swallow stdout on success

[ghost]

This PR adds an option so that Dune may swallow the output of build actions when they succeed. We do that in Jenga, so we need a similar option in Dune.

In practice, as soon as your build is large enough you want a successful build to print nothing, otherwise it's all just noise.

This option is allowed (by decreasing precedence):

• on the command line
• in the workspace file
• in the config file

[bobot]

It is nice to idea, just some questions about design:

• Why restricting the feature to stdout. As for redirection should the three possibilities stdout/stderr/outputs be available?
• Why nothing at the action level?
• A version of with-stdout-to that reprint what it redirected in case of error with-stdout-to! seems still needed.

It doesn't have to go in this PR.

[ghost]

I was mostly adapting what we had at Jane Street, but happy to explore on the design :)

Regarding stderr, I was planning to also follow what we have inside Jane Street, where we consider that if a command has a non-empty stderr then it is failing. Under an option of course, as it might be too extreme for people who don't develop in big mono-repos.

Why restricting the feature to stdout. As for redirection should the three possibilities stdout/stderr/outputs be available?

Ignoring stderr by default doesn't feel right. In Jenga, we consider that a non-empty stderr means failure, and that makes sense to me.

Why nothing at the action level?

Something at the action level makes sense. For this PR, I was actually wondering whether this behaviour should be at the level of individual (run ...) or at the level of full actions. The former has the advantage that it will play nicely with another PR I have in mind to add a (parallel ...) construction to the action language. We need that to speed up inline tests but keep a good user experience.

A version of with-stdout-to that reprint what it redirected in case of error with-stdout-to! seems still needed.

Any reason not to make it the default behaviour?

[ghost]

This is ready for review. The behaviour is currently per-process, which means that with the current action:

(progn
  (system "echo 1")
  (system "echo 2; exit 1"))

with --swallow-stdout-on-success, only 2 will be printed and 1 will be swallowed. This is a bit odd, but in line with the current implementation and how we report errors. I plan to change this in #4435, but the current PR should be enough for making progress on the switch to dune inside Jane Street.

I also changed the way we initialise the Source_tree module. The new code is simpler IMO: we just have a plain Settings.t Memo.Build.t Fdecl.t that it set only once. We no longer have to think about the "current run" or the forward declaration being set multiple times.

[snowleopard]

Looks good, although my head was spinning when reviewing the code around Merged_with_stdout. I wish that part could be made simpler.

[ghost]

Indeed, I'll try to simplify this in #4435