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dir: replace exponential algorithm with a linear one

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dir's read_directory_recursive() naturally operates recursively in order
to walk the directory tree.  Treating of directories is sometimes weird
because there are so many different permutations about how to handle
directories.  Some examples:

   * 'git ls-files -o --directory' only needs to know that a directory
     itself is untracked; it doesn't need to recurse into it to see what
     is underneath.

   * 'git status' needs to recurse into an untracked directory, but only
     to determine whether or not it is empty.  If there are no files
     underneath, the directory itself will be omitted from the output.
     If it is not empty, only the directory will be listed.

   * 'git status --ignored' needs to recurse into untracked directories
     and report all the ignored entries and then report the directory as
     untracked -- UNLESS all the entries under the directory are
     ignored, in which case we don't print any of the entries under the
     directory and just report the directory itself as ignored.  (Note
     that although this forces us to walk all untracked files underneath
     the directory as well, we strip them from the output, except for
     users like 'git clean' who also set DIR_KEEP_TRACKED_CONTENTS.)

   * For 'git clean', we may need to recurse into a directory that
     doesn't match any specified pathspecs, if it's possible that there
     is an entry underneath the directory that can match one of the
     pathspecs.  In such a case, we need to be careful to omit the
     directory itself from the list of paths (see commit 404ebceda0
     ("dir: also check directories for matching pathspecs", 2019-09-17))

Part of the tension noted above is that the treatment of a directory can
change based on the files within it, and based on the various settings
in dir->flags.  Trying to keep this in mind while reading over the code,
it is easy to think in terms of "treat_directory() tells us what to do
with a directory, and read_directory_recursive() is the thing that
recurses".  Since we need to look into a directory to know how to treat
it, though, it is quite easy to decide to (also) recurse into the
directory from treat_directory() by adding a read_directory_recursive()
call.  Adding such a call is actually fine, IF we make sure that
read_directory_recursive() does not also recurse into that same
directory.

Unfortunately, commit df5bcdf83a ("dir: recurse into untracked dirs
for ignored files", 2017-05-18), added exactly such a case to the code,
meaning we'd have two calls to read_directory_recursive() for an
untracked directory.  So, if we had a file named
   one/two/three/four/five/somefile.txt
and nothing in one/ was tracked, then 'git status --ignored' would
call read_directory_recursive() twice on the directory 'one/', and
each of those would call read_directory_recursive() twice on the
directory 'one/two/', and so on until read_directory_recursive() was
called 2^5 times for 'one/two/three/four/five/'.

Avoid calling read_directory_recursive() twice per level by moving a
lot of the special logic into treat_directory().

Since dir.c is somewhat complex, extra cruft built up around this over
time.  While trying to unravel it, I noticed several instances where the
first call to read_directory_recursive() would return e.g.
path_untracked for some directory and a later one would return e.g.
path_none, despite the fact that the directory clearly should have been
considered untracked.  The code happened to work due to the side-effect
from the first invocation of adding untracked entries to dir->entries;
this allowed it to get the correct output despite the supposed override
in return value by the later call.

I am somewhat concerned that there are still bugs and maybe even
testcases with the wrong expectation.  I have tried to carefully
document treat_directory() since it becomes more complex after this
change (though much of this complexity came from elsewhere that probably
deserved better comments to begin with).  However, much of my work felt
more like a game of whackamole while attempting to make the code match
the existing regression tests than an attempt to create an
implementation that matched some clear design.  That seems wrong to me,
but the rules of existing behavior had so many special cases that I had
a hard time coming up with some overarching rules about what correct
behavior is for all cases, forcing me to hope that the regression tests
are correct and sufficient.  Such a hope seems likely to be ill-founded,
given my experience with dir.c-related testcases in the last few months:

  Examples where the documentation was hard to parse or even just wrong:
   * 3aca58045f (git-clean.txt: do not claim we will delete files with
                   -n/--dry-run, 2019-09-17)
   * 09487f2cba (clean: avoid removing untracked files in a nested git
                   repository, 2019-09-17)
   * e86bbcf987 (clean: disambiguate the definition of -d, 2019-09-17)
  Examples where testcases were declared wrong and changed:
   * 09487f2cba (clean: avoid removing untracked files in a nested git
                   repository, 2019-09-17)
   * e86bbcf987 (clean: disambiguate the definition of -d, 2019-09-17)
   * a2b13367fe (Revert "dir.c: make 'git-status --ignored' work within
                   leading directories", 2019-12-10)
  Examples where testcases were clearly inadequate:
   * 502c386ff9 (t7300-clean: demonstrate deleting nested repo with an
                   ignored file breakage, 2019-08-25)
   * 7541cc5302 (t7300: add testcases showing failure to clean specified
                   pathspecs, 2019-09-17)
   * a5e916c745 (dir: fix off-by-one error in match_pathspec_item,
                   2019-09-17)
   * 404ebceda0 (dir: also check directories for matching pathspecs,
                   2019-09-17)
   * 09487f2cba (clean: avoid removing untracked files in a nested git
                   repository, 2019-09-17)
   * e86bbcf987 (clean: disambiguate the definition of -d, 2019-09-17)
   * 452efd11fb (t3011: demonstrate directory traversal failures,
                   2019-12-10)
   * b9670c1f5e (dir: fix checks on common prefix directory, 2019-12-19)
  Examples where "correct behavior" was unclear to everyone:
    https://lore.kernel.org/git/20190905154735.29784-1-newren@gmail.com/
  Other commits of note:
   * 902b90cf42 (clean: fix theoretical path corruption, 2019-09-17)

