_artifactcache.py: Refactored to use utils._get_volume_size()

This will benefit from a better UtilError being raised, and
and turns the artifact cache's local function into a one liner.

The loop which finds the first existing directory in the
given path has been removed, being meaningless due to the
call to os.makedirs() in ArtifactCache.__init__().

The local function was renamed to _get_cache_volume_size() and
no longer takes any arguments, which is more suitable for the
function as it serves as a testing override surface for
unittest.mock().

The following test cases which use the function to override
the ArtifactCache behavior have been updated to use the new
overridable function name:

   tests/artifactcache/cache_size.py
   tests/artifactcache/expiry.py

3 files changed, 19 insertions, 20 deletions

diff --git a/buildstream/_artifactcache.py b/buildstream/_artifactcache.py
index b1afdf377..725b5fbee 100644
--- a/buildstream/_artifactcache.py
+++ b/buildstream/_artifactcache.py
@@ -307,7 +307,7 @@ class ArtifactCache():
     # it is greater than the actual cache size.
     #
     # Returns:
-    #     (int) An approximation of the artifact cache size.
+    #     (int) An approximation of the artifact cache size, in bytes.
     #
     def get_cache_size(self):
 
@@ -848,19 +848,16 @@ class ArtifactCache():
         else:
             headroom = 2e9
 
-        artifactdir_volume = self.context.artifactdir
-        while not os.path.exists(artifactdir_volume):
-            artifactdir_volume = os.path.dirname(artifactdir_volume)
-
         try:
-            cache_quota = utils._parse_size(self.context.config_cache_quota, artifactdir_volume)
+            cache_quota = utils._parse_size(self.context.config_cache_quota,
+                                            self.context.artifactdir)
         except utils.UtilError as e:
             raise LoadError(LoadErrorReason.INVALID_DATA,
                             "{}\nPlease specify the value in bytes or as a % of full disk space.\n"
                             "\nValid values are, for example: 800M 10G 1T 50%\n"
                             .format(str(e))) from e
 
-        available_space, total_size = self._get_volume_space_info_for(artifactdir_volume)
+        total_size, available_space = self._get_cache_volume_size()
         cache_size = self.get_cache_size()
 
         # Ensure system has enough storage for the cache_quota
@@ -903,19 +900,21 @@ class ArtifactCache():
         self._cache_quota = cache_quota - headroom
         self._cache_lower_threshold = self._cache_quota / 2
 
-    # _get_volume_space_info_for
-    #
-    # Get the available space and total space for the given volume
+    # _get_cache_volume_size()
     #
-    # Args:
-    #     volume: volume for which to get the size
+    # Get the available space and total space for the volume on
+    # which the artifact cache is located.
     #
     # Returns:
-    #     A tuple containing first the availabe number of bytes on the requested
-    #     volume, then the total number of bytes of the volume.
-    def _get_volume_space_info_for(self, volume):
-        stat = os.statvfs(volume)
-        return stat.f_bsize * stat.f_bavail, stat.f_bsize * stat.f_blocks
+    #    (int): The total number of bytes on the volume
+    #    (int): The number of available bytes on the volume
+    #
+    # NOTE: We use this stub to allow the test cases
+    #       to override what an artifact cache thinks
+    #       about it's disk size and available bytes.
+    #
+    def _get_cache_volume_size(self):
+        return utils._get_volume_size(self.context.artifactdir)
 
 
 # _configured_remote_artifact_cache_specs():
diff --git a/tests/artifactcache/cache_size.py b/tests/artifactcache/cache_size.py
index 11c8f6194..63ab9ad07 100644
--- a/tests/artifactcache/cache_size.py
+++ b/tests/artifactcache/cache_size.py
@@ -80,7 +80,7 @@ def test_quota_over_1024T(cli, tmpdir):
     _yaml.dump({'name': 'main'}, str(project.join("project.conf")))
 
     volume_space_patch = mock.patch(
-        "buildstream._artifactcache.ArtifactCache._get_volume_space_info_for",
+        "buildstream._artifactcache.ArtifactCache._get_cache_volume_size",
         autospec=True,
         return_value=(1025 * TiB, 1025 * TiB)
     )
diff --git a/tests/artifactcache/expiry.py b/tests/artifactcache/expiry.py
index d7bafe7e8..2230b70bd 100644
--- a/tests/artifactcache/expiry.py
+++ b/tests/artifactcache/expiry.py
@@ -358,9 +358,9 @@ def test_invalid_cache_quota(cli, datafiles, tmpdir, quota, err_domain, err_reas
         total_space = 10000
 
     volume_space_patch = mock.patch(
-        "buildstream._artifactcache.ArtifactCache._get_volume_space_info_for",
+        "buildstream._artifactcache.ArtifactCache._get_cache_volume_size",
         autospec=True,
-        return_value=(free_space, total_space),
+        return_value=(total_space, free_space),
     )
 
     cache_size_patch = mock.patch(