Re: [libvirt] [Qemu-devel] [PATCH 2/2] qapi: deprecate implicit filters

Kevin Wolf <kwolf(a)redhat.com&gt; writes: > Am 15.08.2019 um 18:07 hat John Snow geschrieben: >> >> >> On 8/15/19 6:49 AM, Kevin Wolf wrote: >> > Am 14.08.2019 um 21:27 hat John Snow geschrieben: >> >> >> >> >> >> On 8/14/19 6:07 AM, Vladimir Sementsov-Ogievskiy wrote: >> >>> To get rid of implicit filters related workarounds in future let's >> >>> deprecate them now. >> >>> >> >>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov(a)virtuozzo.com&gt; >> >>> --- >> >>> qemu-deprecated.texi | 7 +++++++ >> >>> qapi/block-core.json | 6 ++++-- >> >>> include/block/block_int.h | 10 +++++++++- >> >>> blockdev.c | 10 ++++++++++ >> >>> 4 files changed, 30 insertions(+), 3 deletions(-) >> >>> >> >>> diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi >> >>> index 2753fafd0b..8222440148 100644 >> >>> --- a/qemu-deprecated.texi >> >>> +++ b/qemu-deprecated.texi >> >>> @@ -183,6 +183,13 @@ the 'wait' field, which is only applicable to sockets in server mode >> >>> >> >>> Use blockdev-mirror and blockdev-backup instead. >> >>> >> >>> +@subsection implicit filters (since 4.2) >> >>> + >> >>> +Mirror and commit jobs inserts filters, which becomes implicit if user >> >>> +omitted filter-node-name parameter. So omitting it is deprecated, set it >> >>> +always. Note, that drive-mirror don't have this parameter, so it will >> >>> +create implicit filter anyway, but drive-mirror is deprecated itself too. >> >>> + >> >>> @section Human Monitor Protocol (HMP) commands >> >>> >> >>> @subsection The hub_id parameter of 'hostfwd_add' / 'hostfwd_remove' (since 3.1) >> >>> diff --git a/qapi/block-core.json b/qapi/block-core.json >> >>> index 4e35526634..0505ac9d8b 100644 >> >>> --- a/qapi/block-core.json >> >>> +++ b/qapi/block-core.json >> >>> @@ -1596,7 +1596,8 @@ >> >>> # @filter-node-name: the node name that should be assigned to the >> >>> # filter driver that the commit job inserts into the graph >> >>> # above @top. If this option is not given, a node name is >> >>> -# autogenerated. (Since: 2.9) >> >>> +# autogenerated. Omitting this option is deprecated, it will >> >>> +# be required in future. (Since: 2.9) >> >>> # >> >>> # @auto-finalize: When false, this job will wait in a PENDING state after it has >> >>> # finished its work, waiting for @block-job-finalize before >> >>> @@ -2249,7 +2250,8 @@ >> >>> # @filter-node-name: the node name that should be assigned to the >> >>> # filter driver that the mirror job inserts into the graph >> >>> # above @device. If this option is not given, a node name is >> >>> -# autogenerated. (Since: 2.9) >> >>> +# autogenerated. Omitting this option is deprecated, it will >> >>> +# be required in future. (Since: 2.9) >> >>> # >> >>> # @copy-mode: when to copy data to the destination; defaults to 'background' >> >>> # (Since: 3.0) >> >>> diff --git a/include/block/block_int.h b/include/block/block_int.h >> >>> index 3aa1e832a8..624da0b4a2 100644 >> >>> --- a/include/block/block_int.h >> >>> +++ b/include/block/block_int.h >> >>> @@ -762,7 +762,15 @@ struct BlockDriverState { >> >>> bool sg; /* if true, the device is a /dev/sg* */ >> >>> bool probed; /* if true, format was probed rather than specified */ >> >>> bool force_share; /* if true, always allow all shared permissions */ >> >>> - bool implicit; /* if true, this filter node was automatically inserted */ >> >>> + >> >>> + /* >> >>> + * @implicit field is deprecated, don't set it to true for new filters. >> >>> + * If true, this filter node was automatically inserted and user don't >> >>> + * know about it and unprepared for any effects of it. So, implicit >> >>> + * filters are workarounded and skipped in many places of the block >> >>> + * layer code. >> >>> + */ >> >>> + bool implicit; >> >>> >> >>> BlockDriver *drv; /* NULL means no media */ >> >>> void *opaque; >> >>> diff --git a/blockdev.c b/blockdev.c >> >>> index 36e9368e01..b3cfaccce1 100644 >> >>> --- a/blockdev.c >> >>> +++ b/blockdev.c >> >>> @@ -3292,6 +3292,11 @@ void qmp_block_commit(bool has_job_id, const char *job_id, const char *device, >> >>> BlockdevOnError on_error = BLOCKDEV_ON_ERROR_REPORT; >> >>> int job_flags = JOB_DEFAULT; >> >>> >> >>> + if (!has_filter_node_name) { >> >>> + warn_report("Omitting filter-node-name parameter is deprecated, it " >> >>> + "will be required in future"); >> >>> + } >> >>> + >> >>> if (!has_speed) { >> >>> speed = 0; >> >>> } >> >>> @@ -3990,6 +3995,11 @@ void qmp_blockdev_mirror(bool has_job_id, const char *job_id, >> >>> Error *local_err = NULL; >> >>> int ret; >> >>> >> >>> + if (!has_filter_node_name) { >> >>> + warn_report("Omitting filter-node-name parameter is deprecated, it " >> >>> + "will be required in future"); >> >>> + } >> >>> + >> >>> bs = qmp_get_root_bs(device, errp); >> >>> if (!bs) { >> >>> return; >> >>> >> >> >> >> This might be OK to do right away, though. >> >> >> >> I asked Markus this not too long ago; do we want to amend the QAPI >> >> schema specification to allow commands to return with "Warning" strings, >> >> or "Deprecated" stings to allow in-band deprecation notices for cases >> >> like these? >> >> >> >> example: >> >> >> >> { "return": {}, >> >> "deprecated": True, >> >> "warning": "Omitting filter-node-name parameter is deprecated, it will >> >> be required in the future" >> >> } >> >> >> >> There's no "error" key, so this should be recognized as success by >> >> compatible clients, but they'll definitely see the extra information. >> >> >> >> Part of my motivation is to facilitate a more aggressive deprecation of >> >> legacy features by ensuring that we are able to rigorously notify users >> >> through any means that they need to adjust their scripts. >> > >> > Who would read this, though? In the best case it ends up deep in a >> > libvirt log that nobody will look at because there was no error. In the >> > more common case, the debug level is configured so that QMP traffic >> > isn't even logged. >> > >> > Kevin >> > >> >> I believe you are right, but I also can't shake the feeling that this >> attitude ensures that we'll never find a way to expose this information >> to the end-user. Is this not too defeatist? > > I think the discussed approach that seemed most likely to me to succeed > was adding a command line option that makes QEMU just crash if you use a > deprecated feature, and enable that in libvirt test cases (or possibly > even any non-release builds, though maybe it's a bit harsh there). Yup. BoF minutes: "The one way to get people read log files is crashing their application." >> I think deprecation notices in the QMP stream has two benefits: >> >> 1) Any direct usages via qmp-shell or manual JSON connection are likely >> to see this message in development or testing. I feel the usage of QEMU >> directly is more likely to increase with time as other stacks seek to >> work around libvirt. >> >> [Whether or not they should is another question, but I believe the >> current reality to be that people are trying to.] > > I don't know about other people, but as a human user, I don't care about > deprecation notices. As long as something works, I use it, and once I > get an error message back, I'll use something else. > > If I manually enter drive_mirror and get a warning back, that doesn't > tell me that libvirt still does the same thing and needs to be fixed. It > just tells me that in the future I might need to change the commands > that I use manually. > > I guess this would still prevent adding new libvirt features that build > on deprecated QEMU features because some manual testing will be involved > there. But was this ever a problem? You're right in that relying on *humans* to read the machine-readable deprecation notice probaly won't work for old client code trying to use newly deprecated QMP. It should work for new client code trying to use already deprecated QMP. >> 2) Programmatic deprecation notices can't be presented to a user at all >> if we don't send them; at least this way it becomes libvirt's problem >> over what to do with them. Perhaps even just in testing and regression >> suites libvirt can assert that it sees no deprecation warnings (or >> whitelist certain ones it knows about.) >> >> In the case of libvirt, it's not even necessarily about making sure the >> end user sees it, because it isn't even necessarily the user's fault -- >> it's libvirt's. This is a sure-fire programmatic way to communicate >> compatibility changes to libvirt. > > If libvirt uses this to make test cases fail, it could work. Yes. However, ensuring tests fail whenever libvirt receives a deprecation notice via QMP seems harder than having them pass --future to QEMU to make it crash instead of sending such a notice. Let's assume all libvirt ever does with deprecation notices is logging them. Would that solve the problem of reliably alerting libvirt developers to deprecation issues? Nope. But it could help occasionally.