[PATCH v4 6/8] dir: avoid unnecessary traversal into ignored directory

From: Elijah Newren <newren@gmail.com>

The show_other_directories case in treat_directory() tried to handle both excludes and untracked files with the same logic, and mishandled both the excludes and the untracked files in the process, in different ways. Split that logic apart, and then focus on the logic for the excludes; a subsequent commit will address the logic for untracked files.

For show_other_directories, an excluded directory means that every path underneath that directory will also be excluded. Given that the calling code requested to just show directories when everything under a directory had the same state (that's what the "DIR_SHOW_OTHER_DIRECTORIES" flag means), we generally do not need to traverse into such directories and can just immediately mark them as ignored (i.e. as path_excluded). The only reason we cannot just immediately return path_excluded is the DIR_HIDE_EMPTY_DIRECTORIES flag and the possibility that the ignored directory is an empty directory. The code previously treated DIR_SHOW_IGNORED_TOO in most cases as an exception as well, which was wrong. It can sometimes reduce the number of cases where we need to recurse (namely if DIR_SHOW_IGNORED_TOO_MODE_MATCHING is also set), but should not be able to increase the number of cases where we need to recurse. Fix the logic accordingly.

Some sidenotes about possible confusion with dir.c:

* "ignored" often refers to an untracked ignore", i.e. a file which is
  not tracked which matches one of the ignore/exclusion rules.  But you
  can also have a "tracked ignore", a tracked file that happens to match
  one of the ignore/exclusion rules and which dir.c has to worry about
  since "git ls-files -c -i" is supposed to list them.

* The dir code often uses "ignored" and "excluded" interchangeably,
  which you need to keep in mind while reading the code.

* "exclude" is used multiple ways in the code:

  * As noted above, "exclude" is often a synonym for "ignored".

  * The logic for parsing .gitignore files was re-used in
    .git/info/sparse-checkout, except there it is used to mark paths that
    the user wants to *keep*.  This was mostly addressed by commit
    65edd96aec ("treewide: rename 'exclude' methods to 'pattern'",
    2019-09-03), but every once in a while you'll find a comment about
    "exclude" referring to these patterns that might in fact be in use
    by the sparse-checkout machinery for inclusion rules.

  * The word "EXCLUDE" is also used for pathspec negation, as in
      (pathspec->items[3].magic & PATHSPEC_EXCLUDE)
    Thus if a user had a .gitignore file containing
      *~
      *.log
      !settings.log
    And then ran
      git add -- 'settings.*' ':^settings.log'
    Then :^settings.log is a pathspec negation making settings.log not
    be requested to be added even though all other settings.* files are
    being added.  Also, !settings.log in the gitignore file is a negative
    exclude pattern meaning that settings.log is normally a file we
    want to track even though all other *.log files are ignored.

Sometimes it feels like dir.c needs its own glossary with its many definitions, including the multiply-defined terms.

Reported-by: Jason Gore <Jason.Gore@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
 dir.c            | 44 +++++++++++++++++++++++++++++---------------
 t/t7300-clean.sh |  2 +-
 2 files changed, 30 insertions(+), 16 deletions(-)