Fix git-related commands not working in worktrees

If spack is checked out in a git worktree (see [1]), all git-related
commands fail because the `spack_is_git_repo()`-check is not thorough
enough.

When developing in a feature-branch in a seperate worktree, this is
annoying as all unittests regarding git-related spack commands fail,
cluttering the test results with false-positives.

[1]: https://git-scm.com/docs/git-worktree

Change-Id: I94b573a2c0e058e9ccc169e7ee6561626fbb06fd

2 files changed, 87 insertions, 3 deletions

diff --git a/lib/spack/spack/cmd/__init__.py b/lib/spack/spack/cmd/__init__.py
index 594b5e3a8e..5172bdee07 100644
--- a/lib/spack/spack/cmd/__init__.py
+++ b/lib/spack/spack/cmd/__init__.py
@@ -9,6 +9,7 @@ import os
 import re
 import sys
 import argparse
+import ruamel.yaml as yaml
 
 import six
 
@@ -16,7 +17,7 @@ import llnl.util.tty as tty
 from llnl.util.lang import attr_setdefault, index_by
 from llnl.util.tty.colify import colify
 from llnl.util.tty.color import colorize
-from llnl.util.filesystem import working_dir
+from llnl.util.filesystem import join_path
 
 import spack.config
 import spack.error
@@ -26,6 +27,7 @@ import spack.spec
 import spack.store
 import spack.util.spack_json as sjson
 import spack.util.string
+from ruamel.yaml.error import MarkedYAMLError
 
 
 # cmd has a submodule called "list" so preserve the python list module
@@ -433,8 +435,23 @@ def display_specs(specs, args=None, **kwargs):
 
 def spack_is_git_repo():
     """Ensure that this instance of Spack is a git clone."""
-    with working_dir(spack.paths.prefix):
-        return os.path.isdir('.git')
+    return is_git_repo(spack.paths.prefix)
+
+
+def is_git_repo(path):
+    dotgit_path = join_path(path, '.git')
+    if os.path.isdir(dotgit_path):
+        # we are in a regular git repo
+        return True
+    if os.path.isfile(dotgit_path):
+        # we might be in a git worktree
+        try:
+            with open(dotgit_path, "rb") as f:
+                dotgit_content = yaml.load(f)
+            return os.path.isdir(dotgit_content.get("gitdir", dotgit_path))
+        except MarkedYAMLError:
+            pass
+    return False
 
 
 class PythonNameError(spack.error.SpackError):
diff --git a/lib/spack/spack/test/cmd/is_git_repo.py b/lib/spack/spack/test/cmd/is_git_repo.py
new file mode 100644
index 0000000000..724925e5e4
--- /dev/null
+++ b/lib/spack/spack/test/cmd/is_git_repo.py
@@ -0,0 +1,67 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from __future__ import print_function
+
+import spack
+import pytest
+
+from llnl.util.filesystem import mkdirp
+
+from spack.util.executable import which
+from spack.version import ver
+
+
+git = which("git")
+git_required_version = '2.17.0'
+
+
+def check_git_version():
+    """Check if git version is new enough for worktree functionality.
+    Return True if requirements are met.
+
+    The latest required functionality is `worktree remove` which was only added
+    in 2.17.0.
+
+    Refer:
+    https://github.com/git/git/commit/cc73385cf6c5c229458775bc92e7dbbe24d11611
+    """
+    git_version = ver(git('--version', output=str).lstrip('git version '))
+    return git_version >= ver(git_required_version)
+
+
+pytestmark = pytest.mark.skipif(
+    not git or not check_git_version(),
+    reason="we need git to test if we are in a git repo"
+)
+
+
+@pytest.fixture(scope="function")
+def git_tmp_worktree(tmpdir):
+    """Create new worktree in a temporary folder and monkeypatch
+    spack.paths.prefix to point to it.
+    """
+    worktree_root = str(tmpdir.join("tmp_worktree"))
+    mkdirp(worktree_root)
+
+    git("worktree", "add", "--detach", worktree_root, "HEAD")
+
+    yield worktree_root
+
+    git("worktree", "remove", "--force", worktree_root)
+
+
+def test_is_git_repo_in_worktree(git_tmp_worktree):
+    """Verify that spack.cmd.spack_is_git_repo() can identify a git repository
+    in a worktree.
+    """
+    assert spack.cmd.is_git_repo(git_tmp_worktree)
+
+
+def test_spack_is_git_repo_nongit(tmpdir, monkeypatch):
+    """Verify that spack.cmd.spack_is_git_repo() correctly returns False if we
+    are in a non-git directory.
+    """
+    assert not spack.cmd.is_git_repo(str(tmpdir))