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

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

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"
            )


> diff --git a/tests/avocado/avocado_qemu/__init__.py b/tests/avocado/avocado_qemu/__init__.py
> index ed4853c805..43b8c8848c 100644
> --- a/tests/avocado/avocado_qemu/__init__.py
> +++ b/tests/avocado/avocado_qemu/__init__.py
> @@ -296,7 +296,7 @@ def require_accelerator(self, accelerator):
>                          "available" % accelerator)
>  
>      def _new_vm(self, name, *args):
> -        self._sd = tempfile.TemporaryDirectory(prefix="avo_qemu_sock_")
> +        self._sd = tempfile.TemporaryDirectory(prefix="qemu_")

This bit is fine.

>          vm = QEMUMachine(self.qemu_bin, base_temp_dir=self.workdir,
>                           sock_dir=self._sd.name, log_dir=self.logdir)
>          self.log.debug('QEMUMachine "%s" created', name)

With regards,
Daniel
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