Revert "bot_update: Refactor branch to detach and checkout."

This reverts commit b99e61c5.

Reason for revert:
Doesn't work for refs/branch-heads/*

Original change's description:
> bot_update: Refactor branch to detach and checkout.
> 
> Refactor _git_checkout to use information from
> get_target_branch_and_revision to know what branch to checkout after
> cloning.
> 
> Bug: 1104182
> Change-Id: Ib3ba57ca0b6803f172b85121c2a4b123f17bfb8c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2303291
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: If51423df99fd9c164f8e42e0220ee2d9bc2a39f6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1104182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2310293Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -588,30 +588,54 @@ def get_total_disk_space():
     return (total, free)
 
 
-def get_target_branch_and_revision(solution_name, git_url, revisions):
-  solution_name = solution_name.strip('/')
-  configured = revisions.get(solution_name) or revisions.get(git_url)
-
-  if configured is None or COMMIT_HASH_RE.match(configured):
-    # TODO(crbug.com/1104182): Get the default branch instead of assuming
-    # 'master'.
-    branch = 'origin/master'
-    revision = configured or 'HEAD'
-    return branch, revision
-  elif ':' in configured:
-    branch, revision = configured.split(':', 1)
+def _get_target_branch_and_revision(solution_name, git_url, revisions):
+  normalized_name = solution_name.strip('/')
+  if normalized_name in revisions:
+    configured = revisions[normalized_name]
+  elif git_url in revisions:
+    configured = revisions[git_url]
   else:
-    branch = configured
-    revision = 'HEAD'
+    return 'master', 'HEAD'
+
+  parts = configured.split(':', 1)
+  if len(parts) == 2:
+    # Support for "branch:revision" syntax.
+    return parts
+  if COMMIT_HASH_RE.match(configured):
+    return 'master', configured
+  return configured, 'HEAD'
+
+
+def get_target_pin(solution_name, git_url, revisions):
+  """Returns revision to be checked out if it is pinned, else None."""
+  _, revision = _get_target_branch_and_revision(
+      solution_name, git_url, revisions)
+  if COMMIT_HASH_RE.match(revision):
+    return revision
+  return None
 
-  if branch.startswith('refs/heads/'):
-    branch = 'refs/remotes/origin/' + branch[len('refs/heads/'):]
-  elif branch.startswith('refs/branch-heads/'):
-    branch = 'refs/remotes/branch-heads/' + branch[len('refs/branch-heads/'):]
-  elif not branch.startswith(('refs/', 'origin/')):
-    branch = 'origin/' + branch
 
-  return branch, revision
+def force_solution_revision(solution_name, git_url, revisions, cwd):
+  branch, revision = _get_target_branch_and_revision(
+      solution_name, git_url, revisions)
+  if revision and revision.upper() != 'HEAD':
+    treeish = revision
+  else:
+    # TODO(machenbach): This won't work with branch-heads, as Gerrit's
+    # destination branch would be e.g. refs/branch-heads/123. But here
+    # we need to pass refs/remotes/branch-heads/123 to check out.
+    # This will also not work if somebody passes a local refspec like
+    # refs/heads/master. It needs to translate to refs/remotes/origin/master
+    # first. See also https://crbug.com/740456 .
+    if branch.startswith(('refs/', 'origin/')):
+      treeish = branch
+    else:
+      treeish = 'origin/' + branch
+
+  # Note that -- argument is necessary to ensure that git treats `treeish`
+  # argument as revision or ref, and not as a file/directory which happens to
+  # have the exact same name.
+  git('checkout', '--force', treeish, '--', cwd=cwd)
 
 
 def _has_in_git_cache(revision_sha1, refs, git_cache_dir, url):
@@ -696,10 +720,8 @@ def _git_checkout(sln, sln_dir, revisions, refs, no_fetch_tags, git_cache_dir,
         'GIT_TRACE_PERFORMANCE': 'true',
     }
 
-  branch, revision = get_target_branch_and_revision(name, url, revisions)
-  pin = revision if COMMIT_HASH_RE.match(revision) else None
-
   # Step 1: populate/refresh cache, if necessary.
+  pin = get_target_pin(name, url, revisions)
   if not pin:
     # Refresh only once.
     git(*populate_cmd, env=env)
@@ -753,10 +775,7 @@ def _git_checkout(sln, sln_dir, revisions, refs, no_fetch_tags, git_cache_dir,
         # to master in contrast with the non-clone case, which results in a
         # detached HEAD. This prevents fetching the default branch so detach the
         # HEAD after cloning.
-        # Note that the '--' argument is needed to ensure that git treats
-        # 'pin or branch' as revision or ref, and not as file/directory which
-        # happens to have the exact same name.
-        git('checkout', pin or branch, '--detach', '--', cwd=sln_dir)
+        git('checkout', 'HEAD', '--detach', cwd=sln_dir)
         _git_disable_gc(sln_dir)
       else:
         _git_disable_gc(sln_dir)
@@ -773,10 +792,7 @@ def _git_checkout(sln, sln_dir, revisions, refs, no_fetch_tags, git_cache_dir,
       if sys.platform.startswith('win'):
         _maybe_break_locks(sln_dir, tries=3)
 
-      # Note that the '--' argument is needed to ensure that git treats
-      # 'pin or branch' as revision or ref, and not as file/directory which
-      # happens to have the exact same name.
-      git('checkout', '--force', pin or branch, '--', cwd=sln_dir)
+      force_solution_revision(name, url, revisions, sln_dir)
       git('clean', '-dff', cwd=sln_dir)
       return
     except SubprocessFailed as e: