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From b41fc6d57fae80ac2a431bca22862985f003fe88 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
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Date: Fri, 8 Jan 2021 07:40:54 -0500
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Subject: [PATCH 03/10] error: Improve error.h's big comment
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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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RH-Author: Marc-André Lureau <marcandre.lureau@redhat.com>
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Message-id: <20210108074101.290008-4-marcandre.lureau@redhat.com>
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Patchwork-id: 100522
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O-Subject: [RHEL-8.3.0.z qemu-kvm PATCH 03/10] error: Improve error.h's big comment
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Bugzilla: 1913818
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RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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From: Marc-André Lureau <marcandre.lureau@redhat.com>
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From: Markus Armbruster <armbru@redhat.com>
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Add headlines to the big comment.
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Explain examples for NULL, &error_abort and &error_fatal argument
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better.
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Tweak rationale for error_propagate_prepend().
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Signed-off-by: Markus Armbruster <armbru@redhat.com>
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Message-Id: <20200707160613.848843-3-armbru@redhat.com>
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Reviewed-by: Eric Blake <eblake@redhat.com>
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Reviewed-by: Greg Kurz <groug@kaod.org>
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(cherry picked from commit 9aac7d486cc792191c25c30851f501624b0c2751)
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Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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 include/qapi/error.h | 51 +++++++++++++++++++++++++++++++-------------
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 1 file changed, 36 insertions(+), 15 deletions(-)
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diff --git a/include/qapi/error.h b/include/qapi/error.h
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index 83c38f9a188..3351fe76368 100644
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--- a/include/qapi/error.h
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+++ b/include/qapi/error.h
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@@ -15,6 +15,8 @@
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 /*
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  * Error reporting system loosely patterned after Glib's GError.
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  *
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+ * = Creating errors =
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+ *
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  * Create an error:
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  *     error_setg(&err, "situation normal, all fouled up");
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  *
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@@ -27,6 +29,8 @@
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  *     error_setg(&err, "invalid quark\n" // WRONG!
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  *                "Valid quarks are up, down, strange, charm, top, bottom.");
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  *
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+ * = Reporting and destroying errors =
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+ *
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  * Report an error to the current monitor if we have one, else stderr:
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  *     error_report_err(err);
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  * This frees the error object.
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@@ -40,6 +44,30 @@
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  *     error_free(err);
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  * Note that this loses hints added with error_append_hint().
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  *
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+ * Call a function ignoring errors:
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+ *     foo(arg, NULL);
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+ * This is more concise than
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+ *     Error *err = NULL;
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+ *     foo(arg, &err;;
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+ *     error_free(err); // don't do this
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+ *
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+ * Call a function aborting on errors:
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+ *     foo(arg, &error_abort);
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+ * This is more concise and fails more nicely than
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+ *     Error *err = NULL;
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+ *     foo(arg, &err;;
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+ *     assert(!err); // don't do this
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+ *
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+ * Call a function treating errors as fatal:
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+ *     foo(arg, &error_fatal);
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+ * This is more concise than
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+ *     Error *err = NULL;
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+ *     foo(arg, &err;;
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+ *     if (err) { // don't do this
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+ *         error_report_err(err);
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+ *         exit(1);
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+ *     }
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+ *
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  * Handle an error without reporting it (just for completeness):
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  *     error_free(err);
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  *
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@@ -47,6 +75,11 @@
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  * reporting it (primarily useful in testsuites):
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  *     error_free_or_abort(&err;;
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  *
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+ * = Passing errors around =
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+ *
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+ * Errors get passed to the caller through the conventional @errp
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+ * parameter.
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+ *
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  * Pass an existing error to the caller:
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  *     error_propagate(errp, err);
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  * where Error **errp is a parameter, by convention the last one.
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@@ -54,11 +87,10 @@
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  * Pass an existing error to the caller with the message modified:
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  *     error_propagate_prepend(errp, err,
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  *                             "Could not frobnicate '%s': ", name);
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- *
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- * Avoid
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- *     error_propagate(errp, err);
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+ * This is more concise than
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+ *     error_propagate(errp, err); // don't do this
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  *     error_prepend(errp, "Could not frobnicate '%s': ", name);
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- * because this fails to prepend when @errp is &error_fatal.
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+ * and works even when @errp is &error_fatal.
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  *
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  * Create a new error and pass it to the caller:
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  *     error_setg(errp, "situation normal, all fouled up");
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@@ -70,15 +102,6 @@
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  *         handle the error...
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  *     }
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  *
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- * Call a function ignoring errors:
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- *     foo(arg, NULL);
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- *
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- * Call a function aborting on errors:
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- *     foo(arg, &error_abort);
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- *
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- * Call a function treating errors as fatal:
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- *     foo(arg, &error_fatal);
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- *
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  * Receive an error and pass it on to the caller:
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  *     Error *err = NULL;
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  *     foo(arg, &err;;
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@@ -86,8 +109,6 @@
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  *         handle the error...
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  *         error_propagate(errp, err);
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  *     }
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- * where Error **errp is a parameter, by convention the last one.
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- *
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  * Do *not* "optimize" this to
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  *     foo(arg, errp);
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  *     if (*errp) { // WRONG!
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-- 
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2.27.0
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