dir: replace exponential algorithm with a linear one

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 404ebceda01c ("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 df5bcdf83aeb ("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: * 3aca58045f4f (git-clean.txt: do not claim we will delete files with -n/--dry-run, 2019-09-17) * 09487f2cbad3 (clean: avoid removing untracked files in a nested git repository, 2019-09-17) * e86bbcf987fa (clean: disambiguate the definition of -d, 2019-09-17) Examples where testcases were declared wrong and changed: * 09487f2cbad3 (clean: avoid removing untracked files in a nested git repository, 2019-09-17) * e86bbcf987fa (clean: disambiguate the definition of -d, 2019-09-17) * a2b13367fe55 (Revert "dir.c: make 'git-status --ignored' work within leading directories", 2019-12-10) Examples where testcases were clearly inadequate: * 502c386ff944 (t7300-clean: demonstrate deleting nested repo with an ignored file breakage, 2019-08-25) * 7541cc530239 (t7300: add testcases showing failure to clean specified pathspecs, 2019-09-17) * a5e916c7453b (dir: fix off-by-one error in match_pathspec_item, 2019-09-17) * 404ebceda01c (dir: also check directories for matching pathspecs, 2019-09-17) * 09487f2cbad3 (clean: avoid removing untracked files in a nested git repository, 2019-09-17) * e86bbcf987fa (clean: disambiguate the definition of -d, 2019-09-17) * 452efd11fbf6 (t3011: demonstrate directory traversal failures, 2019-12-10) * b9670c1f5e6b (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: * 902b90cf42bc (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>