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) diff --git a/HACKING b/HACKING index 12fbc8a..058aa8f 100644 --- a/HACKING +++ b/HACKING @@ -157,3 +157,58 @@ painful. These are: * you may assume that integers are 2s complement representation * you may assume that right shift of a signed integer duplicates the sign bit (ie it is an arithmetic shift, not a logical shift) + +7. Error handling and reporting + +7.1 Reporting errors to the human user + +Do not use printf(), fprintf() or monitor_printf(). Instead, use +error_report() or error_vreport() from error-report.h. This ensures the +error is reported in the right place (current monitor or stderr), and in +a uniform format. + +Use error_printf() & friends to print additional information. + +error_report() prints the current location. In certain common cases +like command line parsing, the current location is tracked +automatically. To manipulate it manually, use the loc_*() from +error-report.h. + +7.2 Propagating errors + +An error can't always be reported to the user right where it's detected, +but often needs to be propagated up the call chain to a place that can +handle it. This can be done in various ways. + +The most flexible one is Error objects. See error.h for usage +information. + +Use the simplest suitable method to communicate success / failure to +callers. Stick to common methods: non-negative on success / -1 on +error, non-negative / -errno, non-null / null, or Error objects. + +Example: when a function returns a non-null pointer on success, and it +can fail only in one way (as far as the caller is concerned), returning +null on failure is just fine, and certainly simpler and a lot easier on +the eyes than propagating an Error object through an Error ** parameter. + +Example: when a function's callers need to report details on failure +only the function really knows, use Error **, and set suitable errors. + +Do not report an error to the user when you're also returning an error +for somebody else to handle. Leave the reporting to the place that +consumes the error returned. + +7.3 Handling errors + +Calling exit() is fine when handling configuration errors during +startup. It's problematic during normal operation. In particular, +monitor commands should never exit(). + +Do not call exit() or abort() to handle an error that can be triggered +by the guest (e.g., some unimplemented corner case in guest code +translation or device emulation). Guests should not be able to +terminate QEMU. + +Note that &error_fatal is just another way to exit(1), and &error_abort +is just another way to abort().