source.py: Remove BST_KEY_REQUIRES_STAGE

Refactored this to remove unneeded complexity in the code base,
as described here:

  https://lists.apache.org/thread.html/r4b9517742433f07c79379ba5b67932cfe997c1e64965a9f1a2b613fc%40%3Cdev.buildstream.apache.org%3E

Changes:

  * source.py: Added private Source._cache_directory() context manager

    We also move the assertion about nodes which are safe to write to
    a bit lower in Source._set_ref(), as this was unnecessarily early.

    When tracking a workspace, the ref will be none and will turn out
    to be none afterwards, it is not a problem that a workspace's node
    is a synthetic one, as tracking will never affect it.

  * local plugin: Implement get_unique_key() and stage() using
    the new context manager in order to optimize staging and
    cache key calculations here.

  * workspace plugin: Implement get_unique_key() and stage() using
    the new context manager in order to optimize staging and
    cache key calculations here.

  * trackqueue.py: No special casing with Source._is_trackable()

4 files changed, 137 insertions, 90 deletions

diff --git a/src/buildstream/_scheduler/queues/trackqueue.py b/src/buildstream/_scheduler/queues/trackqueue.py
index d9c31ace1..5b3f05b57 100644
--- a/src/buildstream/_scheduler/queues/trackqueue.py
+++ b/src/buildstream/_scheduler/queues/trackqueue.py
@@ -39,9 +39,8 @@ class TrackQueue(Queue):
         return TrackQueue._track_element
 
     def status(self, element):
-        # We can skip elements entirely if they have no trackable
-        # sources.
-        if not any(source._is_trackable() for source in element.sources()):
+        # We can skip elements without any sources
+        if not any(element.sources()):
 
             # But we still have to mark them as tracked
             element._tracking_done()
diff --git a/src/buildstream/plugins/sources/local.py b/src/buildstream/plugins/sources/local.py
index ffcae4993..15962b4eb 100644
--- a/src/buildstream/plugins/sources/local.py
+++ b/src/buildstream/plugins/sources/local.py
@@ -37,19 +37,16 @@ details on common configuration options for sources.
 """
 
 import os
-from buildstream.storage.directory import Directory
-from buildstream import Source, SourceError
+from buildstream import Source, SourceError, Directory
 
 
 class LocalSource(Source):
     # pylint: disable=attribute-defined-outside-init
 
     BST_MIN_VERSION = "2.0"
-
     BST_STAGE_VIRTUAL_DIRECTORY = True
-    BST_KEY_REQUIRES_STAGE = True
 
-    __unique_key = None
+    __digest = None
 
     def configure(self, node):
         node.validate_keys(["path", *Source.COMMON_CONFIG_KEYS])
@@ -65,6 +62,21 @@ class LocalSource(Source):
     def is_cached(self):
         return True
 
+    def get_unique_key(self):
+        #
+        # As a core plugin, we use some private API to optimize file hashing.
+        #
+        # * Use Source._cache_directory() to prepare a Directory
+        # * Do the regular staging activity into the Directory
+        # * Use the hash of the cached digest as the unique key
+        #
+        if not self.__digest:
+            with self._cache_directory() as directory:
+                self.__do_stage(directory)
+                self.__digest = directory._get_digest()
+
+        return self.__digest.hash
+
     # We dont have a ref, we're a local file...
     def load_ref(self, node):
         pass
@@ -80,8 +92,38 @@ class LocalSource(Source):
         pass  # pragma: nocover
 
     def stage(self, directory):
-        # directory should always be a Directory object
+        #
+        # We've already prepared the CAS while resolving the cache key which
+        # will happen before staging.
+        #
+        # Now just retrieve the previously cached content to stage.
+        #
         assert isinstance(directory, Directory)
+        assert self.__digest is not None
+        with self._cache_directory(digest=self.__digest) as cached_directory:
+            directory.import_files(cached_directory)
+
+    def init_workspace(self, directory):
+        #
+        # FIXME: We should be able to stage the workspace from the content
+        #        cached in CAS instead of reimporting from the filesystem
+        #        to the new workspace directory with this special case, but
+        #        for some reason the writable bits are getting lost on regular
+        #        files through the transition.
+        #
+        self.__do_stage(directory)
+
+    # As a core element, we speed up some scenarios when this is used for
+    # a junction, by providing the local path to this content directly.
+    #
+    def _get_local_path(self):
+        return self.fullpath
+
+    # Staging is implemented internally, we preemptively put it in the CAS
+    # as a side effect of resolving the cache key, at stage time we just
+    # do an internal CAS stage.
+    #
+    def __do_stage(self, directory):
         with self.timed_activity("Staging local files into CAS"):
             if os.path.isdir(self.fullpath) and not os.path.islink(self.fullpath):
                 result = directory.import_files(self.fullpath)
@@ -93,9 +135,6 @@ class LocalSource(Source):
                     "Failed to stage source: files clash with existing directory", reason="ensure-stage-dir-fail"
                 )
 
