[PATCH 2/8] within_depth: fix return for empty path

From: Jeff King <peff@peff.net>

The within_depth function is used to check whether pathspecs limited by a max-depth parameter are acceptable. It takes a path to check, a maximum depth, and a "base" depth. It counts the components in the path (by counting slashes), adds them to the base, and compare them to the maximum.

However, if the base does not have any slashes at all, we always return "true". If the base depth is 0, then this is correct; no matter what the maximum is, we are always within it. However, if the base depth is greater than 0, then we might return an erroneous result.

This ends up not causing any user-visible bugs in the current code. The call sites in dir.c always pass a base depth of 0, so are unaffected. But tree_entry_interesting uses this function differently: it will pass the prefix of the current entry, along with a "1" if the entry is a directory, in essence checking whether items inside the entry would be of interest. It turns out not to make a difference in behavior, but the reasoning is complex.

Given a tree like:

  file
  a/file
  a/b/file

walking the tree and calling tree_entry_interesting will yield the following results:

  (with max_depth=0):
      file: yes
         a: yes
    a/file: no
       a/b: no

  (with max_depth=1):
      file: yes
         a: yes
    a/file: yes
       a/b: no

So we have inconsistent behavior in considering directories interesting. If they are at the edge of our depth but at the root, we will recurse into them, but then find all of their entries uninteresting (e.g., in the first case, we will look at "a" but find "a/*" uninteresting). But if they are at the edge of our depth and not at the root, then we will not recurse (in the second example, we do not even bother entering "a/b").

This turns out not to matter because the only caller which uses max-depth pathspecs is cmd_grep, which only cares about blob entries. From its perspective, it is exactly the same to not recurse into a subtree, or to recurse and find that it contains no matching entries. Not recursing is merely an optimization.

It is debatable whether tree_entry_interesting should consider such an entry interesting. The only caller does not care if it sees the tree itself, and can benefit from the optimization. But if we add a "max-depth" limiter to regular diffs, then a diff with DIFF_OPT_TREE_IN_RECURSIVE would probably want to show the tree itself, but not what it contains.

This patch just fixes within_depth, which means we consider such entries uninteresting (and makes the current caller happy). If we want to change that in the future, then this fix is still the correct first step, as the current behavior is simply inconsistent.

Signed-off-by: Jeff King <peff@peff.net>
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 dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)