[Monotone-devel] rewrite of checkout to use editable_working_tree

[Derek Scherger]

I've just pushed a new branch net.venge.monotone.editable_tree.checkout 
that rewrites checkout in terms of editable_working_tree. The point of 
this is to allow checkout to take advantage of the workspace conflict 
resolution machinery, once it is written.

Of course I am assuming that people do want checkout to be able to deal 
with workspace conflicts and not just clobber anything that gets in its 
way. I guess we'll see whether this is a good assumption or not . ;)

Hopefully this will also add some detail to what the "workspace conflict 
resolution machinery" should eventually look like. There are at least 3 
conflict cases that checkout may encounter:

1. create_dir_node will fail if _MTN/tmp/<nid> already exists in the 
workspace. I'm not sure how we would respond to such an error, other 
than requesting that the user clean up their _MTN/tmp dir?

2. create_file_node will fail if _MTN/tmp/<nid> already exists in the 
workspace, although this failure is somewhat pointless at the moment. 
The get_linesep_conv and get_charset_conv hooks require that files 
aren't created until their path is known, which isn't the case until 
attach_node executes, so the actual creation is deferred to attach_node 
which seems a bit ugly. Ideally, we should be able to do away with these 
hooks if character set and line ending style are specified as 
attributes. Again, I'm not sure how we'd respond to this error.

3. attach_node will fail if the path it wants to attach a node to 
already exists in the workspace that checkout is creating. Currently 
checkout simply clobbers files as required, although I believe it will 
fail if a file is blocked by a directory or vice-versa. I'll start 
another thread to discuss how attach_node conflicts might be handled.

These are just three cases that come to mind but it won't surprise me if 
there are other things that can go wrong.

This new version of checkout appears to be functional, only a few tests 
needed updating, but it is considerably slower than what we currently 
have and is also considerably more verbose. Stock checkout of 
net.venge.monotone takes ~1.164s while the new checkout takes ~8.344s on 
my laptop. Stock checkout produces no output at all, while the new 
checkout writes "mtn: adding ..." for each file and directory that it 
creates, 1955 lines in total.

If anyone is set up to do profiling and has a chance I'd be interested 
to see where the time is going. I'll probably have a go at this myself 
at some point too but it may be a while.

Cheers,
Derek