labath added inline comments.
================
Comment at: lit/tools/lldb-mi/target/inputs/target-select-so-path.py:8-11
+def get_free_port():
+ s = socket.socket()
+ s.bind(('', 0))
+ return s.getsockname()[1]
----------------
apolyakov wrote:
> labath wrote:
> > apolyakov wrote:
> > > labath wrote:
> > > > This is still racy, because the port can be snatched from under you
> > > > between the time you get the free port and the time when lldb-server
> > > > binds to it. If this was the only test doing it then it might be fine,
> > > > but since this is going to be running concurrently with other tests,
> > > > all of which are fetching free ports, the chances of that happening add
> > > > up.
> > > >
> > > > (Also, binding to the wildcard address will trigger a firewall popup on
> > > > some machines.)
> > > There is a problem with getting port from lldb-server. If we run
> > > `lldb-server gdbserver --pipe 0 ocalhost:0`, it'll print port number to
> > > its stdout, but we can't get it using pipes since to do this we need to
> > > wait until lldb-server finishes that isn't what we want.
> > Aha, I see. lldb-server does not really expect you to pass std file
> > descriptor as the --pipe argument. Normally you would create a separate fd
> > and pass that instead. Doing that from python is a bit icky, but doable:
> > ```
> > (r, w) = os.pipe()
> > kwargs = {}
> > if sys.version_info >= (3,0):
> > kwargs["pass_fds"] = [w]
> > llgs = subprocess.Popen(["lldb-server", "gdbserver", "--pipe", str(w),
> > ...], **kwargs)
> > port_bytes = os.read(r, 10)
> > port = int(port_bytes.decode("utf-8").strip('\x00'))
> > ```
> >
> > Alternatively, we could modify lldb-server to print the port number to
> > stdout in addition to any --pipe arguments (sounds like a good addition
> > anyway, as it enables easy free port selection for a shell user), and then
> > you can sniff that text from here.
> `--pipe 0` prints port number exactly to stdout, so there will not be a
> difference for us. It's not so simple to get port from lldb-server's stdout
> in python, so I don't think it will save us.
I think you're looking for this:
```
foo = subprocess.Popen(...)
print "The first line of output is: " + foo.stdout.readline()
```
Btw, using `--pipe 0` works only by accident (0 is the stdin descriptor), and
probably only in a terminal. Once popen() starts redirecting things, `0` will
probably not refer to the thing that `stdout` will read from. `--pipe 1` would
fix that, but then we have the issue that lldb-server will close the `--pipe`
descriptor once it's finished writing the port. That can have surprising
effects as subsequent attempts to write to stdout will fail. (That's why I
suggested a different implementation. Among other things, outputting something
like "lldb-server listening on 127.0.0.1:4747" will make it easier to separate
out the port from other things that lldb-server happens to write to stdout.)
https://reviews.llvm.org/D49739
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