tests: Add a test for the interactive failure shell

This test checks that the interactive failure shell stages the cached
build tree.

dev-requirements.txt

@@ -9,3 +9,4 @@ pytest-pep8
 pytest-pylint
 pytest-xdist
 pytest-timeout
+pexpect

tests/integration/build-tree.py

 import os
 import pytest
 import shutil
+import pexpect
 
 from tests.testutils import cli, cli_integration, create_artifact_share
 
@@ -29,6 +30,31 @@ def test_buildtree_staged(cli_integration, tmpdir, datafiles):
     res.assert_success()
 
 
+# Check that a failed build causes a shell that stages the buildtree
+@pytest.mark.datafiles(DATA_DIR)
+def test_buildtree_staged_interactive(cli_integration, tmpdir, datafiles):
+    # i.e. tests that cached build trees are staged by `bst shell --build`
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    element_name = 'build-shell/buildtree-fail.bst'
+
+    # Start an interactive session and intercept the interactive prompt
+    child = pexpect.spawn('bst', ['--no-colors', 'build', element_name], cwd=project)
+
+    # Ensure starting a shell is a valid option
+    child.expect('\(s\)hell')
+
+    # Wait for the interactive prompt
+    child.expect('Choice: \[continue\]:')
+
+    # Start a shell and wait for the shell's prompt
+    child.sendline('s')
+    child.expect('\]\$')
+
+    # Read the contents of the file we added
+    child.sendline('cat test')
+    child.expect('Hi')
+
+
 # Check that build shells work when pulled from a remote cache
 # This is to roughly simulate remote execution
 @pytest.mark.datafiles(DATA_DIR)