_artifactcache.py: Get the logs of an artifact

A CasBasedDirectory object of an artifacts logs can be obtained
with ArtifactCache.get_artifacts_log(). This ultimately calls
CASCache.get_top_level_dir() to obtained a CasBasedDirectory
of an artifact's subdirectory (or subdirectories).

buildstream/_artifactcache.py

@@ -833,6 +833,20 @@ class ArtifactCache():
 
         self.cas.link_ref(oldref, newref)
 
+    # get_artifact_logs():
+    #
+    # Get the logs of an existing artifact
+    #
+    # Args:
+    #     ref (str): The ref of the artifact
+    #
+    # Returns:
+    #     logsdir (CasBasedDirectory): A CasBasedDirectory containing the artifact's logs
+    #
+    def get_artifact_logs(self, ref):
+        descend = ["logs"]
+        return self.cas.get_toplevel_dir(ref, descend)
+
     ################################################
     #               Local Private Methods          #
     ################################################

buildstream/_cas/cascache.py

@@ -32,6 +32,7 @@ from .._protos.buildstream.v2 import buildstream_pb2
 
 from .. import utils
 from .._exceptions import CASCacheError
+from ..storage._casbaseddirectory import CasBasedDirectory
 
 from .casremote import BlobNotFound, _CASBatchRead, _CASBatchUpdate
 
@@ -587,6 +588,22 @@ class CASCache():
         reachable = set()
         self._reachable_refs_dir(reachable, tree, update_mtime=True)
 
+    # get_toplevel_dir()
+    #
+    # Return a CasBasedDirectory object of the specified sub_directories
+    #
+    # Args:
+    #     ref (str): The artifact ref
+    #     descend (list): A list of strings of artifact subdirectories
+    #
+    # Returns:
+    #     (CasBasedDirectory): The CasBasedDirectory object
+    #
+    def get_toplevel_dir(self, ref, descend):
+        cache_id = self.resolve_ref(ref, update_mtime=True)
+        vdir = CasBasedDirectory(self, cache_id).descend(descend)
+        return vdir
+
     ################################################
     #             Local Private Methods            #
     ################################################