Re: [PATCH v2 1/1] python/machine: Fix AF_UNIX path too long on macOS

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jul 21, 2022 at 07:44:21PM -0700, Peter Delevoryas wrote:
> On Mon, Jul 18, 2022 at 09:56:17AM +0100, Daniel P. Berrangé wrote:
> > On Sat, Jul 16, 2022 at 10:34:34AM -0700, Peter Delevoryas wrote:
> > > On macOS, private $TMPDIR's are the default. These $TMPDIR's are
> > > generated from a user's unix UID and UUID [1], which can create a
> > > relatively long path:
> > > 
> > >     /var/folders/d7/rz20f6hd709c1ty8f6_6y_z40000gn/T/
> > > 
> > > QEMU's avocado tests create a temporary directory prefixed by
> > > "avo_qemu_sock_", and create QMP sockets within _that_ as well.
> > > The QMP socket is unnecessarily long, because a temporary directory
> > > is created for every QEMUMachine object.
> > > 
> > >     /avo_qemu_sock_uh3w_dgc/qemu-37331-10bacf110-monitor.sock
> > 
> > 
> > Looking at this again, I realize my suggestion for dealing with the
> > second part of the path was mistaken.
> > 
> > The "qemu-37331-10bacf110-monitor.sock" part is combining two
> > pieces.
> > 
> > First the result of
> > 
> >   f"qemu-{os.getpid()}-{id(self):02x}"
> > 
> > is
> > 
> >   qemu-37331-10bacf110
> > 
> > and the code later than appends '-monitor.sock'
> > 
> > So...
> > 
> > > 
> > > The path limit for unix sockets on macOS is 104: [2]
> > > 
> > >     /*
> > >      * [XSI] Definitions for UNIX IPC domain.
> > >      */
> > >     struct  sockaddr_un {
> > 
> > >         unsigned char   sun_len;        /* sockaddr len including null */
> > >         sa_family_t     sun_family;     /* [XSI] AF_UNIX */
> > >         char            sun_path[104];  /* [XSI] path name (gag) */
> > >     };
> > > 
> > > This results in avocado tests failing on macOS because the QMP unix
> > > socket can't be created, because the path is too long:
> > > 
> > >     ERROR| Failed to establish connection: OSError: AF_UNIX path too long
> > > 
> > > This change reduces the size of both paths, and removes the unique
> > > identification information from the socket name, since it seems to be
> > > unnecessary.
> > > 
> > > This commit produces paths like the following:
> > > 
> > >     pdel@pdel-mbp:/var/folders/d7/rz20f6hd709c1ty8f6_6y_z40000gn/T
> > >     $ tree qemu*
> > >     qemu_oc7h7f3u
> > >     ├── qmp-console.sock
> > >     └── qmp-monitor.sock
> > > 
> > > [1] https://apple.stackexchange.com/questions/353832/why-is-mac-osx-temp-directory-in-weird-path
> > > [2] /Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk/usr/include/sys/un.h
> > > 
> > > Signed-off-by: Peter Delevoryas <peter@pjd.dev>
> > > ---
> > >  python/qemu/machine/machine.py         | 2 +-
> > >  tests/avocado/avocado_qemu/__init__.py | 2 +-
> > >  2 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/python/qemu/machine/machine.py b/python/qemu/machine/machine.py
> > > index 37191f433b..b1823966b3 100644
> > > --- a/python/qemu/machine/machine.py
> > > +++ b/python/qemu/machine/machine.py
> > > @@ -157,7 +157,7 @@ def __init__(self,
> > >          self._wrapper = wrapper
> > >          self._qmp_timer = qmp_timer
> > >  
> > > -        self._name = name or f"qemu-{os.getpid()}-{id(self):02x}"
> > > +        self._name = name or "qmp"
> > 
> > ...my suggestion here was wrong.
> > 
> > We don't need the os.getpid() unoiqueness because the tmpdir already
> > ensures that is safe, but keeping 'id(self)' is a good idea, if the
> > test case creates multiple machines concurrently. Bearing in mind we
> > later append '-monitor.sock' we don't need 'qmp' in the self._name.
> > 
> > So on reflection I think I should have suggested using:
> > 
> >     self._name = name or f"{id(self):02x}"
> > 
> > And *in addition*, a few lines later change:
> > 
> >             self._monitor_address = os.path.join(
> >                 self.sock_dir, f"{self._name}-monitor.sock"
> >             )
> > 
> > To
> > 
> >             self._monitor_address = os.path.join(
> >                 self.sock_dir, f"{self._name}.qmp"
> >             )
> >
> 
> Finally getting back to this (sorry, been working on other stuff), and I noticed
> the console socket is just below this:
> 
>     self._console_address = os.path.join(
>         self.sock_dir, f"{self._name}-console.sock"
>     )
> 
> So I probably shouldn't do the "-monitor.sock" change right?

I'd suggest changing this one to   f"{self._name}.con" at the
same time.


With regards,
Daniel
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