Serve rebuilds for every file individually when a lot of files change

[azerupi]

When a lot of source files change and we were running mdbook serve, it will rebuild the book for each file that changed. This is particularly painful when using git and changing branches. During those rebuilds, the served book is broken.

Simply quitting mdbook and relaunching the command solves the problem, but it is an annoying paper-cut.

[Michael-F-Bryan]

I just noticed this myself when I added a "todo: write this" note to all the unwritten chapters in something I'm working on. It's a real pain when the mdbook watch/mdbook serve tries to rebuild for every updated file when really it should only do the update once.

The actual culprit is our trigger_on_change() function in src/bin/watch.rs because it'll re-run the function for every change notification, but I'm not 100% sure how we can deal with that. We'd need some sort of "debounce" mechanism to collate a bunch of notifications arriving at the same time into a single "stuff changed" message.

What are your thoughts on something like this?

loop {
  let _ = rx.recv();  // block until the first change comes in
  sleep(50ms);  
  let _ = rx.try_iter().count(); // collect all other notifications received while sleeping
  rebuild();
}

[azerupi]

That could work. Notify seems to send an event for every file changed, even within the 1 second debounce time we give it. So flushing the iterator should do the trick. However, we should decently document/comment this portion of code because it is not obvious what it does. 😉

I also think that last() could be used instead of count on the iterator if we do not use the number of changes detected in logs or other things. Both consume the iterator, so I think it should work the same but I find last() more obvious in its intent.

[Michael-F-Bryan]

Both consume the iterator, so I think it should work the same but I find last() more obvious in its intent.

Agreed. Although it could also be turned into some sort of collect() so we can log which files were updated and how... Or is that overkill?

[azerupi]

Yes, absolutely! It comes down to what you want to log.

[cauebs]

I forgot to mention, but I've opened #870 to fix this.