utils._get_volume_size: use shutil.disk_usageaevri/psutil_volume

This is win32-friendly, less to read, and fixes a latent bug.

Avoid dealing with low-level platform specifics by using the
higher-level shutil.disk_usage() function to calc total and available
bytes for us.

shutil is also more correct - it uses f_frsize to convert from blocks to
bytes, instead of f_bsize. I verified that the numbers match with the
output from `df -k`, whereas the old implementation did not. Here are
the meanings from `man statvfs`:

    f_frsize    The size in bytes of the minimum unit of allocation on
                this file system. (This corresponds to the f_bsize
                member of struct statfs.)

    f_bsize     The preferred length of I/O requests for files on this
                file system. (Corresponds to the f_iosize member of
                struct statfs.)

Link to shutil.disk_usage() implementation:
https://github.com/python/cpython/blob/3.5/Lib/shutil.py#L980

1 files changed, 2 insertions, 3 deletions

diff --git a/src/buildstream/utils.py b/src/buildstream/utils.py
index f509ce998..775a11142 100644
--- a/src/buildstream/utils.py
+++ b/src/buildstream/utils.py
@@ -642,12 +642,11 @@ def _get_dir_size(path):
 #
 def _get_volume_size(path):
     try:
-        stat_ = os.statvfs(path)
+        usage = shutil.disk_usage(path)
     except OSError as e:
         raise UtilError("Failed to retrieve stats on volume for path '{}': {}"
                         .format(path, e)) from e
-
-    return stat_.f_bsize * stat_.f_blocks, stat_.f_bsize * stat_.f_bavail
+    return usage.total, usage.free
 
 
 # _parse_size():