Merge branch 'mh/ref-remove-empty-directory'

Deletion of a branch "foo/bar" could remove .git/refs/heads/foo
once there no longer is any other branch whose name begins with
"foo/", but we didn't do so so far.  Now we do.

* mh/ref-remove-empty-directory: (23 commits)
  files_transaction_commit(): clean up empty directories
  try_remove_empty_parents(): teach to remove parents of reflogs, too
  try_remove_empty_parents(): don't trash argument contents
  try_remove_empty_parents(): rename parameter "name" -> "refname"
  delete_ref_loose(): inline function
  delete_ref_loose(): derive loose reference path from lock
  log_ref_write_1(): inline function
  log_ref_setup(): manage the name of the reflog file internally
  log_ref_write_1(): don't depend on logfile argument
  log_ref_setup(): pass the open file descriptor back to the caller
  log_ref_setup(): improve robustness against races
  log_ref_setup(): separate code for create vs non-create
  log_ref_write(): inline function
  rename_tmp_log(): improve error reporting
  rename_tmp_log(): use raceproof_create_file()
  lock_ref_sha1_basic(): use raceproof_create_file()
  lock_ref_sha1_basic(): inline constant
  raceproof_create_file(): new function
  safe_create_leading_directories(): set errno on SCLD_EXISTS
  safe_create_leading_directories_const(): preserve errno
  ...

cache.h

@@ -1072,8 +1072,9 @@ int adjust_shared_perm(const char *path);
 
 /*
  * Create the directory containing the named path, using care to be
- * somewhat safe against races.  Return one of the scld_error values
- * to indicate success/failure.
+ * somewhat safe against races. Return one of the scld_error values to
+ * indicate success/failure. On error, set errno to describe the
+ * problem.
  *
  * SCLD_VANISHED indicates that one of the ancestor directories of the
  * path existed at one point during the function call and then
@@ -1097,6 +1098,49 @@ enum scld_error {
 enum scld_error safe_create_leading_directories(char *path);
 enum scld_error safe_create_leading_directories_const(const char *path);
 
+/*
+ * Callback function for raceproof_create_file(). This function is
+ * expected to do something that makes dirname(path) permanent despite
+ * the fact that other processes might be cleaning up empty
+ * directories at the same time. Usually it will create a file named
+ * path, but alternatively it could create another file in that
+ * directory, or even chdir() into that directory. The function should
+ * return 0 if the action was completed successfully. On error, it
+ * should return a nonzero result and set errno.
+ * raceproof_create_file() treats two errno values specially:
+ *
+ * - ENOENT -- dirname(path) does not exist. In this case,
+ *             raceproof_create_file() tries creating dirname(path)
+ *             (and any parent directories, if necessary) and calls
+ *             the function again.
+ *
+ * - EISDIR -- the file already exists and is a directory. In this
+ *             case, raceproof_create_file() removes the directory if
+ *             it is empty (and recursively any empty directories that
+ *             it contains) and calls the function again.
+ *
+ * Any other errno causes raceproof_create_file() to fail with the
+ * callback's return value and errno.
+ *
+ * Obviously, this function should be OK with being called again if it
+ * fails with ENOENT or EISDIR. In other scenarios it will not be
+ * called again.
+ */
+typedef int create_file_fn(const char *path, void *cb);
+
+/*
+ * Create a file in dirname(path) by calling fn, creating leading
+ * directories if necessary. Retry a few times in case we are racing
+ * with another process that is trying to clean up the directory that
+ * contains path. See the documentation for create_file_fn for more
+ * details.
+ *
+ * Return the value and set the errno that resulted from the most
+ * recent call of fn. fn is always called at least once, and will be
+ * called more than once if it returns ENOENT or EISDIR.
+ */
+int raceproof_create_file(const char *path, create_file_fn fn, void *cb);
+
 int mkdir_in_gitdir(const char *path);
 extern char *expand_user_path(const char *path);
 const char *enter_repo(const char *path, int strict);