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 <s...@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@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/systemd-devel >
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