Re: Bug in ftdetect?

Christian Brabandt wrote:

> On Do, 25 Mai 2017, Bram Moolenaar wrote:
>
> > Christian Brabandt wrote:
> >
> > > On Mi, 24 Mai 2017, fREW Schmidt wrote:
> > >
> > > > Yes. "If I replace the contents of /usr/share/vim/vim80/syntax/vb.vim with `let
> > > > b:current_syntax = "vb"` everything is fast and, as I would hope, the support
> > > > files are not loaded N times per buffer but instead exactly once." 
> > >
> > > Well, I see the problem, although I cannot reproduce it.
> > >
> > > The markdown syntax script sources the html syntax script and that one
> > > does include a couple of other syntax scripts, like css.vim,
> > > javascript.vim and also vb.vim. However from my testing vb.vim is not
> > > guilty alone. If I uncomment vb.vim, the overall loading time only
> > > shrinks by 0.2 seconds or so (it needs here a total of around 7
> > > seconds). So you would need to disable all other to be included syntax
> > > scripts as well (for a test, you could set main_syntax='java').
> >
> > I haven't read the whole thread, but I suspect that the trick is to set
> > b:current_syntax (to anything), which will cause further syntax scripts to bail
> > out. Anyway, this is all working around the actual problem.
>
> Not for html, which explicitly resets b:current_syntax to be able to
> include several other syntax scripts and even provides the main_syntax
> global variable to make this work with other syntax scripts.

Well, it must be used somehwere to explain why making the file empty has
a different effect from only setting b:current_syntax in the file.

> > > However, I think you might have found a bug here. Your ftdetect script
> > > does this:
> > >
> > > autocmd BufNew,BufNewFile,BufRead *.md :set filetype=markdown
> > >
> > > The :args command does fill the argument list, calls ex_next() which
> > > calls eventually alist_set() and then iterates over the argument list
> > > and calls adlist_add(). This will finally call buflist_add() which in
> > > turn calls buflist_new(), which triggers the BufNew autocommand, which
> > > then triggers the FileType and finally the Syntax autocommands. So in
> > > the end, we are iterating over the whole list of arguments and call
> > > autocommands, for buffers that are not even going to be displayed next.
> > >
> > > So I wonder, if we can't do a little bit better, similar to how vimgrep
> > > sets the eventignore option to prevent at least the expensive FileType
> > > autocommand:
> >
> > I'm quite sure this will make other things fail. Just don't use
> > BufNew for detecting the filetype, that is the cause of the problem.
> > The file type should only be detected when the buffer is loaded.
> > Doing it earlier indeed causes the overhead experienced.
>
> Indeed, however, that doesn't mean, we shouldn't try to avoid this
> problem. Vimgrep does the same.

Avoid what problem? Detecting the filetype in cases it isn't used? Not
sure how we can predict that. For :vimgrep it's obvious, and it then
has to carefully jump to hoops later for when a buffer ends up not being
unloaded. Probably can't be turned into a generic solution, since we
don't know what happens next.

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