Re: [PATCH v2 6/6] migration: Deprecate fd: for file migration

On Fri, Apr 26, 2024 at 10:14:08AM -0300, Fabiano Rosas wrote: > The fd: URI can currently trigger two different types of migration, a > TCP migration using sockets and a file migration using a plain > file. This is in conflict with the recently introduced (8.2) QMP > migrate API that takes structured data as JSON-like format. We cannot > keep the same backend for both types of migration because with the new > API the code is more tightly coupled to the type of transport. This > means a TCP migration must use the 'socket' transport and a file > migration must use the 'file' transport. > > If we keep allowing fd: when using a file, this creates an issue when > the user converts the old-style (fd:) to the new style ("transport": > "socket") invocation because the file descriptor in question has > previously been allowed to be either a plain file or a socket. > > To avoid creating too much confusion, we can simply deprecate the fd: > + file usage, which is thought to be rarely used currently and instead > establish a 1:1 correspondence between fd: URI and socket transport, > and file: URI and file transport. > > Signed-off-by: Fabiano Rosas <farosas(a)suse.de&gt; > --- > docs/about/deprecated.rst | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst > index 0fb5c82640..813f7996fe 100644 > --- a/docs/about/deprecated.rst > +++ b/docs/about/deprecated.rst > @@ -464,3 +464,17 @@ both, older and future versions of QEMU. > The ``blacklist`` config file option has been renamed to ``block-rpcs`` > (to be in sync with the renaming of the corresponding command line > option). > + > +Migration > +--------- > + > +``fd:`` URI when used for file migration (since 9.1) > +'''''''''''''''''''''''''''''''''''''''''''''''''''' > + > +The ``fd:`` URI can currently provide a file descriptor that > +references either a socket or a plain file. These are two different > +types of migration. In order to reduce ambiguity, the ``fd:`` URI > +usage of providing a file descriptor to a plain file has been > +deprecated in favor of explicitly using the ``file:`` URI with the > +file descriptor being passed as an ``fdset``. Refer to the ``add-fd`` > +command documentation for details on the ``fdset`` usage. Wanna do some warn_report() when detected non-socket fds alongside? Looks like we previously do this for all deprecations.

What's the plan when it's support removed? I'm imaginging that we sanity check fstat() + S_ISSOCK on the fd and fail otherwise? In that case we can have the code there, dump warn_report(), then switch to failing qmp migrate (and incoming side) later on?