Add initial `bst push` command

This is mainly useful for testing artifact caches and such. Most users
will hopefully be able to make use of artifact caches populated by
automated build machines, but right now it's unlikely that most people
will be pushing artifacts around.

buildstream/_frontend/main.py

@@ -208,6 +208,37 @@ def track(app, target, variant, deps, except_):
         sys.exit(-1)
 
 
+##################################################################
+#                           Push Command                         #
+##################################################################
+@cli.command(short_help="Push a built artifact")
+@click.option('--variant',
+              help='A variant of the specified target')
+@click.option('--deps', '-d', default='none',
+              type=click.Choice(['none', 'all']),
+              help='The dependencies to fetch (default: plan)')
+@click.argument('target')
+@click.pass_obj
+def push(app, target, variant, deps):
+    """Push a built artifact to the configured remote artifact cache.
+
+    Specify `--deps` to control which artifacts to push:
+
+    \b
+        none:  No dependencies, just the element itself
+        all:   All dependencies
+    """
+    app.initialize(target, variant)
+    try:
+        to_push = app.pipeline.deps_elements(deps)
+        app.pipeline.push(app.scheduler, to_push)
+        click.echo("")
+    except _BstError as e:
+        click.echo("")
+        click.echo("ERROR: {}".format(e))
+        sys.exit(-1)
+
+
 ##################################################################
 #                           Show Command                         #
 ##################################################################

buildstream/_pipeline.py

@@ -630,6 +630,43 @@ class Pipeline():
 
         self.open_workspace(scheduler, workspace_dir, source_index, no_checkout,
                             track, False)
+    # push()
+    #
+    # Pushes elements in the pipeline
+    #
+    # Args:
+    #    scheduler (Scheduler): The scheduler to run this pipeline on
+    #    elements (list): List of elements to push
+    #
+    def push(self, scheduler, elements):
+
+        if not self.artifacts.can_push():
+            self.message(self.target, MessageType.FAIL, "Not configured for pushing artifacts")
+
+        plan = elements
+        self.assert_consistent(plan)
+        self.session_elements = len(plan)
+
+        push = PushQueue()
+        push.enqueue(plan)
+        queues = [push]
+
+        self.message(self.target, MessageType.START, "Pushing {} artifacts".format(len(plan)))
+        elapsed, status = scheduler.run(queues)
+        pushed = len(push.processed_elements)
+
+        if status == SchedStatus.ERROR:
+            self.message(self.target, MessageType.FAIL, "Push failed", elapsed=elapsed)
+            raise PipelineError()
+        elif status == SchedStatus.TERMINATED:
+            self.message(self.target, MessageType.WARN,
+                         "Terminated after pushing {} elements".format(pushed),
+                         elapsed=elapsed)
+            raise PipelineError()
+        else:
+            self.message(self.target, MessageType.SUCCESS,
+                         "Pushed {} complete".format(pushed),
+                         elapsed=elapsed)
 
     # remove_elements():
     #