[DISCUSS] Add --use-buildtrees support to bst artifacts checkout

[Nikita Ostrenkov]

Hi BuildStream developers,

I would like to discuss a possible small feature addition to "bst artifacts
checkout".

Currently, "bst shell" supports the "--use-buildtrees" option, which makes
it possible to open a shell with the existing build tree.

I was wondering whether it would make sense to support a similar option in
"bst artifacts checkout", so that the build tree contents could also be
checked out directly.

I took a quick look at the implementation, and it seems that most of the
required infrastructure already exists, so the change itself appears
relatively straightforward.

Before working on a proper patch/MR, I wanted to ask:

- Has this been discussed before?
- Is something similar already planned?
- Are there architectural or UX reasons why this functionality should not
be added to "bst artifacts checkout"?

To make the idea more concrete, I am also attaching a small example patch
showing roughly what I have in mind.

If the idea sounds acceptable, I would be happy to work on a proper
contribution.

Thanks!

diff --git a/src/buildstream/_frontend/cli.py b/src/buildstream/_frontend/cli.py
index e9e6ddfb9..2b8139ab1 100644
--- a/src/buildstream/_frontend/cli.py
+++ b/src/buildstream/_frontend/cli.py
@@ -1053,6 +1053,16 @@ def source_track(app, elements, deps, except_, cross_junctions):
 @click.option(
     "--ignore-project-source-remotes", is_flag=True, help="Ignore remote source cache servers recommended by projects"
 )
+@click.option(
+    "--use-buildtree",
+    "-t",
+    "cli_buildtree",
+    is_flag=True,
+    help=(
+        "Stage a buildtree. Will fail if a buildtree is not available. "
+        "pull-buildtrees configuration is needed if the buildtree is not available locally."
+    ),
+)
 @click.argument("element", required=False, type=click.Path(readable=False))
 @click.pass_obj
 def source_checkout(
@@ -1067,6 +1077,7 @@ def source_checkout(
     build_scripts,
     source_remotes,
     ignore_project_source_remotes,
+    cli_buildtree: bool,
 ):
     """Checkout sources of an element to the specified location
 
@@ -1103,6 +1114,7 @@ def source_checkout(
             include_build_scripts=build_scripts,
             source_remotes=source_remotes,
             ignore_project_source_remotes=ignore_project_source_remotes,
+            usebuildtree=cli_buildtree,
         )
 
 
diff --git a/src/buildstream/_stream.py b/src/buildstream/_stream.py
index a475bdb41..661f7c972 100644
--- a/src/buildstream/_stream.py
+++ b/src/buildstream/_stream.py
@@ -695,6 +695,7 @@ class Stream:
         tar: bool = False,
         artifact_remotes: Iterable[RemoteSpec] = (),
         ignore_project_artifact_remotes: bool = False,
+        usebuildtree: bool = False,
     ):
 
         elements = self._load(
@@ -728,7 +729,7 @@ class Stream:
                 _PipelineSelection.NONE: _Scope.NONE,
                 _PipelineSelection.ALL: _Scope.ALL,
             }
-            with element._prepare_sandbox(scope=scope[selection], integrate=integrate) as sandbox:
+            with element._prepare_sandbox(scope=scope[selection], integrate=integrate, usebuildtree=usebuildtree) as sandbox:
                 # Copy or move the sandbox to the target directory
                 virdir = sandbox.get_virtual_directory()
                 self._export_artifact(tar, location, compression, element, hardlinks, virdir)