fix: print "Leaving Directory"

[rgrinberg]

Print "Leaving Directory '%s'" in addition to "Entering Directory"

Fixes #138

The original PR didn't flush to stdout so the output was garbled.

There's still a small issue with dune exec. Because we're execing a program, we're not unable to write the "Leaving Directory" after the program exits. @emillon @cpitclaudel should we leave it this way or just get rid of the "Leaving" for such cases?

[emillon]

I don't have the best understanding of why it's necessary and how it works, but if this fixes some editor integration that's fine by me.
Regarding dune exec, 2 remarks:

• we're calling Stdlib.do_at_exit in Proc.restore_cwd_and_execve. This means that this is going to print Leaving directory early. is it fine?
• processes exec'ing into each other is usually not a great idea for portability anyway. I'd prefer if we could use create_process or spawn and wait for the process (which would do the right thing on windows too), that would allow us to print the message after the process exits. We probably can't do that comfortably before dune 4.0.

[rgrinberg]

I don't have the best understanding of why it's necessary and how it works, but if this fixes some editor integration that's fine by me.

Yes, this fixes editor integration. Editors will use a regex to set the base directory for all errors so running dune will screw up interpreting errors for subsequent programs

we're calling Stdlib.do_at_exit in Proc.restore_cwd_and_execve. This means that this is going to print Leaving directory early. is it fine?

I suppose it's fine because we aren't changing the directory for the executed process. My original comment didn't appreciate that.

processes exec'ing into each other is usually not a great idea for portability anyway. I'd prefer if we could use create_process or spawn and wait for the process (which would do the right thing on windows too), that would allow us to print the message after the process exits. We probably can't do that comfortably before dune 4.0.

Note that we always run the process in the CWD so it in fact we should print before exec'ing.

We could introduce dune run instead of dune exec. exec is already well defined and I'm not so confident in our ability of doing dune run and handling signals correctly.

[cpitclaudel]

Thanks a lot for working on this!

should we leave it this way or just get rid of the "Leaving" for such cases?

I think either way is fine for dune exec, but…

Note that we always run the process in the CWD so it in fact we should print before exec'ing.

… it looks like in fact printing before exec is both feasible and right, so maybe the above question solved itself faster than my github response time? :)

[rgrinberg]

@emillon this is ready.