X wcfs: Run fusermount and friends with /bin:/usr/bin always on path

For example runUnitTest in ERP5 runs test program with $PATH set without those
directores. This leads to failures like

      File "/srv/slapgrid/slappart16/srv/testnode/djk/soft/950bed0366ff018b829e9bbb7dc3326f/parts/wendelin.core/wcfs/__init__.py", line 533, in _is_mountpoint
        mounted = (0 == subprocess.call(["mountpoint", "-q", path]))
      File "/srv/slapgrid/slappart16/srv/testnode/djk/shared/python2.7/a8bdae732e550b385eb04a06ed09d383/lib/python2.7/subprocess.py", line 172, in call
        return Popen(*popenargs, **kwargs).wait()
      File "/srv/slapgrid/slappart16/srv/testnode/djk/shared/python2.7/a8bdae732e550b385eb04a06ed09d383/lib/python2.7/subprocess.py", line 394, in __init__
        errread, errwrite)
      File "/srv/slapgrid/slappart16/srv/testnode/djk/shared/python2.7/a8bdae732e550b385eb04a06ed09d383/lib/python2.7/subprocess.py", line 1047, in _execute_child
        raise child_exception
    OSError: [Errno 2] No such file or directory

where those system programs are not found.

wcfs/__init__.py

@@ -489,9 +489,8 @@ def _mkdir_p(path, mode=0o777): # -> created(bool)
 # _fuse_unmount calls `fusermount -u` + logs details if unmount failed.
 @func
 def _fuse_unmount(mntpt):
-    try:
-        subprocess.check_call(["fusermount", "-u", mntpt], close_fds=True)
-    except subprocess.CalledProcessError:
+    ret = _sysproccall(["fusermount", "-u", mntpt], close_fds=True)
+    if ret != 0:
         # unmount failed, usually due to "device is busy".
         # Log which files are still opened and reraise
         def _():
@@ -503,7 +502,7 @@ def _fuse_unmount(mntpt):
             # if there are other uncleaned wcfs mountpoints.
             # (lsof stats all filesystems on startup)
             # XXX -> better use `fuser -m <mntpt>` (it says it will limit search to files only under mntpt) ?
-            lsof = subprocess.Popen(["lsof", "-w", "+D", mntpt],
+            lsof = _sysproc(["lsof", "-w", "+D", mntpt],
                                 stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
             out, _ = lsof.communicate()
             log.warn(out)
@@ -530,10 +529,33 @@ def _is_mountpoint(path):    # -> bool
     if not S_ISDIR(_.st_mode):
         return False
 
-    mounted = (0 == subprocess.call(["mountpoint", "-q", path]))
+    mounted = (0 == _sysproccall(["mountpoint", "-q", path]))
     return mounted
 
 
+# _sysproc creates subprocess.Popen for "system" command.
+#
+# System commands are those that reside either in /bin or /usr/bin and which
+# should be found even if $PATH does no contain those directories. For example
+# runUnitTest in ERP5 sets $PATH without /bin, and this way executing
+# fusermount via subprocess.Popen instead of _sysproc would fail.
+def _sysproc(argv, **kw): # -> subprocess.Popen
+    env = kw.get('env', None)
+    if env is None:
+        env = os.environ
+    env = env.copy()
+    path = env.get('PATH', '')
+    if path:
+        path += ':'
+    path += '/bin:/usr/bin'
+    env['PATH'] = path
+    return subprocess.Popen(argv, env=env, **kw)
+
+# _sysproccall is to _sysproc like subprocess.call is to subprocess.Popen.
+def _sysproccall(argv, **kw): # -> retcode
+    return _sysproc(argv, **kw).wait()
+
+
 # _procwait waits for a process (subprocess.Popen) to terminate.
 def _procwait(ctx, proc):
     _waitfor(ctx, lambda: proc.poll() is not None)