Re: [Qemu-devel] [PATCH v8 06/13] qemu.py: make sure we only remove files we create

[Amador Pahim]

On Thu, Sep 14, 2017 at 9:46 PM, Eduardo Habkost <address@hidden> wrote:
> On Thu, Sep 14, 2017 at 09:38:13PM +0200, Amador Pahim wrote:
>> On Tue, Sep 5, 2017 at 5:18 AM, Fam Zheng <address@hidden> wrote:
>> > On Fri, 09/01 13:28, Amador Pahim wrote:
>> >> To launch a VM, we need to create basically two files: the monitor
>> >> socket (if it's a UNIX socket) and the qemu log file.
>> >>
>> >> For the qemu log file, we currently just open the path, which will
>> >> create the file if it does not exist or overwrite the file if it does
>> >> exist.
>> >>
>> >> For the monitor socket, if it already exists, we are currently removing
>> >> it, even if it's not created by us.
>> >>
>> >> This patch moves to pre_launch() the responsibility to make sure we only
>> >> create files that are not pre-existent and to populate a list of
>> >> controlled files. This list will then be used as the reference of
>> >> files to remove during the cleanup (post_shutdown()).
>> >>
>> >> Signed-off-by: Amador Pahim <address@hidden>
>> >> ---
>> >>  scripts/qemu.py | 30 +++++++++++++++++++++++-------
>> >>  1 file changed, 23 insertions(+), 7 deletions(-)
>> >>
>> >> diff --git a/scripts/qemu.py b/scripts/qemu.py
>> >> index 3ebe5ee0a4..c26e1412f9 100644
>> >> --- a/scripts/qemu.py
>> >> +++ b/scripts/qemu.py
>> >> @@ -41,6 +41,7 @@ class QEMUMachine(object):
>> >>              monitor_address = os.path.join(test_dir, name + 
>> >> "-monitor.sock")
>> >>          self._monitor_address = monitor_address
>> >>          self._qemu_log_path = os.path.join(test_dir, name + ".log")
>> >> +        self._qemu_log_fd = None
>> >>          self._popen = None
>> >>          self._binary = binary
>> >>          self._args = list(args) # Force copy args in case we modify them
>> >> @@ -50,6 +51,7 @@ class QEMUMachine(object):
>> >>          self._socket_scm_helper = socket_scm_helper
>> >>          self._debug = debug
>> >>          self._qemu_full_args = None
>> >> +        self._created_files = []
>> >>
>> >>      # This can be used to add an unused monitor instance.
>> >>      def add_monitor_telnet(self, ip, port):
>> >> @@ -128,30 +130,44 @@ class QEMUMachine(object):
>> >>                  '-display', 'none', '-vga', 'none']
>> >>
>> >>      def _pre_launch(self):
>> >> -        self._qmp = qmp.qmp.QEMUMonitorProtocol(self._monitor_address, 
>> >> server=True,
>> >> -                                                debug=self._debug)
>> >> +        try:
>> >> +            self._qmp = 
>> >> qmp.qmp.QEMUMonitorProtocol(self._monitor_address,
>> >> +                                                    server=True,
>> >> +                                                    debug=self._debug)
>> >> +        except:
>> >> +            raise
>> >
>> > What's the point of "except: raise"? It seems useless.
>>
>> The point is to execute the block in the else only when no exception
>> happens. When some exception happens, I want to raise it without
>> executing the else block.
>
> Isn't this exactly what Python does when an exception is raised
> with no "try" block?

Sure, cleaning this up.

>
>
>>
>> >
>> >> +        else:
>> >> +            if not isinstance(self._monitor_address, tuple):
>> >> +                self._created_files.append(self._monitor_address)
>> >> +
>> >> +        try:
>> >> +            flags = os.O_CREAT | os.O_EXCL | os.O_WRONLY
>> >> +            os.open(self._qemu_log_path, flags)
>> >
>> > Why change to os.open() instead of open()?
>>
>> I want to create the file only if it does not exist. The open() flag
>> 'x' is available only in python 3.3. For python <3.3, we need the
>> os.open() to have that feature.
>
> I'm not sure this extra complexity is really necessary.  We could
> fix all that by using mkdtemp() and deleting the temporary
> directory on shutdown.

I thought about that, but I foresee the question: hat happens if
between the mkdtemp and the file creation (i.e. self._qemu_log_path)
someone goes in that directory and creates a file with the same name
of the self._qemu_log_path? Are we going to overwrite it? Ok, very
unlikely, but possible. This extra step takes care of that.

>
>>
>> >
>> >> +        except:
>> >> +            raise
>> >> +        else:
>> >> +            self._created_files.append(self._qemu_log_path)
>> >> +            self._qemu_log_fd = open(self._qemu_log_path, 'wb')
>> >>
>> >>      def _post_launch(self):
>> >>          self._qmp.accept()
>> >>
>> >>      def _post_shutdown(self):
>> >> -        if not isinstance(self._monitor_address, tuple):
>> >> -            self._remove_if_exists(self._monitor_address)
>> >> -        self._remove_if_exists(self._qemu_log_path)
>> >> +        while self._created_files:
>> >> +            self._remove_if_exists(self._created_files.pop())
>> >>
>> >>      def launch(self):
>> >>          '''Launch the VM and establish a QMP connection'''
>> >>          self._iolog = None
>> >>          self._qemu_full_args = None
>> >>          devnull = open(os.path.devnull, 'rb')
>> >> -        qemulog = open(self._qemu_log_path, 'wb')
>> >>          try:
>> >>              self._pre_launch()
>> >>              self._qemu_full_args = (self._wrapper + [self._binary] +
>> >>                                      self._base_args() + self._args)
>> >>              self._popen = subprocess.Popen(self._qemu_full_args,
>> >>                                             stdin=devnull,
>> >> -                                           stdout=qemulog,
>> >> +                                           stdout=self._qemu_log_fd,
>> >>                                             stderr=subprocess.STDOUT,
>> >>                                             shell=False)
>> >>              self._post_launch()
>> >> --
>> >> 2.13.5
>> >>
>> >
>> > Fam
>
> --
> Eduardo