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709dde |
From d931195ef5cccd6a4e6fceeba37809b1712c97ad Mon Sep 17 00:00:00 2001
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709dde |
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
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709dde |
Date: Fri, 8 Jan 2021 07:40:55 -0500
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709dde |
Subject: [PATCH 04/10] error: Document Error API usage rules
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709dde |
MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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709dde |
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709dde |
RH-Author: Marc-André Lureau <marcandre.lureau@redhat.com>
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709dde |
Message-id: <20210108074101.290008-5-marcandre.lureau@redhat.com>
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709dde |
Patchwork-id: 100523
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709dde |
O-Subject: [RHEL-8.3.0.z qemu-kvm PATCH 04/10] error: Document Error API usage rules
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709dde |
Bugzilla: 1913818
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709dde |
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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709dde |
RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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709dde |
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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709dde |
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709dde |
From: Marc-André Lureau <marcandre.lureau@redhat.com>
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709dde |
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709dde |
From: Markus Armbruster <armbru@redhat.com>
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709dde |
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709dde |
This merely codifies existing practice, with one exception: the rule
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709dde |
advising against returning void, where existing practice is mixed.
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709dde |
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709dde |
When the Error API was created, we adopted the (unwritten) rule to
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709dde |
return void when the function returns no useful value on success,
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709dde |
unlike GError, which recommends to return true on success and false on
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709dde |
error then.
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709dde |
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709dde |
When a function returns a distinct error value, say false, a checked
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709dde |
call that passes the error up looks like
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709dde |
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709dde |
if (!frobnicate(..., errp)) {
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709dde |
handle the error...
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709dde |
}
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709dde |
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709dde |
When it returns void, we need
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709dde |
Error *err = NULL;
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709dde |
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frobnicate(..., &err;;
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if (err) {
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709dde |
handle the error...
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error_propagate(errp, err);
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}
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709dde |
Not only is this more verbose, it also creates an Error object even
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709dde |
when @errp is null, &error_abort or &error_fatal.
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709dde |
People got tired of the additional boilerplate, and started to ignore
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709dde |
the unwritten rule. The result is confusion among developers about
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709dde |
the preferred usage.
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709dde |
Make the rule advising against returning void official by putting it
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709dde |
in writing. This will hopefully reduce confusion.
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709dde |
Update the examples accordingly.
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709dde |
The remainder of this series will update a substantial amount of code
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709dde |
to honor the rule.
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709dde |
Signed-off-by: Markus Armbruster <armbru@redhat.com>
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709dde |
Reviewed-by: Eric Blake <eblake@redhat.com>
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Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Reviewed-by: Greg Kurz <groug@kaod.org>
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Message-Id: <20200707160613.848843-4-armbru@redhat.com>
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709dde |
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(cherry picked from commit e3fe3988d7851cac30abffae06d2f555ff7bee62)
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709dde |
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
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709dde |
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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709dde |
include/qapi/error.h | 52 +++++++++++++++++++++++++++++++++++++++-----
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1 file changed, 46 insertions(+), 6 deletions(-)
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diff --git a/include/qapi/error.h b/include/qapi/error.h
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index 3351fe76368..08d48e74836 100644
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--- a/include/qapi/error.h
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709dde |
+++ b/include/qapi/error.h
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709dde |
@@ -15,6 +15,33 @@
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/*
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709dde |
* Error reporting system loosely patterned after Glib's GError.
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*
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+ * = Rules =
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709dde |
+ *
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709dde |
+ * - Functions that use Error to report errors have an Error **errp
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709dde |
+ * parameter. It should be the last parameter, except for functions
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+ * taking variable arguments.
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709dde |
+ *
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+ * - You may pass NULL to not receive the error, &error_abort to abort
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+ * on error, &error_fatal to exit(1) on error, or a pointer to a
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+ * variable containing NULL to receive the error.
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+ *
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+ * - Separation of concerns: the function is responsible for detecting
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709dde |
+ * errors and failing cleanly; handling the error is its caller's
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709dde |
+ * job. Since the value of @errp is about handling the error, the
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709dde |
+ * function should not examine it.
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709dde |
+ *
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709dde |
+ * - On success, the function should not touch *errp. On failure, it
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709dde |
+ * should set a new error, e.g. with error_setg(errp, ...), or
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709dde |
+ * propagate an existing one, e.g. with error_propagate(errp, ...).
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709dde |
+ *
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709dde |
+ * - Whenever practical, also return a value that indicates success /
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709dde |
+ * failure. This can make the error checking more concise, and can
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709dde |
+ * avoid useless error object creation and destruction. Note that
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709dde |
+ * we still have many functions returning void. We recommend
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709dde |
+ * • bool-valued functions return true on success / false on failure,
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709dde |
+ * • pointer-valued functions return non-null / null pointer, and
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709dde |
+ * • integer-valued functions return non-negative / negative.
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709dde |
+ *
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709dde |
* = Creating errors =
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709dde |
*
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709dde |
* Create an error:
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709dde |
@@ -95,14 +122,13 @@
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709dde |
* Create a new error and pass it to the caller:
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709dde |
* error_setg(errp, "situation normal, all fouled up");
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709dde |
*
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709dde |
- * Call a function and receive an error from it:
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709dde |
- * Error *err = NULL;
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709dde |
- * foo(arg, &err;;
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709dde |
- * if (err) {
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709dde |
+ * Call a function, receive an error from it, and pass it to the caller
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709dde |
+ * - when the function returns a value that indicates failure, say
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709dde |
+ * false:
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709dde |
+ * if (!foo(arg, errp)) {
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709dde |
* handle the error...
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709dde |
* }
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709dde |
- *
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709dde |
- * Receive an error and pass it on to the caller:
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709dde |
+ * - when it does not, say because it is a void function:
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709dde |
* Error *err = NULL;
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709dde |
* foo(arg, &err;;
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709dde |
* if (err) {
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709dde |
@@ -120,6 +146,20 @@
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709dde |
* foo(arg, errp);
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709dde |
* for readability.
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709dde |
*
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709dde |
+ * Receive an error, and handle it locally
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709dde |
+ * - when the function returns a value that indicates failure, say
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709dde |
+ * false:
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709dde |
+ * Error *err = NULL;
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709dde |
+ * if (!foo(arg, &err)) {
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709dde |
+ * handle the error...
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709dde |
+ * }
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709dde |
+ * - when it does not, say because it is a void function:
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709dde |
+ * Error *err = NULL;
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709dde |
+ * foo(arg, &err;;
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
709dde |
+ * if (err) {
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709dde |
+ * handle the error...
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
709dde |
+ * }
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709dde |
+ *
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709dde |
* Receive and accumulate multiple errors (first one wins):
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709dde |
* Error *err = NULL, *local_err = NULL;
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709dde |
* foo(arg, &err;;
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709dde |
--
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709dde |
2.27.0
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709dde |
|