[systemd-devel] Reasoning behind sd_bus_error argument to sd_bus_call?

[Daan De Meyer]

I completely agree that for errors returned by the service, a D-Bus error
is a lot better. However, from what I understand of sd-bus, any errors
returned by the service are encoded in the reply returned by sd_bus_call
and you use sd_bus_message_is_method_error and sd_bus_message_get_error on
the reply to get the actual service error. Where does that leave the
sd_bus_error argument of sd_bus_call? Is it simply another way to get the
error? It seems to be always be set when a local or remote error occurs,
but it can only contain information that I can get by checking the return
value of the function or by checking whether the reply object passed to
sd_bus_call contains an error.

How I would imagine using sd_bus_call:

r = sd_bus_call(..., reply, ...);
if (r < 0) {
  // Local error
}

if (sd_bus_message_is_method_error(reply)) {
  const sd_bus_error *error = sd_bus_message_get_error(reply);
  // Service error
}

But if this is the intended usage, what's the use of the sd_bus_error
argument of sd_bus_call since the above code already handles both the local
error and the remote service error failure paths?

Daan

On Wed, 18 Mar 2020 at 11:57, Simon McVittie <smcv at collabora.com> wrote:

> On Tue, 17 Mar 2020 at 20:17:05 +0100, Daan De Meyer wrote:
> > I'm documenting sd_bus_call and its async variant and I was wondering
> about the
> > sd_bus_error output parameter that's passed to it. [...] I don't
> > see immediately see the benefit of the sd_bus_error parameter in a D-Bus
> client
> > since I can simply check the return value instead which seems to contain
> the
> > same information looking at the implementation.
>
> The return value is a single int, which according to systemd conventions
> is probably a negative errno value. That's a lot less information than
> a D-Bus error (systemd sd_bus_error, libdbus DBusError or equivalent):
> D-Bus errors consist of a machine-readable name (namespaced by a reversed
> domain name) and a human-readable message.
>
> For the information about *whether* an error occurred, sure, you get the
> same information, but for information about *which* error occurred and why,
> a sd_bus_error is a lot better.
>
> Let's pretend your D-Bus client is interacting with a D-Bus service that
> resembles systemd-timedated. An errno value can give you, at best,
> something like this (where *** marks the part that came from the service's
> reply):
>
>     my-client: Error: Unable to set time zone to America/Gotham:
>     ***No such file or directory (errno 2)***
>
> whereas a D-Bus error (sd_bus_error) from a well-implemented service can
> give you something a lot more detailed. For example, after you ispect
> the sd_bus_error, you might find that the error above was either of these:
>
>     my-client: Error: Unable to set time zone to America/Gotham:
>     ***No time zone file for "America/Gotham" found (tried
>     "/usr/share/zoneinfo/America/Gotham",
>     "/usr/local/share/zoneinfo/America/Gotham")
>     (error code com.example.NotTimedated.Error.NoSuchTimezone)***
>
>     my-client: Error: Unable to set time zone to America/Gotham:
>     ***No time zone data installed (tried "/usr/share/zoneinfo",
>     "/usr/local/share/zoneinfo")
>     (error code com.example.NotTimedated.Error.TzdataNotInstalled)***
>
> In this example a programmatic client would also be able
> to respond differently to the distinct machine-readable
> errors com.example.NotTimedated.Error.NoSuchTimezone and
> com.example.NotTimedated.Error.TzdataNotInstalled if it wanted to;
> for example it could respond to the second error by trying to use
> PackageKit to install tzdata, which obviously wouldn't be appropriate
> for the first error.
>
> D-Bus errors were inspired by GLib's GError, which is basically a triple
> { domain: interned string, code: int, message: string }, where the domain
> provides extensible uniqueness, and the code is a member of an enum
> determined by the domain.
>
>     smcv
> _______________________________________________
> systemd-devel mailing list
> systemd-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/systemd-devel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/systemd-devel/attachments/20200318/1bc8f2f4/attachment.htm>