document and automate llfuse requirement

docs/installation.rst

@@ -105,7 +105,9 @@ following dependencies first:
 * liblz4_
 * some Python dependencies, pip will automatically install them for you
 * optionally, the llfuse_ Python package is required if you wish to mount an
-  archive as a FUSE filesystem. FUSE >= 2.8.0 is required for llfuse.
+  archive as a FUSE filesystem. llfuse has been tested in version 0.40.x,
+  0.41.x might also work, >= 0.42.x does not work (due to API changes in
+  llfuse). FUSE >= 2.8.0 is required for llfuse 0.40.
 
 In the following, the steps needed to install the dependencies are listed for a
 selection of platforms. If your distribution is not covered by these

requirements.d/fuse.txt

@@ -0,0 +1,4 @@
+# low-level FUSE support library for "borg mount"
+# see comments setup.py about this version requirement.
+llfuse<0.41
+

setup.py

@@ -21,6 +21,12 @@ on_rtd = os.environ.get('READTHEDOCS')
 # Also, we might use some rather recent API features.
 install_requires = ['msgpack-python>=0.4.6', ]
 
+extras_require = {
+    # llfuse 0.40 (tested, proven, ok)
+    # llfuse 0.41 (unknown, maybe ok)
+    # llfuse 0.42 (tested, does not work, incompatible api changes)
+    'fuse': ['llfuse<0.41', ],
+}
 
 from setuptools import setup, Extension
 from setuptools.command.sdist import sdist
@@ -260,4 +266,5 @@ setup(
     ext_modules=ext_modules,
     setup_requires=['setuptools_scm>=1.7'],
     install_requires=install_requires,
+    extras_require=extras_require,
 )