extract WriteSourceFileStep out from WriteFileStep

[andrewrk]

Extracted from #14647.

zig/lib/std/Build/WriteFileStep.zig

Lines 5 to 10 in b3af5d0

	//! However, this step has an additional capability of writing data to paths
	//! relative to the package root, effectively mutating the package's source
	//! files. Be careful with the latter functionality; it should not be used
	//! during the normal build process, but as a utility run by a developer with
	//! intention to update source files, which will then be committed to version
	//! control.

zig/lib/std/Build/WriteFileStep.zig

Lines 148 to 186 in b3af5d0

	// Writing to source files is kind of an extra capability of this
	// WriteFileStep - arguably it should be a different step. But anyway here
	// it is, it happens unconditionally and does not interact with the other
	// files here.
	var any_miss = false;
	for (wf.output_source_files.items) |output_source_file| {
	if (fs.path.dirname(output_source_file.sub_path)) |dirname| {
	b.build_root.handle.makePath(dirname) catch |err| {
	return step.fail("unable to make path '{}{s}': {s}", .{
	b.build_root, dirname, @errorName(err),
	});
	};
	}
	switch (output_source_file.contents) {
	.bytes => |bytes| {
	b.build_root.handle.writeFile(output_source_file.sub_path, bytes) catch |err| {
	return step.fail("unable to write file '{}{s}': {s}", .{
	b.build_root, output_source_file.sub_path, @errorName(err),
	});
	};
	any_miss = true;
	},
	.copy => |file_source| {
	const source_path = file_source.getPath(b);
	const prev_status = fs.Dir.updateFile(
	fs.cwd(),
	source_path,
	b.build_root.handle,
	output_source_file.sub_path,
	.{},
	) catch |err| {
	return step.fail("unable to update file from '{s}' to '{}{s}': {s}", .{
	source_path, b.build_root, output_source_file.sub_path, @errorName(err),
	});
	};
	any_miss = any_miss or prev_status == .stale;
	},
	}
	}

This should be a different step, not part of WriteFileStep.

[matklad]

Once WriteSourceFileStep, it would be good to add a mode to it to just check that the files are up-to-date and fail the build if they aren't. So, as developer, you run:

$ vim source-of-truth
$ zig build generate-sources
$ git add .
$ git commit -m "refresh generated source" && git push

and then CI could optionally run

$ zig build generate-sources -Dgenerate-sources-mode=check

which would fail the build if you forgot to commit generated sources.

Concrete use-case for TigerBeetle:

• We'd love to use WriteSourceFileStep to generate an .h file from Zig's source of truth and to include the generated file in the repository, so that consumers of our pre-compiled C library wouldn't have to run zig build process to get the header
• But we need to also check that the .h file in the repo is indeed in sync with the Zig sources.

Piggy-backing on the existing issue, as it seems like this could be tacked in the process of extracting the code, if this looks like a good idea

[lishaduck]

This seems to have been done in #20580.

[alexrp]

d1c14f2