-    def _get_local_path(self):
-        return self.fullpath
-
 
 # Plugin entry point
 def setup():
diff --git a/src/buildstream/plugins/sources/workspace.py b/src/buildstream/plugins/sources/workspace.py
index 44d0889b3..df24abb91 100644
--- a/src/buildstream/plugins/sources/workspace.py
+++ b/src/buildstream/plugins/sources/workspace.py
@@ -37,30 +37,21 @@ workspace. The node constructed would be specified as follows:
 
 import os
 
-from buildstream.storage.directory import Directory
-from buildstream import Source, SourceError
+from buildstream import Source, SourceError, Directory, MappingNode
 from buildstream.types import SourceRef
-from buildstream.node import MappingNode
 
 
 class WorkspaceSource(Source):
     # pylint: disable=attribute-defined-outside-init
 
     BST_MIN_VERSION = "2.0"
-
     BST_STAGE_VIRTUAL_DIRECTORY = True
-    BST_KEY_REQUIRES_STAGE = True
 
-    # Cached unique key
-    __unique_key = None
     # the digest of the Directory following the import of the workspace
     __digest = None
     # the cache key of the last workspace build
     __last_build = None
 
-    def track(self) -> SourceRef:  # pylint: disable=arguments-differ
-        return None
-
     def configure(self, node: MappingNode) -> None:
         node.validate_keys(["path", "last_build", "kind"])
         self.path = node.get_str("path")
@@ -75,6 +66,21 @@ class WorkspaceSource(Source):
     def is_resolved(self):
         return os.path.exists(self._get_local_path())
 
+    def get_unique_key(self):
+        #
+        # As a core plugin, we use some private API to optimize file hashing.
+        #
+        # * Use Source._cache_directory() to prepare a Directory
+        # * Do the regular staging activity into the Directory
+        # * Use the hash of the cached digest as the unique key
+        #
+        if not self.__digest:
+            with self._cache_directory() as directory:
+                self.__do_stage(directory)
+                self.__digest = directory._get_digest()
+
+        return self.__digest.hash
+
     def get_ref(self) -> None:
         return None
 
@@ -93,7 +99,29 @@ class WorkspaceSource(Source):
     def fetch(self) -> None:  # pylint: disable=arguments-differ
         pass  # pragma: nocover
 
-    def stage(self, directory: Directory) -> None:
+    def stage(self, directory):
+        #
+        # We've already prepared the CAS while resolving the cache key which
+        # will happen before staging.
+        #
+        # Now just retrieve the previously cached content to stage.
+        #
+        assert isinstance(directory, Directory)
+        assert self.__digest is not None
+        with self._cache_directory(digest=self.__digest) as cached_directory:
+            directory.import_files(cached_directory)
+
+    # As a core element, we speed up some scenarios when this is used for
+    # a junction, by providing the local path to this content directly.
+    #
+    def _get_local_path(self) -> str:
+        return self.path
+
+    # Staging is implemented internally, we preemptively put it in the CAS
+    # as a side effect of resolving the cache key, at stage time we just
+    # do an internal CAS stage.
+    #
+    def __do_stage(self, directory: Directory) -> None:
         assert isinstance(directory, Directory)
         with self.timed_activity("Staging local files"):
             result = directory.import_files(self.path, properties=["mtime"])
@@ -103,9 +131,6 @@ class WorkspaceSource(Source):
                     "Failed to stage source: files clash with existing directory", reason="ensure-stage-dir-fail"
                 )
 
-    def _get_local_path(self) -> str:
-        return self.path
-
 
 # Plugin entry point
 def setup() -> WorkspaceSource:
diff --git a/src/buildstream/source.py b/src/buildstream/source.py
index 245c3ca99..ce1cf8434 100644
--- a/src/buildstream/source.py
+++ b/src/buildstream/source.py
@@ -312,11 +312,6 @@ class Source(Plugin):
     to.
     """
 
-    BST_KEY_REQUIRES_STAGE = False
-    """Whether the source will require staging in order to efficiently generate
-    a unique key.
-    """
-
     def __init__(
         self,
         context: "Context",
@@ -364,7 +359,6 @@ class Source(Plugin):
         self.__mirror_directory = None  # type: Optional[str]
 
         self._configure(self.__config)
-        self.__digest = None
 
         self.__is_cached = None
 
@@ -723,23 +717,6 @@ class Source(Plugin):
     #############################################################
     #            Private Methods used in BuildStream            #
     #############################################################
-    # Stage files at the localpath into the cascache
-    #
-    # Returns:
-    #   the hash of the cas directory
-    def _stage_into_cas(self) -> str:
-        # FIXME: this should not be called for sources with digests already set
-        # since they will already have been staged into the cache. However,
-        # _get_unique_key is sometimes called outside of _generate_key
-        if self.__digest is None:
-            cas_dir = CasBasedDirectory(self._get_context().get_cascache())
-            self.stage(cas_dir)
-            digest = cas_dir._get_digest()
-            self.__digest = digest
-        else:
-            # XXX: an assignment to please mypy
-            digest = self.__digest
-        return digest.hash
 
     # Wrapper around preflight() method
     #
@@ -813,14 +790,8 @@ class Source(Plugin):
     # 'directory' option
     #
     def _stage(self, directory):
-        if self.BST_KEY_REQUIRES_STAGE:
-            # _get_unique_key should be called before _stage
-            assert self.__digest is not None
-            cas_dir = CasBasedDirectory(self._get_context().get_cascache(), digest=self.__digest)
-            directory.import_files(cas_dir)
-        else:
-            self.validate_cache()
-            self.stage(directory)
+        self.validate_cache()
+        self.stage(directory)
 
     # Wrapper for init_workspace()
     def _init_workspace(self, directory):
@@ -835,10 +806,7 @@ class Source(Plugin):
     # Wrapper for get_unique_key() api
     #
     def _get_unique_key(self):
-        if self.BST_KEY_REQUIRES_STAGE:
-            return self._stage_into_cas()
-        else:
-            return self.get_unique_key()
+        return self.get_unique_key()
 
     # _project_refs():
     #
@@ -954,20 +922,6 @@ class Source(Plugin):
         if project is toplevel and not node:
             node = provenance._node
 
-        # Ensure the node is not from a junction
-        if not toplevel.ref_storage == ProjectRefStorage.PROJECT_REFS and provenance._project is not toplevel:
-            if provenance._project is project:
-                self.warn("{}: Not persisting new reference in junctioned project".format(self))
-            elif provenance._project is None:
-                assert provenance._filename == ""
-                assert provenance._shortname == ""
-                raise SourceError("{}: Error saving source reference to synthetic node.".format(self))
-            else:
-                raise SourceError(
-                    "{}: Cannot track source in a fragment from a junction".format(provenance._shortname),
-                    reason="tracking-junction-fragment",
-                )
-
         #
         # Step 2 - Set the ref in memory, and determine changed state
         #
@@ -988,6 +942,21 @@ class Source(Plugin):
             # tracking in the future.  For now, this is quite safe.
             return False
 
+        # Ensure the node is not from a junction
+        if not toplevel.ref_storage == ProjectRefStorage.PROJECT_REFS and provenance._project is not toplevel:
+            if provenance._project is project:
+                self.warn("{}: Not persisting new reference in junctioned project".format(self))
+            elif provenance._project is None:
+                assert provenance._filename == ""
+                assert provenance._shortname == ""
+
+                raise SourceError("{}: Error saving source reference to synthetic node.".format(self))
+            else:
+                raise SourceError(
+                    "{}: Cannot track source in a fragment from a junction".format(provenance._shortname),
+                    reason="tracking-junction-fragment",
+                )
+
         actions = {}
         for k, v in clean.items():
             if k not in to_modify:
@@ -1081,11 +1050,6 @@ class Source(Plugin):
     #   previous_sources_dir (str): directory where previous sources are staged
     #
     def _track(self, previous_sources_dir: str = None) -> SourceRef:
-        if self.BST_KEY_REQUIRES_STAGE:
-            # ensure that these sources have a key after tracking
-            self._generate_key()
-            return None
-
         if self.BST_REQUIRES_PREVIOUS_SOURCES_TRACK:
             new_ref = self.__do_track(previous_sources_dir=previous_sources_dir)
         else:
@@ -1107,16 +1071,6 @@ class Source(Plugin):
 
         return new_ref
 
-    # _is_trackable()
-    #
-    # Returns:
-    #   (bool): Whether this source is trackable
-    #
-    def _is_trackable(self) -> bool:
-        """Report whether this source can be tracked."""
-        # sources that require staging to generate keys cannot be tracked
-        return not self.BST_KEY_REQUIRES_STAGE
-
     # _requires_previous_sources()
     #
     # If a plugin requires access to previous sources at track or fetch time,
@@ -1162,6 +1116,36 @@ class Source(Plugin):
     def _element_name(self):
         return self.__element_name
 
+    # _cache_directory()
+    #
+    # A context manager to cache and retrieve content.
+    #
+    # If the digest is not specified, then a new directory is prepared, the
+    # content of which can later be addressed by accessing it's digest,
+    # using the private API Directory._get_digest().
+    #
+    # The hash of the Digest of the cached directory is suitable for use as a
+    # cache key, and the Digest object can be reused later on to do the
+    # staging operation.
+    #
+    # This context manager was added specifically to optimize cases where
+    # we have project or host local data to stage into CAS, such as local
+    # sources and workspaces.
+    #
+    # Args:
+    #    digest: A Digest of previously cached content.
+    #
+    # Yields:
+    #    (Directory): A handle on the cached content directory
+    #
+    @contextmanager
+    def _cache_directory(self, digest=None):
+        context = self._get_context()
+        cache = context.get_cascache()
+        cas_dir = CasBasedDirectory(cache, digest=digest)
+
+        yield cas_dir
+
     #############################################################
     #                   Local Private Methods                   #
     #############################################################