fix(ocamllsp): fix in pp handling the file permissions

[jboillot]

Hello,
In the PR that made Merlin a dependency #1070 a bug was introduced that prevents from using preprocessor programs as https://github.com/ocaml-community/cppo.
This is because the file permissions are not provided in the right base and that the output files are currently not created if they do not exist yet.
I do not know if the addition of the flag Unix.O_CREAT could cause an issue with some pp or ppx.

[voodoos]

I think this needs to be fixed on Merlin's side: in the same way as for -ppx the -pp flag expect an arbitrary command, with it's arguments, so we should really handle this the same as ppxs (but we changed that behavior when refactoring).

I will do that asap.

[voodoos]

However the proposed change looks reasonable: having input files be read-only and allowing creating new files.

[voodoos]

After having a look on Merlin's side I think things are all-right there. I added a test for preprocessing.

The issue doesn't show with ppxes because the driver is responsible for creating/writing to the file, it is not relying on redirecting stdout.

The changes look good to me.

@jboillot where you able to test the changes on your project ?

It can be done using opam pin https://github.com/jboillot/ocaml-lsp.git#bugfix/ocamllsp-pp-output-file-creation.

@rgrinberg I guess this deserves a patch release. I can take care of it if you want (and if I have the rights).

[rgrinberg]

Yes, give it a try.

Where's the test though?

[voodoos]

The test I mentioned is on Merlin side. I am looking at how I could do the same in ocaml-lsp but it's not as simple without cram and the cli interface.

https://github.com/ocaml/merlin/pull/1630/files

[jboillot]

@voodoos I was able to test the changes on a few files of a big OCaml project with cppo as the only pp (and a few other ppxes).

[voodoos]

I added a test that was effectively failing without the changes in this PR.

[voodoos]

Thanks a lot @jboillot !