Allow checking out artifact metadata with `bst checkout`sam/bst-checkout-metadata

This commit adds a new `bst checkout --metadata` mode which checks out
the artifact's build logs and metadata into a user specified directory.
This is instead of the normal mode which checks out and optionally
integrates the artifact's contents.

3 files changed, 57 insertions, 2 deletions

diff --git a/buildstream/_frontend/main.py b/buildstream/_frontend/main.py
index 3ac6b5222..91fc750ac 100644
--- a/buildstream/_frontend/main.py
+++ b/buildstream/_frontend/main.py
@@ -504,16 +504,21 @@ def shell(app, element, sysroot, build, command):
               help="Overwrite files existing in checkout directory")
 @click.option('--integrate/--no-integrate', default=True, is_flag=True,
               help="Whether to run integration commands")
+@click.option('--metadata', '-m', default=False, is_flag=True,
+              help="Check out the artifact's metadata, instead of the contents")
 @click.argument('element',
                 type=click.Path(dir_okay=False, readable=True))
 @click.argument('directory', type=click.Path(file_okay=False))
 @click.pass_obj
-def checkout(app, element, directory, force, integrate):
+def checkout(app, element, directory, force, integrate, metadata):
     """Checkout a built artifact to the specified directory
     """
     app.initialize([element])
     try:
-        app.pipeline.checkout(directory, force, integrate)
+        if metadata:
+            app.pipeline.checkout_metadata(directory, force)
+        else:
+            app.pipeline.checkout(directory, force, integrate)
         click.echo("")
     except BstError as e:
         click.echo("")
diff --git a/buildstream/_pipeline.py b/buildstream/_pipeline.py
index 90ed899ea..aa0299c52 100644
--- a/buildstream/_pipeline.py
+++ b/buildstream/_pipeline.py
@@ -490,6 +490,38 @@ class Pipeline():
                 except OSError as e:
                     raise PipelineError("Failed to copy files: {}".format(e)) from e
 
+    # checkout_metadata()
+    #
+    # Checkout the metadata from the pipeline target artifact to the specified directory
+    #
+    # Args:
+    #    directory (str): The directory to checkout the artifact to
+    #    force (bool): Force overwrite files which exist in `directory`
+    #
+    def checkout_metadata(self, directory, force):
+        # We only have one target in a checkout command
+        target = self.targets[0]
+
+        try:
+            os.makedirs(directory, exist_ok=True)
+        except OSError as e:
+            raise PipelineError("Failed to create metadata checkout directory: {}".format(e)) from e
+
+        if not os.access(directory, os.W_OK):
+            raise PipelineError("Directory {} not writable".format(directory))
+
+        if not force and os.listdir(directory):
+            raise PipelineError("Checkout directory is not empty: {}"
+                                .format(directory))
+
+        metadata_dirs = target.metadata_dirs()
+        with target.timed_activity("Copying metadata files to {}".format(directory)):
+            for metadata_type, metadata_dir in metadata_dirs.items():
+                try:
+                    utils.copy_files(metadata_dir, os.path.join(directory, metadata_type))
+                except OSError as e:
+                    raise PipelineError("Failed to copy files: {}".format(e)) from e
+
     # open_workspace
     #
     # Open a project workspace.
diff --git a/buildstream/element.py b/buildstream/element.py
index 86dd2b8dc..d2976cfe2 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -543,6 +543,24 @@ class Element(Plugin):
 
         return None
 
+    def metadata_dirs(self):
+        """Return a map of the paths that contain the artifact's metadata.
+
+        The paths will point inside the artifact cache and must be treated
+        as read-only.
+
+        Returns:
+           (dict): A dictionary mapping each type of metadata to the
+           location where it is stored.
+        """
+
+        self._assert_cached()
+        artifact_base = self.__artifacts.extract(self)
+        return {
+            'logs': os.path.join(artifact_base, 'logs'),
+            'meta': os.path.join(artifact_base, 'meta'),
+        }
+
     #############################################################
     #                  Abstract Element Methods                 #
     #############################################################