clean: optimize and document cases where we recurse into subdirectories

Commit 6b1db43109 ("clean: teach clean -d to preserve ignored paths",
2017-05-23) added the following code block (among others) to git-clean:
    if (remove_directories)
        dir.flags |= DIR_SHOW_IGNORED_TOO | DIR_KEEP_UNTRACKED_CONTENTS;
The reason for these flags is well documented in the commit message, but
isn't obvious just from looking at the code.  Add some explanations to
the code to make it clearer.

Further, it appears git-2.26 did not correctly handle this combination
of flags from git-clean.  With both these flags and without
DIR_SHOW_IGNORED_TOO_MODE_MATCHING set, git is supposed to recurse into
all untracked AND ignored directories.  git-2.26.0 clearly was not doing
that.  I don't know the full reasons for that or whether git < 2.27.0
had additional unknown bugs because of that misbehavior, because I don't
feel it's worth digging into.  As per the huge changes and craziness
documented in commit 8d92fb2927 ("dir: replace exponential algorithm
with a linear one", 2020-04-01), the old algorithm was a mess and was
thrown out.  What I can say is that git-2.27.0 correctly recurses into
untracked AND ignored directories with that combination.

However, in clean's case we don't need to recurse into ignored
directories; that is just a waste of time.  Thus, when git-2.27.0
started correctly handling those flags, we got a performance regression
report.  Rather than relying on other bugs in fill_directory()'s former
logic to provide the behavior of skipping ignored directories, make use
of the DIR_SHOW_IGNORED_TOO_MODE_MATCHING value specifically added in
commit eec0f7f2b7 ("status: add option to show ignored files
differently", 2017-10-30) for this purpose.

Reported-by: Brian Malehorn <bmalehorn@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
1 file changed
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README.md

Build status

Git - fast, scalable, distributed revision control system

Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.

Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.

Please read the file INSTALL for installation instructions.

Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.

See Documentation/gittutorial.txt to get started, then see Documentation/giteveryday.txt for a useful minimum set of commands, and Documentation/git-<commandname>.txt for documentation of each command. If git has been correctly installed, then the tutorial can also be read with man gittutorial or git help tutorial, and the documentation of each command with man git-<commandname> or git help <commandname>.

CVS users may also want to read Documentation/gitcvs-migration.txt (man gitcvs-migration or git help cvs-migration if git is installed).

The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just “subscribe git” in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.

Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.

The maintainer frequently sends the “What's cooking” reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.

The name “git” was given by Linus Torvalds when he wrote the very first version. He described the tool as “the stupid content tracker” and the name as (depending on your mood):

  • random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of “get” may or may not be relevant.
  • stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
  • “global information tracker”: you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
  • “goddamn idiotic truckload of sh*t”: when it breaks