vcs.git: make sure to always be in detached HEAD

closes issue #2037

pip/vcs/git.py

@@ -93,7 +93,8 @@ def check_version(self, dest, rev_options):
 
     def switch(self, dest, url, rev_options):
         self.run_command(['config', 'remote.origin.url', url], cwd=dest)
-        self.run_command(['checkout', '-q'] + rev_options, cwd=dest)
+        self.run_command(
+            ['checkout', '-q', '--detach'] + rev_options, cwd=dest)
 
         self.update_submodules(dest)
 
@@ -111,12 +112,11 @@ def update(self, dest, rev_options):
 
     def obtain(self, dest):
         url, rev = self.get_url_rev()
-        if rev:
-            rev_options = [rev]
-            rev_display = ' (to %s)' % rev
-        else:
-            rev_options = ['origin/master']
-            rev_display = ''
+        if not rev:
+            rev = 'origin/master'
+        rev_options = [rev]
+        rev_display = ' (to %s)' % rev
+
         if self.check_destination(dest, url, rev_options, rev_display):
             logger.info(
                 'Cloning %s%s to %s', url, rev_display, display_path(dest),
@@ -128,7 +128,7 @@ def obtain(self, dest):
                 # Only do a checkout if rev_options differs from HEAD
                 if not self.check_version(dest, rev_options):
                     self.run_command(
-                        ['checkout', '-q'] + rev_options,
+                        ['checkout', '-q', '--detach'] + rev_options,
                         cwd=dest,
                     )
             #: repo may contain submodules

tests/functional/test_install_vcs.py

@@ -144,8 +144,10 @@ def test_git_branch_should_not_be_changed(script, tmpdir):
         expect_error=True,
     )
     source_dir = script.venv_path / 'src' / 'pip-test-package'
-    result = script.run('git', 'branch', cwd=source_dir)
-    assert '* master' in result.stdout, result.stdout
+    # Check we are on master
+    head = script.run('git', 'rev-parse', 'HEAD', cwd=source_dir).stdout
+    master = script.run('git', 'rev-parse', 'master', cwd=source_dir).stdout
+    assert head == master
 
 
 @pytest.mark.network

tests/functional/test_install_vcs_git.py

@@ -133,3 +133,18 @@ def test_check_submodule_addition(script):
         script.venv / 'src/version-pkg/testpkg/static/testfile2'
         in update_result.files_created
     )
+
+
+def test_install_git_detached(script):
+    version_pkg_path = _create_test_package(script)
+    script.pip(
+        'install', '-e',
+        'git+file://' + version_pkg_path + '@master#egg=version_pkg'
+    )
+    source_dir = script.venv_path / 'src' / 'version-pkg'
+    branch = script.run(
+        'git', 'status', '--porcelain', '--branch',
+        cwd=source_dir
+    ).stdout.strip()
+    # Check we are in detached HEAD
+    assert 'HEAD' in branch