Re: [PATCH 2/2 v2] ls-files: add pathspec matching for submodules

Brandon Williams [off-list ref] writes:

-	/* Find common prefix for all pathspec's */
-	max_prefix = common_prefix(&pathspec);
+	/*
+	 * Find common prefix for all pathspec's
+	 * This is used as a performance optimization which unfortunately cannot
+	 * be done when recursing into submodules
+	 */
+	if (recurse_submodules)
+		max_prefix = NULL;
+	else
+		max_prefix = common_prefix(&pathspec);
 	max_prefix_len = max_prefix ? strlen(max_prefix) : 0;

This is OK for now, but for a future enhancement, I think we could
do better than this.  In a superproject with a submodule at "sub/",
the current implementation of the common_prefix() helper would yield
"sub/a/" when given "sub/a/x" and "sub/a/y" (a pathspec with two
elements), which we want to avoid.

But somebody should be able to notice, before "sub/a/" is given to
max_prefix here, that "sub/" is the leaf level in our repository and
reduce the max_prefix to it.  dir.c::common_prefix_len() might be 
a place we could do so, but I didn't think about the ramifications
of doing so for other callers of common_prefix() or when we are not
recursing into submodules.  Doing it in the caller here, i.e.

	max_prefix = common_prefix(&pathspec);
        if (recurse_submodules)
        	max_prefix = chomp_at_submodule_boundary(max_prefix);

is certainly safer.

If the superproject has submodules at "a/b/{sub1,sub2,...}", this
matters more.  We do want to notice that we won't have to scan
outside "a/b/" of the index given "a/b/sub1" and "a/b/sub2" as a
pathspec.

The common_prefix_len() function also looks beyond symbolic links,
which is another thing that we may want to think about.  In a
repository with a symbolic link "link" pointing somewhere else, when
you give "link/a/x" and "link/a/y" (a pathspec with two elements),
we would get "link/a/" as a common prefix, but we won't find
anything underneath "link" in our index.  In such a case, leaving
the common prefix to "link/a/" _might_ allow us to notice that no
pathspec elements can ever match, so not noticing that the common
prefix points beyond a symbolic link might be a feature.  I dunno.