clean before building

[mbostock]

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Fixes #1538. Now observable build is always clean.

Leaving as draft because I think we want a --clean flag to clean automatically, and without it we prompt you before cleaning the existing output root (if it exists) (and error in CI if the output root is dirty and --clean or --no-clean wasn’t specified).

And maybe --clean isn’t the right name because a “clean” build will still respect the cache so it’s not fully clean? Sigh.

[mcglincy]

Code changes LGTM.

If "clean" seems too inaccurate, maybe something like "clean-dist" (and then later "clean-cache", "clean-all"?) could be used.

[mbostock]

Thinking about this some more, and I think cleaning the output directory is the right default, and probably doesn’t even need a flag. For precedence, Vite’s build command has an emptyOutDir option that likewise defaults to true, but only if the output directory is within the project root (so as to avoid accidentally deleting something important, such as /). I’ll add that to the PR and commit.

[srenatus]

Sorry for posting to an old issue, but it seems like the best spot:

Thinking about this some more, and I think cleaning the output directory is the right default, and probably doesn’t even need a flag.

I've come here looking for a flag 😅 The problem is this: I'm using framework in a concourse pipeline, and the output directory is declared as the task's output, so that only that data is moved to the next task. Concourse seems to do that by mounting something into the output location, and that something cannot be removed:

> observable
> observable "build"

(node:2607) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
Warning: the config file is missing the root option, which specifies the path to
the source root. The recommended source root is "src"; however, since docs
exists and was previously the default for this option, we will use "docs". You
can suppress this warning by specifying root: "docs" in the config file.

Unexpected error: EBUSY: resource busy or locked, rmdir 'dist'

Tip: To see the full stack trace, run with the --debug flag.


If you think this is a bug, please file an issue at
↳ https://github.com/observablehq/framework/issues

I think I can just use another directory within the output directory, so it's probably not a big deal.