buildman: Use git worktrees instead of git clones when possible

This patch makes buildman create linked working trees instead of clones
of the source repository, but keeps updating the older clones of the
repository that might already exist. These worktrees share "everything
except working directory specific files such as HEAD, index, etc." with
the source repository. See the git-worktree(1) manual page for more
information.

If git-worktree isn't available, silently falls back to cloning the
repository.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>

2 files changed, 42 insertions, 8 deletions

diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index dbb75b3..c939468 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -1541,41 +1541,73 @@ class Builder:
         """Prepare the working directory for a thread.
 
         This clones or fetches the repo into the thread's work directory.
+        Optionally, it can create a linked working tree of the repo in the
+        thread's work directory instead.
 
         Args:
             thread_num: Thread number (0, 1, ...)
-            setup_git: True to set up a git repo clone
+            setup_git:
+               'clone' to set up a git clone
+               'worktree' to set up a git worktree
         """
         thread_dir = self.GetThreadDir(thread_num)
         builderthread.Mkdir(thread_dir)
         git_dir = os.path.join(thread_dir, '.git')
 
-        # Clone the repo if it doesn't already exist
-        # TODO(sjg@chromium): Perhaps some git hackery to symlink instead, so
-        # we have a private index but uses the origin repo's contents?
+        # Create a worktree or a git repo clone for this thread if it
+        # doesn't already exist
         if setup_git and self.git_dir:
             src_dir = os.path.abspath(self.git_dir)
-            if os.path.exists(git_dir):
+            if os.path.isdir(git_dir):
+                # This is a clone of the src_dir repo, we can keep using
+                # it but need to fetch from src_dir.
                 Print('\rFetching repo for thread %d' % thread_num,
                       newline=False)
                 gitutil.Fetch(git_dir, thread_dir)
                 terminal.PrintClear()
-            else:
+            elif os.path.isfile(git_dir):
+                # This is a worktree of the src_dir repo, we don't need to
+                # create it again or update it in any way.
+                pass
+            elif os.path.exists(git_dir):
+                # Don't know what could trigger this, but we probably
+                # can't create a git worktree/clone here.
+                raise ValueError('Git dir %s exists, but is not a file '
+                                 'or a directory.' % git_dir)
+            elif setup_git == 'worktree':
+                Print('\rChecking out worktree for thread %d' % thread_num,
+                      newline=False)
+                gitutil.AddWorktree(src_dir, thread_dir)
+                terminal.PrintClear()
+            elif setup_git == 'clone' or setup_git == True:
                 Print('\rCloning repo for thread %d' % thread_num,
                       newline=False)
                 gitutil.Clone(src_dir, thread_dir)
                 terminal.PrintClear()
+            else:
+                raise ValueError("Can't setup git repo with %s." % setup_git)
 
     def _PrepareWorkingSpace(self, max_threads, setup_git):
         """Prepare the working directory for use.
 
-        Set up the git repo for each thread.
+        Set up the git repo for each thread. Creates a linked working tree
+        if git-worktree is available, or clones the repo if it isn't.
 
         Args:
             max_threads: Maximum number of threads we expect to need.
-            setup_git: True to set up a git repo clone
+            setup_git: True to set up a git worktree or a git clone
         """
         builderthread.Mkdir(self._working_dir)
+        if setup_git and self.git_dir:
+            src_dir = os.path.abspath(self.git_dir)
+            if gitutil.CheckWorktreeIsAvailable(src_dir):
+                setup_git = 'worktree'
+                # If we previously added a worktree but the directory for it
+                # got deleted, we need to prune its files from the repo so
+                # that we can check out another in its place.
+                gitutil.PruneWorktrees(src_dir)
+            else:
+                setup_git = 'clone'
         for thread in range(max_threads):
             self._PrepareThread(thread, setup_git)
 
diff --git a/tools/buildman/func_test.py b/tools/buildman/func_test.py
index 418677f..3dd2e6e 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -319,6 +319,8 @@ class TestFunctional(unittest.TestCase):
             return command.CommandResult(return_code=0)
         elif sub_cmd == 'checkout':
             return command.CommandResult(return_code=0)
+        elif sub_cmd == 'worktree':
+            return command.CommandResult(return_code=0)
 
         # Not handled, so abort
         print('git', git_args, sub_cmd, args)