However, on the positive side, it does make the code much faster.  For
the following simple shell loop in an empty repository:

  for depth in $(seq 10 25)
  do
    dirs=$(for i in $(seq 1 $depth) ; do printf 'dir/' ; done)
    rm -rf dir
    mkdir -p $dirs
    >$dirs/untracked-file
    /usr/bin/time --format="$depth: %e" git status --ignored >/dev/null
  done

I saw the following timings, in seconds (note that the numbers are a
little noisy from run-to-run, but the trend is very clear with every
run):

    10: 0.03
    11: 0.05
    12: 0.08
    13: 0.19
    14: 0.29
    15: 0.50
    16: 1.05
    17: 2.11
    18: 4.11
    19: 8.60
    20: 17.55
    21: 33.87
    22: 68.71
    23: 140.05
    24: 274.45
    25: 551.15

For the above run, using strace I can look for the number of untracked
directories opened and can verify that it matches the expected
2^($depth+1)-2 (the sum of 2^1 + 2^2 + 2^3 + ... + 2^$depth).

After this fix, with strace I can verify that the number of untracked
directories that are opened drops to just $depth, and the timings all
drop to 0.00.  In fact, it isn't until a depth of 190 nested directories
that it sometimes starts reporting a time of 0.01 seconds and doesn't
consistently report 0.01 seconds until there are 240 nested directories.
The previous code would have taken
  17.55 * 2^220 / (60*60*24*365) = 9.4 * 10^59 YEARS
to have completed the 240 nested directories case.  It's not often
that you get to speed something up by a factor of 3*10^69.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Elijah Newren 2020-04-01 04:17:42 +00:00 committed by Junio C Hamano
parent 0bbd0e8b52
commit 8d92fb2927
1 changed files with 151 additions and 67 deletions
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218
dir.c
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@ -1659,7 +1659,13 @@ static enum path_treatment treat_directory(struct dir_struct *dir,
const char *dirname, int len, int baselen, int excluded,
const struct pathspec *pathspec)
{
int nested_repo = 0;
/*
* WARNING: From this function, you can return path_recurse or you
* can call read_directory_recursive() (or neither), but
* you CAN'T DO BOTH.
*/
enum path_treatment state;
int nested_repo = 0, old_ignored_nr, check_only, stop_early;
/* The "len-1" is to strip the final '/' */
enum exist_status status = directory_exists_in_index(istate, dirname, len-1);
@ -1711,18 +1717,135 @@ static enum path_treatment treat_directory(struct dir_struct *dir,
/* This is the "show_other_directories" case */
if (!(dir->flags & DIR_HIDE_EMPTY_DIRECTORIES))
return excluded ? path_excluded : path_untracked;
untracked = lookup_untracked(dir->untracked, untracked,
dirname + baselen, len - baselen);
/*
* If we have a pathspec which could match something _below_ this
* directory (e.g. when checking 'subdir/' having a pathspec like
* 'subdir/some/deep/path/file' or 'subdir/widget-*.c'), then we
* need to recurse.
*/
if (pathspec) {
int ret = do_match_pathspec(istate, pathspec, dirname, len,
0 /* prefix */, NULL /* seen */,
DO_MATCH_LEADING_PATHSPEC);
if (ret == MATCHED_RECURSIVELY_LEADING_PATHSPEC)
return path_recurse;
}
/*
* If this is an excluded directory, then we only need to check if
* the directory contains any files.
* Other than the path_recurse case immediately above, we only need
* to recurse into untracked/ignored directories if either of the
* following bits is set:
* - DIR_SHOW_IGNORED_TOO (because then we need to determine if
* there are ignored directories below)
* - DIR_HIDE_EMPTY_DIRECTORIES (because we have to determine if
* the directory is empty)
*/
return read_directory_recursive(dir, istate, dirname, len,
untracked, 1, excluded, pathspec);
if (!(dir->flags & (DIR_SHOW_IGNORED_TOO | DIR_HIDE_EMPTY_DIRECTORIES)))
return excluded ? path_excluded : path_untracked;
/*
* ...and even if DIR_SHOW_IGNORED_TOO is set, we can still avoid
* recursing into ignored directories if the path is excluded and
* DIR_SHOW_IGNORED_TOO_MODE_MATCHING is also set.
*/
