cascache: move tmp directory one level up

The tmp directory is filled when adding an artifact with temporary
files. This causes problems on calculate_cache_size() because we could
try to a attemp to do a stat() on a temporary file that meanwhile has
been reaped.

Handle this by not considering the tmp directory when calculating the
cache size.

buildstream/_artifactcache/cascache.py

@@ -56,7 +56,10 @@ class CASCache(ArtifactCache):
         super().__init__(context)
 
         self.casdir = os.path.join(context.artifactdir, 'cas')
-        os.makedirs(os.path.join(self.casdir, 'tmp'), exist_ok=True)
+        os.makedirs(self.casdir, exist_ok=True)
+
+        self.tmpdir = os.path.join(context.artifactdir, 'tmp')
+        os.makedirs(self.tmpdir, exist_ok=True)
 
         self._enable_push = enable_push
 
@@ -394,7 +397,7 @@ class CASCache(ArtifactCache):
         try:
             h = hashlib.sha256()
             # Always write out new file to avoid corruption if input file is modified
-            with tempfile.NamedTemporaryFile(dir=os.path.join(self.casdir, 'tmp')) as out:
+            with tempfile.NamedTemporaryFile(dir=self.tmpdir) as out:
                 # Set mode bits to 0644
                 os.chmod(out.name, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
 
@@ -764,7 +767,7 @@ class CASCache(ArtifactCache):
             # already in local cache
             return
 
-        with tempfile.NamedTemporaryFile(dir=os.path.join(self.casdir, 'tmp')) as out:
+        with tempfile.NamedTemporaryFile(dir=self.tmpdir) as out:
             self._fetch_blob(remote, tree, out)
 
             directory = remote_execution_pb2.Directory()
@@ -778,7 +781,7 @@ class CASCache(ArtifactCache):
                     # already in local cache
                     continue
 
-                with tempfile.NamedTemporaryFile(dir=os.path.join(self.casdir, 'tmp')) as f:
+                with tempfile.NamedTemporaryFile(dir=self.tmpdir) as f:
                     self._fetch_blob(remote, filenode.digest, f)
 
                     digest = self.add_object(path=f.name)