Can we add a 'label_matches()' and label filtering functions?

[Aaron Wood]

For some developer-overrides configuration use-cases in Fuchsia, I'd like to add a new function to GN:

some_label = "//baz:bar"
result = label_matches(some_label, [ "//foo/bar/*", "//baz:*" ])

# result == true

And along with this, two related functions that are similar to the file-path filtering functions.

filter_include_labels(labels, patterns)

filter_exclude_labels(labels, patterns)

I have a prototype up at: https://gn-review.git.corp.google.com/c/gn/+/16680

The motivation for the first function (label_matches()) is to allow us (Fuchsia) to implement developer overrides of configs / optimization for specified targets within the build graph, in particular for Rust compilation:

args.gn:

rust_rbe_enable = true

rust_incremental_targets = [

  "//src/lib/assembly/*",
  "//src/connectivity/netstack/netstack3/*",

]

The above would allow the build to use the RBE remote builders by default, and then would switch those targets that match the rust_incremental_targets to use rustc's incremental recompilation options that utilize a local cache to greatly speed up compilation (but isn't compatible with remote builders).

The filter_include|exclude_labels() methods are motivated as a nicety for processing a list of targets at once, and would allow the replacement of matching against file paths in our BUILDCONFIG.gn when doing variant selection.

[David Turner]

Thanks Aaron,

I second this addition, it would simplify and improve the performance of our `gn gen` passes when enabling build-variants (as we need to perform manual filtering will foreach() loops are that are very slow in practice.

- Digit

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[Dirk Pranke]

Hi Aaron, 

I think this sounds reasonable enough. Let's give this a few more days to see if anyone else objects, and then we can proceed with the change.

-- Dirk

[Aaron Wood]

Ok.  I'll pick the CL back up and address the comments I have from digit@ and some other cleanup it needs, then.