if (excluded &&
(dir->flags & DIR_SHOW_IGNORED_TOO) &&
(dir->flags & DIR_SHOW_IGNORED_TOO_MODE_MATCHING))
return path_excluded;
/*
* If we have we don't want to know the all the paths under an
* untracked or ignored directory, we still need to go into the
* directory to determine if it is empty (because an empty directory
* should be path_none instead of path_excluded or path_untracked).
*/
check_only = ((dir->flags & DIR_HIDE_EMPTY_DIRECTORIES) &&
!(dir->flags & DIR_SHOW_IGNORED_TOO));
/*
* However, there's another optimization possible as a subset of
* check_only, based on the cases we have to consider:
* A) Directory matches no exclude patterns:
* * Directory is empty => path_none
* * Directory has an untracked file under it => path_untracked
* * Directory has only ignored files under it => path_excluded
* B) Directory matches an exclude pattern:
* * Directory is empty => path_none
* * Directory has an untracked file under it => path_excluded
* * Directory has only ignored files under it => path_excluded
* In case A, we can exit as soon as we've found an untracked
* file but otherwise have to walk all files. In case B, though,
* we can stop at the first file we find under the directory.
*/
stop_early = check_only && excluded;
/*
* If /every/ file within an untracked directory is ignored, then
* we want to treat the directory as ignored (for e.g. status
* --porcelain), without listing the individual ignored files
* underneath. To do so, we'll save the current ignored_nr, and
* pop all the ones added after it if it turns out the entire
* directory is ignored.
*/
old_ignored_nr = dir->ignored_nr;
/* Actually recurse into dirname now, we'll fixup the state later. */
untracked = lookup_untracked(dir->untracked, untracked,
dirname + baselen, len - baselen);
state = read_directory_recursive(dir, istate, dirname, len, untracked,
check_only, stop_early, pathspec);
/* There are a variety of reasons we may need to fixup the state... */
if (state == path_excluded) {
/* state == path_excluded implies all paths under
* dirname were ignored...
*
* if running e.g. `git status --porcelain --ignored=matching`,
* then we want to see the subpaths that are ignored.
*
* if running e.g. just `git status --porcelain`, then
* we just want the directory itself to be listed as ignored
* and not the individual paths underneath.
*/
int want_ignored_subpaths =
((dir->flags & DIR_SHOW_IGNORED_TOO) &&
(dir->flags & DIR_SHOW_IGNORED_TOO_MODE_MATCHING));
if (want_ignored_subpaths) {
/*
* with --ignored=matching, we want the subpaths
* INSTEAD of the directory itself.
*/
state = path_none;
} else {
int i;
for (i = old_ignored_nr + 1; i<dir->ignored_nr; ++i)
FREE_AND_NULL(dir->ignored[i]);
dir->ignored_nr = old_ignored_nr;
}
}
/*
* If there is nothing under the current directory and we are not
* hiding empty directories, then we need to report on the
* untracked or ignored status of the directory itself.
*/
if (state == path_none && !(dir->flags & DIR_HIDE_EMPTY_DIRECTORIES))
state = excluded ? path_excluded : path_untracked;
/*
* We can recurse into untracked directories that don't match any
* of the given pathspecs when some file underneath the directory
* might match one of the pathspecs. If so, we should make sure
* to note that the directory itself did not match.
*/
if (pathspec &&
!match_pathspec(istate, pathspec, dirname, len,
0 /* prefix */, NULL,
0 /* do NOT special case dirs */))
state = path_none;
return state;
}
/*
@ -1870,6 +1993,11 @@ static enum path_treatment treat_path_fast(struct dir_struct *dir,
int baselen,
const struct pathspec *pathspec)
{
/*
* WARNING: From this function, you can return path_recurse or you
* can call read_directory_recursive() (or neither), but
* you CAN'T DO BOTH.
*/
strbuf_setlen(path, baselen);
if (!cdir->ucd) {
strbuf_addstr(path, cdir->file);
@ -1904,7 +2032,6 @@ static enum path_treatment treat_path(struct dir_struct *dir,
const struct pathspec *pathspec)
{
int has_path_in_index, dtype, excluded;
