[Qemu-devel] QError conversion problems: putting errors in context

[Markus Armbruster]

Our QError conversions were pretty straightforward so far.  For example,
when we found

                monitor_printf(mon, "device is not removable\n");

in eject_device(), we created the obvious QError class for it:

#define QERR_DEVICE_NOT_REMOVABLE \
    "{ 'class': 'DeviceNotRemovable', 'data': { 'device': %s } }"

with the obvious pretty-print template:

    {
        .error_fmt = QERR_DEVICE_NOT_REMOVABLE,
        .desc      = "Device %(device) is not removable",
    },

and replaced the print with

                qemu_error_new(QERR_DEVICE_NOT_REMOVABLE,
                               bdrv_get_device_name(bs));

This even improved the message from "device is not removable" to "device
hda is not removable".  Commit 2c2a6bb8.


We also ran into cases like this one, in qdev_device_add():

-        qemu_error("Device \"%s\" not found.  Try -device '?' for a list.\n",
-                   driver);
+        qemu_error_new(QERR_DEVICE_NOT_FOUND, driver);

Here, the pretty-print template became

        .desc      = "The %(device) device has not been found",

Note the loss of the helpful "Try -device '?' for a list." part.  That's
because the same error is used in other places, where that piece of
advice doesn't apply.  Commit 3ced9f7a.


For similar reasons, "Invalid CPU index" changed to the more generic
"Invalid parameter index" in monitor command cpu (commit cc0c4185).
More examples in my private tree: set_link's "invalid link status 'off';
only 'up' or 'down' valid" becomes "Invalid parameter up_or_down", and
migrate's "migration already in progress" becomes "Already in progress".


Most of the conversions touched only monitor code.  Things get much more
complicated for code shared between the monitor and other places.  For
instance, there's this one in qdev_device_add():

        qemu_error("-device: no driver specified\n");

Of course, this message is pretty pathetic already, in several ways:

* A command line can contain several -device, and the error message
  leaves finding the offending one to the user.

* It's even worse with configuration files.  FILE:LINENR is par for that
  course.

* It talks about -device even when we're in the monitor or a
  configuration file.  To trigger it in the monitor, try "device_add
  a=b".

>From QMP's point of view, the appropriate error to use is this one:

#define QERR_MISSING_PARAMETER \
    "{ 'class': 'MissingParameter', 'data': { 'name': %s } }"

It's pretty-print template is

        .desc      = "Parameter %(name) is missing",

This changes the message "-device: no driver specified" to "Parameter
driver is missing".  We go from bad to worse.


I think we have a few related but nevertheless distinct issues here:

* We need to identify the error message's object.  Proper identification
  depends on the context in which the code is executing:

  - Command line: the option, with its argument, if any.

  - Config file: filename, line number.

  - Monitor: command (this is trivial)

  This is what's lacking in the "no driver specified" example, even
  before conversion to QError.

  Dragging context information along all over the place so we can use it
  to make better error messages would be cumbersome and invasive.  There
  must be a better way.

  Note that qemu_error_new() already has a primitive notion of context:
  it distinguishes between human monitor, QMP monitor, and anything
  else.  This could be refined, so it can add suitable context
  information to the error without encumbering its callers.  For
  instance, a hypothetical error message "pants on fire", emitted with

      qemu_error_new(QERR_PANTS_ON_FILE)

  should become

  - "qemu-system-x86_64: -device pants,color=black: pants on fire" in
    the context of command line option "-device pants,color=black",

  - "vm.cfg:123: pants on fire" in the context of configuration file
    vm.cfg, line 123,

  - "pants on fire" in the human monitor, and

  - { "error": { "class": "PantsOnFire", "data": {},
                 "desc": "pants on fire" } }
    in QMP.

* Occasionally, we'd like to add help, but not for all uses of the same
  error, and not in all contexts.

  This is the "Try -device '?' for a list" example.  We'd like to print
  that if we're working for -device (user qdev_device_add(), context
  command line).  And maybe "Try device_add ? for a list" if we're
  working for device_add (same user, context human monitor).

  I believe this is rare enough to permit an ad hoc solution, like this:

        qemu_error_new(QERR_DEVICE_NOT_FOUND, driver);
        printf_if_cmdline("Try -device '?' for a list.\n");
        printf_if_human_monitor("Try device_add ? for a list.\n");

  Wouldn't work for the monitor right now, because the actual printing
  of the error gets delayed until the handler returns, but that's
  fixable.

* Nice human-readable messages sometimes need information that should
  not go into QMP.

  This is the "Migration already in progress" example.  Since different
  things can be in progress, it is tempting to do

    #define QERR_ALREADY_IN_PROGRESS \
        "{ 'class': 'AlreadyInProgress', 'data': { 'activity': %s } }"

  with pretty-print template

    .desc      = "%(activity) already in progress"

  and use it like this:

    qemu_error_new(QERR_FAILED, "migration");

  But that puts a rather pointless 'activity' member into the data
  object.

  An obvious way to avoid that is to adopt a convention like "if the key
  starts with '_', it's internal, and not to be passed over QMP".

  Another way (suggested by Anthony) is to permit overriding the
  pretty-print template somehow.  More flexible, but also more clumsy to
  use.

Comments?