[Update Checkout] Fall back to revision if branch checkout fails.

In workflows featuring git worktrees, it is common for the same branch
to be in use by multiple multiple checkouts.  For example, at the
moment, the "master" branches of swift-format, swift-tensorflow-apis,
and pythonkit (and the "release" branch of ninja) are indicated, in
update-checkout-config.json, by both "master" and "release/5.3".  If one
has a workflow featuring git worktrees, that means that when one runs
<<update_checkout --scheme master>> in one's mainline directory and
<<update_checkout --scheme release/5.3>> in one's release/5.3 directory,
the latter will encounter failures for each of those four projects
because the branch "master" will already be checked out in the mainline
directory's worktrees and so it cannot be checked out in the
release/5.3's directory's worktrees.  The error looks something like:

    /path/to/swift-container/release53/swift-format failed
    (ret=128): ['git', 'checkout', u'master']
    fatal: 'master' is already checked out at
    '/path/to/swift-container/mainline/swift-format'

    /path/to/swift-container/release53/tensorflow-swift-apis failed
    (ret=128): ['git', 'checkout', u'master']
    fatal: 'master' is already checked out at
    '/path/to/swift-container/mainline/tensorflow-swift-apis'

    /path/to/swift-container/release53/pythonkit failed (ret=128):
    ['git', 'checkout', u'master']
    fatal: 'master' is already checked out at
    '/path/to/swift-container/mainline/pythonkit'

    /path/to/swift-container/release53/ninja failed (ret=128):
    ['git', 'checkout', u'release']
    fatal: 'release' is already checked out at
    '/path/to/swift-container/mainline/ninja'

Here, that workflow is enabled.  If <<git checkout branch_name>> fails,
for one of the projects, update_checkout falls back to getting the SHA
for the indicated branch via <<git rev-parse branch_name>> and then
checking out the SHA directly.

Author: nate-chandler

utils/update_checkout/tests/scheme_mock.py

@@ -119,6 +119,8 @@ def setup_mock_remote(base_dir):
     with open(get_config_path(base_dir), 'w') as f:
         json.dump(base_config, f)
 
+    return (LOCAL_PATH, REMOTE_PATH)
+
 
 BASEDIR_ENV_VAR = 'UPDATECHECKOUT_TEST_WORKSPACE_DIR'
 CURRENT_FILE_DIR = os.path.dirname(os.path.abspath(__file__))
@@ -147,7 +149,7 @@ def __init__(self, *args, **kwargs):
 
     def setUp(self):
         create_dir(self.source_root)
-        setup_mock_remote(self.workspace)
+        (self.local_path, self.remote_path) = setup_mock_remote(self.workspace)
 
     def tearDown(self):
         teardown_mock_remote(self.workspace)

utils/update_checkout/tests/test_update_worktree.py

@@ -0,0 +1,65 @@
+# ===--- test_clone.py ----------------------------------------------------===#
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https:#swift.org/LICENSE.txt for license information
+# See https:#swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+#
+# ===----------------------------------------------------------------------===#
+
+import os
+
+from scheme_mock import call_quietly
+
+from . import scheme_mock
+
+WORKTREE_NAME = "feature"
+
+
+def path_for_worktree(workspace_path, worktree_name):
+    return os.path.join(workspace_path, worktree_name)
+
+
+def setup_worktree(workspace_path, local_path, worktree_name):
+    worktree_path = path_for_worktree(workspace_path, worktree_name)
+    os.makedirs(worktree_path)
+    for project in os.listdir(local_path):
+        local_project_path = os.path.join(local_path, project)
+        worktree_project_path = os.path.join(worktree_path, project)
+        call_quietly(['git', 
+                      '-C', local_project_path, 
+                      'worktree', 'add', worktree_project_path])
+
+
+def teardown_worktree(workspace_path, local_path, worktree_name):
+    worktree_path = path_for_worktree(workspace_path, worktree_name)
+    for project in os.listdir(local_path):
+        local_project_path = os.path.join(local_path, project)
+        worktree_project_path = os.path.join(worktree_path, project)
+        call_quietly(['git', 
+                      '-C', local_project_path, 
+                      'worktree', 'remove', worktree_project_path])
+
+
+class WorktreeTestCase(scheme_mock.SchemeMockTestCase):
+
+    def __init__(self, *args, **kwargs):
+        super(WorktreeTestCase, self).__init__(*args, **kwargs)
+
+    def test_worktree(self):
+        self.call([self.update_checkout_path,
+                   '--config', self.config_path,
+                   '--source-root', self.worktree_path,
+                   '--scheme', 'master'])
+
+    def setUp(self):
+        super(WorktreeTestCase, self).setUp()
+        self.worktree_path = os.path.join(self.workspace, WORKTREE_NAME)
+        setup_worktree(self.workspace, self.local_path, WORKTREE_NAME)
+
+    def tearDown(self):
+        teardown_worktree(self.workspace, self.local_path, WORKTREE_NAME)
+        super(WorktreeTestCase, self).tearDown()

utils/update_checkout/update_checkout/update_checkout.py

@@ -158,7 +158,15 @@ def update_single_repository(pool_args):
             if checkout_target:
                 shell.run(['git', 'status', '--porcelain', '-uno'],
                           echo=False)
-                shell.run(['git', 'checkout', checkout_target], echo=True)
+                try:
+                    shell.run(['git', 'checkout', checkout_target], echo=True)
+                except Exception as originalException:
+                    try:
+                        result = shell.run(['git', 'rev-parse', checkout_target])
+                        revision = result[0].strip()
+                        shell.run(['git', 'checkout', revision], echo=True)
+                    except Exception as e:
+                        raise originalException
 
             # It's important that we checkout, fetch, and rebase, in order.
             # .git/FETCH_HEAD updates the not-for-merge attributes based on