path/filepath: ensure Glob() returns in lexical order and comment about that

[joesis]

glob()'s comment mentioned the order.

[minux]

Why should we specify the order? It's quite possible that we can speed Glob up by not sorting the filenames. If you want a specific ordering, you should sort the result manually.

[joesis]

I thought the order was intentionally maintained per the comment of glob() (it's called only by Glob). The only other method in the same package that deals with multiple files, filepath.Walk, maintains lexical order too.

If the order is intentionally unspecified, adding a line of comment would be good to avoid callers from depending on the current implementation.

[dgryski]

I've had Glob() heavy code which profiles showed the call to Sort() is what was dominating the runtime. (I switched to a large in-memory index to virtually eliminate the calls to Glob() for a significant performance win.)

[quentinmit]

I don't think we want to guarantee any particular order. I read glob's comment as specifying the order in which it appends items to the argument, which is not at all guaranteed to be the order that Glob returns. I also don't think we need to, ahem, specify that it is unspecified in a comment. There are many things all over the standard library that have undocumented additional behavior. (See also the recent changes to the implementations of Read in standard library types.)