Comment 14 for bug 1655619

And it gets stranger:
I evaluated the exact command the envelope extension is using in Python on the commandline:

p = Popen('inkscape --query-x --query-id=rect10 "C:\Users\Eduard\AppData\Local\Temp\ink_ext_XXXXXX.svg06TNVY"', shell=True, stdout=PIPE, stderr=PIPE)
rc = p.wait()
res = float(p.stdout.read())
err = p.stderr.read()

And guess what: When exectuing all commands at once "res" is undefined. When executing them one after the other "res" contains the proper value. In both cases no deadlock, though.

Now I'm really starting to suspect that this is some kind of really ugly timing issue with the Popen command (maybe that's why it doesn't occur when bytecode of the "subprocess" module is available?). When you look at [1] there are a *lot* of warnings about using Popen in the way this extension does...

[1] https://docs.python.org/2/library/subprocess.html#popen-objects