X wcfs: Tune _fuse_unmount to include `fusermount -u` error message into raised exception

Previously that message was printed to stderr and it was very confusing:
we had something in the exception message, something in the log and
something in stderr.

Now exception message is self-sufficient and log provides more details.

wcfs/__init__.py

@@ -517,12 +517,11 @@ def _mkdir_p(path, mode=0o777): # -> created(bool)
 # _fuse_unmount calls `fusermount -u` + logs details if unmount failed.
 @func
 def _fuse_unmount(mntpt):
-    ret = _sysproccall(["fusermount", "-u", mntpt], close_fds=True)
+    ret, out = _sysproccallout(["fusermount", "-u", mntpt])
     if ret != 0:
         # unmount failed, usually due to "device is busy".
         # Log which files are still opened and reraise
         def _():
-            log.warn("fuse_unmount %s failed", mntpt)
             log.warn("# lsof %s" % mntpt)
             # -w to avoid lots of
             #  lsof: WARNING: can't stat() fuse.wcfs file system /tmp/wcfs/X
@@ -530,14 +529,16 @@ 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 = _sysproc(["lsof", "-w", "+D", mntpt],
-                                stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-            out, _ = lsof.communicate()
+            ret, out = _sysproccallout(["lsof", "-w", "+D", mntpt])
             log.warn(out)
-            if lsof.returncode:
+            if ret:
                 log.warn("(lsof failed)")
         defer(_)
-        raise RuntimeError("fuse_unmount %s failed  (logged details)" % mntpt)
+
+        out = out.rstrip() # kill trailing \n\n
+        emsg = "fuse_unmount %s: failed: %s" % (mntpt, out)
+        log.warn(emsg)
+        raise RuntimeError("%s\n(more details logged)" % emsg)
 
 # _is_mountpoint returns whether path is a mountpoint
 def _is_mountpoint(path):    # -> bool
@@ -577,12 +578,18 @@ def _sysproc(argv, **kw): # -> subprocess.Popen
         path += ':'
     path += '/bin:/usr/bin'
     env['PATH'] = path
-    return subprocess.Popen(argv, env=env, **kw)
+    return subprocess.Popen(argv, env=env, close_fds=True, **kw)
 
-# _sysproccall is to _sysproc like subprocess.call is to subprocess.Popen.
+# _sysproccall calls _sysproc and waits for spawned program to complete.
 def _sysproccall(argv, **kw): # -> retcode
     return _sysproc(argv, **kw).wait()
 
+# _sysproccallout calls _sysproc, waits for spawned program to complete and returns combined out/err.
+def _sysproccallout(argv, **kw): # -> retcode, output
+    proc = _sysproc(argv, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, **kw)
+    out, _ = proc.communicate()
+    return proc.returncode, out
+
 
 # _procwait waits for a process (subprocess.Popen) to terminate.
 def _procwait(ctx, proc):