Add pip source plugin

`pip` source plugin can stage python packages that are either specified
directly in the element definition or picked up from `requirements.txt`
from previous sources. In order to support the latter use-case
(which is also the primary motivation for this plugin), this plugin
requires access to previous sources and hence is an example of a
Source Transform source.

Also, bump `BST_FORMAT_VERSION` as this patch adds a new core plugin.

6 files changed, 69 insertions, 0 deletions

diff --git a/tests/sources/pip.py b/tests/sources/pip.py
new file mode 100644
index 000000000..8b4c213dc
--- /dev/null
+++ b/tests/sources/pip.py
@@ -0,0 +1,47 @@
+import os
+import pytest
+
+from buildstream._exceptions import ErrorDomain
+from buildstream import _yaml
+from tests.testutils import cli
+
+DATA_DIR = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)),
+    'pip',
+)
+
+
+def generate_project(project_dir, tmpdir):
+    project_file = os.path.join(project_dir, "project.conf")
+    _yaml.dump({'name': 'foo'}, project_file)
+
+
+# Test that without ref, consistency is set appropriately.
+@pytest.mark.datafiles(os.path.join(DATA_DIR, 'no-ref'))
+def test_no_ref(cli, tmpdir, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    generate_project(project, tmpdir)
+    assert cli.get_element_state(project, 'target.bst') == 'no reference'
+
+
+# Test that pip is not allowed to be the first source
+@pytest.mark.datafiles(os.path.join(DATA_DIR, 'first-source-pip'))
+def test_first_source(cli, tmpdir, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    generate_project(project, tmpdir)
+    result = cli.run(project=project, args=[
+        'show', 'target.bst'
+    ])
+    result.assert_main_error(ErrorDomain.ELEMENT, None)
+
+
+# Test that error is raised when neither packges nor requirements files
+# have been specified
+@pytest.mark.datafiles(os.path.join(DATA_DIR, 'no-packages'))
+def test_no_packages(cli, tmpdir, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    generate_project(project, tmpdir)
+    result = cli.run(project=project, args=[
+        'show', 'target.bst'
+    ])
+    result.assert_main_error(ErrorDomain.SOURCE, None)
diff --git a/tests/sources/pip/first-source-pip/target.bst b/tests/sources/pip/first-source-pip/target.bst
new file mode 100644
index 000000000..e5f20ab0b
--- /dev/null
+++ b/tests/sources/pip/first-source-pip/target.bst
@@ -0,0 +1,6 @@
+kind: import
+description: pip should not be allowed to be the first source
+sources:
+- kind: pip
+  packages:
+  - flake8
diff --git a/tests/sources/pip/no-packages/file b/tests/sources/pip/no-packages/file
new file mode 100644
index 000000000..980a0d5f1
--- /dev/null
+++ b/tests/sources/pip/no-packages/file
@@ -0,0 +1 @@
+Hello World!
diff --git a/tests/sources/pip/no-packages/target.bst b/tests/sources/pip/no-packages/target.bst
new file mode 100644
index 000000000..0d8b948c4
--- /dev/null
+++ b/tests/sources/pip/no-packages/target.bst
@@ -0,0 +1,6 @@
+kind: import
+description: The kind of this element is irrelevant.
+sources:
+- kind: local
+  path: file
+- kind: pip
diff --git a/tests/sources/pip/no-ref/file b/tests/sources/pip/no-ref/file
new file mode 100644
index 000000000..980a0d5f1
--- /dev/null
+++ b/tests/sources/pip/no-ref/file
@@ -0,0 +1 @@
+Hello World!
diff --git a/tests/sources/pip/no-ref/target.bst b/tests/sources/pip/no-ref/target.bst
new file mode 100644
index 000000000..ec450b7ef
--- /dev/null
+++ b/tests/sources/pip/no-ref/target.bst
@@ -0,0 +1,8 @@
+kind: import
+description: The kind of this element is irrelevant.
+sources:
+- kind: local
+  path: file
+- kind: pip
+  packages:
+  - flake8