Add `--deps build` option to `bst checkout`

As discussed in
https://mail.gnome.org/archives/buildstream-list/2018-September/msg00064.html,
add `--deps build` option to `bst checkout`. This will allow users to
checkout the all build dependencies of a given element using a single
command.

- _frontend/cli.py: Add `--deps build` option for `bst checkout`.

- element.py: Support `deps='build'` in Element._prepare_sandbox().

- tests/frontend/buildcheckout.py: Ensure `--deps build` works as
  expected.

3 files changed, 17 insertions, 4 deletions

diff --git a/buildstream/_frontend/cli.py b/buildstream/_frontend/cli.py
index b845d00eb..59c917323 100644
--- a/buildstream/_frontend/cli.py
+++ b/buildstream/_frontend/cli.py
@@ -640,7 +640,7 @@ def shell(app, element, sysroot, mount, isolate, build_, command):
 @click.option('--force', '-f', default=False, is_flag=True,
               help="Allow files to be overwritten")
 @click.option('--deps', '-d', default='run',
-              type=click.Choice(['run', 'none']),
+              type=click.Choice(['run', 'build', 'none']),
               help='The dependencies to checkout (default: run)')
 @click.option('--integrate/--no-integrate', default=True, is_flag=True,
               help="Whether to run integration commands")
diff --git a/buildstream/element.py b/buildstream/element.py
index d94f2e8ad..2caac8d3c 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1353,7 +1353,10 @@ class Element(Plugin):
                 if scope == Scope.BUILD:
                     self.stage(sandbox)
                 elif scope == Scope.RUN:
-                    if deps == 'run':
+
+                    if deps == 'build':
+                        dependency_scope = Scope.BUILD
+                    elif deps == 'run':
                         dependency_scope = Scope.RUN
                     else:
                         dependency_scope = Scope.NONE
diff --git a/tests/frontend/buildcheckout.py b/tests/frontend/buildcheckout.py
index d43bac863..03bba0b26 100644
--- a/tests/frontend/buildcheckout.py
+++ b/tests/frontend/buildcheckout.py
@@ -86,7 +86,7 @@ def test_build_invalid_suffix_dep(datafiles, cli, strict, hardlinks):
 
 
 @pytest.mark.datafiles(DATA_DIR)
-@pytest.mark.parametrize("deps", [("run"), ("none")])
+@pytest.mark.parametrize("deps", [("run"), ("none"), ("build")])
 def test_build_checkout_deps(datafiles, cli, deps):
     project = os.path.join(datafiles.dirname, datafiles.basename)
     checkout = os.path.join(cli.directory, 'checkout')
@@ -107,7 +107,17 @@ def test_build_checkout_deps(datafiles, cli, deps):
 
     # Verify output of this element
     filename = os.path.join(checkout, 'etc', 'buildstream', 'config')
-    assert os.path.exists(filename)
+    if deps == "build":
+        assert not os.path.exists(filename)
+    else:
+        assert os.path.exists(filename)
+
+    # Verify output of this element's build dependencies
+    filename = os.path.join(checkout, 'usr', 'include', 'pony.h')
+    if deps == "build":
+        assert os.path.exists(filename)
+    else:
+        assert not os.path.exists(filename)
 
     # Verify output of this element's runtime dependencies
     filename = os.path.join(checkout, 'usr', 'bin', 'hello')