_frontend/cli.py: Fetch all elements when tracking a build plantristan/pipeline-refactor

It makes not sense to type `bst fetch --track --deps all <targets>`,
because tracking will inevitably modify the build plan.

Stream initialization will not cope with this either, instead of
silently doing something which does not make any sense, we add an
assertion that this should not happen.

Unfortunately since `plan` is the default deps type for `bst fetch`,
this is likely to happen so it's important to correct.

This patch adds a warning in the case tracking of the build plan
elements is requested, and converts the request to track all elements
instead.

1 files changed, 7 insertions, 0 deletions

diff --git a/buildstream/_frontend/cli.py b/buildstream/_frontend/cli.py
index 2b5c77f8a..41736ac2f 100644
--- a/buildstream/_frontend/cli.py
+++ b/buildstream/_frontend/cli.py
@@ -275,10 +275,17 @@ def fetch(app, elements, deps, track_, except_, track_cross_junctions):
         plan:  Only dependencies required for the build plan
         all:   All dependencies
     """
+    from .._pipeline import PipelineSelection
+
     if track_cross_junctions and not track_:
         click.echo("ERROR: The --track-cross-junctions option can only be used with --track", err=True)
         sys.exit(-1)
 
+    if track_ and deps == PipelineSelection.PLAN:
+        click.echo("WARNING: --track specified for tracking of a build plan\n\n"
+                   "Since tracking modifies the build plan, all elements will be tracked.", err=True)
+        deps = PipelineSelection.ALL
+
     with app.initialized(session_name="Fetch"):
         app.stream.fetch(elements,
                          selection=deps,