_artifactcache: Add bool return value to pull()

Calling pull() for a potentially unavailable artifact is no longer
considered an error.

3 files changed, 11 insertions, 6 deletions

diff --git a/buildstream/_artifactcache/artifactcache.py b/buildstream/_artifactcache/artifactcache.py
index 352cdb4a9..18fca86e0 100644
--- a/buildstream/_artifactcache/artifactcache.py
+++ b/buildstream/_artifactcache/artifactcache.py
@@ -333,6 +333,9 @@ class ArtifactCache():
     #     key (str): The cache key to use
     #     progress (callable): The progress callback, if any
     #
+    # Returns:
+    #   (bool): True if pull was successful, False if artifact was not available
+    #
     def pull(self, element, key, *, progress=None):
         raise ImplError("Cache '{kind}' does not implement pull()"
                         .format(kind=type(self).__name__))
diff --git a/buildstream/_artifactcache/ostreecache.py b/buildstream/_artifactcache/ostreecache.py
index 97e9088a5..b592ac4f2 100644
--- a/buildstream/_artifactcache/ostreecache.py
+++ b/buildstream/_artifactcache/ostreecache.py
@@ -171,9 +171,14 @@ class OSTreeCache(ArtifactCache):
 
         try:
             # fetch the artifact from highest priority remote using the specified cache key
-            remote = artifact_map.lookup_first(ref)
+            remotes = artifact_map.lookup(ref)
+            if not remotes:
+                return False
+
+            remote = remotes[0]
             remote_name = self._ensure_remote(self.repo, remote.pull_url)
             _ostree.fetch(self.repo, remote=remote_name, ref=ref, progress=progress)
+            return True
         except OSTreeError as e:
             raise ArtifactError("Failed to pull artifact for element {}: {}"
                                 .format(element.name, e)) from e
@@ -429,8 +434,5 @@ class _OSTreeArtifactMap():
     def lookup(self, ref):
         return self._ref_to_remotes.get(ref, [])
 
-    def lookup_first(self, ref):
-        return self._ref_to_remotes.get(ref, [])[0]
-
     def contains(self, ref):
         return ref in self._ref_to_remotes
diff --git a/buildstream/element.py b/buildstream/element.py
index c563b79af..adcccc8cf 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1569,12 +1569,12 @@ class Element(Plugin):
 
         if self.__remotely_strong_cached:
             key = self.__strict_cache_key
-            self.__artifacts.pull(self, key, progress=progress)
+            assert self.__artifacts.pull(self, key, progress=progress)
 
             # update weak ref by pointing it to this newly fetched artifact
             self.__artifacts.link_key(self, key, weak_key)
         elif not context.get_strict() and self.__remotely_cached:
-            self.__artifacts.pull(self, weak_key, progress=progress)
+            assert self.__artifacts.pull(self, weak_key, progress=progress)
 
             # extract strong cache key from this newly fetched artifact
             self._update_state()