enum path_treatment path_treatment;
if (!cdir->d_name)
return treat_path_fast(dir, untracked, cdir, istate, path,
@ -1961,24 +2088,16 @@ static enum path_treatment treat_path(struct dir_struct *dir,
default:
return path_none;
case DT_DIR:
strbuf_addch(path, '/');
path_treatment = treat_directory(dir, istate, untracked,
path->buf, path->len,
baselen, excluded, pathspec);
/*
* If 1) we only want to return directories that
* match an exclude pattern and 2) this directory does
* not match an exclude pattern but all of its
* contents are excluded, then indicate that we should
* recurse into this directory (instead of marking the
* directory itself as an ignored path).
* WARNING: Do not ignore/amend the return value from
* treat_directory(), and especially do not change it to return
* path_recurse as that can cause exponential slowdown.
* Instead, modify treat_directory() to return the right value.
*/
if (!excluded &&
path_treatment == path_excluded &&
(dir->flags & DIR_SHOW_IGNORED_TOO) &&
(dir->flags & DIR_SHOW_IGNORED_TOO_MODE_MATCHING))
return path_recurse;
return path_treatment;
strbuf_addch(path, '/');
return treat_directory(dir, istate, untracked,
path->buf, path->len,
baselen, excluded, pathspec);
case DT_REG:
case DT_LNK:
return excluded ? path_excluded : path_untracked;
@ -2175,14 +2294,10 @@ static enum path_treatment read_directory_recursive(struct dir_struct *dir,
int stop_at_first_file, const struct pathspec *pathspec)
{
/*
* WARNING WARNING WARNING:
*
* Any updates to the traversal logic here may need corresponding
* updates in treat_leading_path(). See the commit message for the
* commit adding this warning as well as the commit preceding it
* for details.
* WARNING: Do NOT recurse unless path_recurse is returned from
* treat_path(). Recursing on any other return value
* can result in exponential slowdown.
*/
struct cached_dir cdir;
enum path_treatment state, subdir_state, dir_state = path_none;
struct strbuf path = STRBUF_INIT;
@ -2204,13 +2319,7 @@ static enum path_treatment read_directory_recursive(struct dir_struct *dir,
dir_state = state;
/* recurse into subdir if instructed by treat_path */
if ((state == path_recurse) ||
((state == path_untracked) &&
(resolve_dtype(cdir.d_type, istate, path.buf, path.len) == DT_DIR) &&
((dir->flags & DIR_SHOW_IGNORED_TOO) ||
(pathspec &&
do_match_pathspec(istate, pathspec, path.buf, path.len,
baselen, NULL, DO_MATCH_LEADING_PATHSPEC) == MATCHED_RECURSIVELY_LEADING_PATHSPEC)))) {
if (state == path_recurse) {
struct untracked_cache_dir *ud;
ud = lookup_untracked(dir->untracked, untracked,
path.buf + baselen,
@ -2294,15 +2403,6 @@ static int treat_leading_path(struct dir_struct *dir,
const char *path, int len,
const struct pathspec *pathspec)
{
/*
* WARNING WARNING WARNING:
*
* Any updates to the traversal logic here may need corresponding
* updates in read_directory_recursive(). See 777b420347 (dir:
* synchronize treat_leading_path() and read_directory_recursive(),
* 2019-12-19) and its parent commit for details.
*/
struct strbuf sb = STRBUF_INIT;
struct strbuf subdir = STRBUF_INIT;
int prevlen, baselen;
@ -2353,23 +2453,7 @@ static int treat_leading_path(struct dir_struct *dir,
strbuf_reset(&subdir);
strbuf_add(&subdir, path+prevlen, baselen-prevlen);
cdir.d_name = subdir.buf;
state = treat_path(dir, NULL, &cdir, istate, &sb, prevlen,
pathspec);
if (state == path_untracked &&
resolve_dtype(cdir.d_type, istate, sb.buf, sb.len) == DT_DIR &&
(dir->flags & DIR_SHOW_IGNORED_TOO ||
do_match_pathspec(istate, pathspec, sb.buf, sb.len,
baselen, NULL, DO_MATCH_LEADING_PATHSPEC) == MATCHED_RECURSIVELY_LEADING_PATHSPEC)) {
if (!match_pathspec(istate, pathspec, sb.buf, sb.len,
0 /* prefix */, NULL,
0 /* do NOT special case dirs */))
state = path_none;
add_path_to_appropriate_result_list(dir, NULL, &cdir,
istate,
&sb, baselen,
pathspec, state);
state = path_recurse;
}
state = treat_path(dir, NULL, &cdir, istate, &sb, prevlen, pathspec);
if (state != path_recurse)
break; /* do not recurse into it */