BridgeJS: Skip writing output files when content is unchanged

[krodak]

Overview

BridgeJSTool writes BridgeJS.swift and BridgeJS.json on every invocation, even when the content hasn't changed. The atomic write creates a new temp file each time, which updates the mtime and makes SPM think the output changed. This triggers a recompile of BridgeJS.swift and a relink on every build, even with zero source changes.

Reproducing

While testing cross-plugin builds after #638, we noticed Compiling Khasm BridgeJS.swift and Linking KhasmWasm.wasm in every no-op build. Traced it to the unconditional .write(to:atomically:) calls in BridgeJSTool - the content was identical but the file got a new timestamp every time.

To reproduce with any BridgeJS project:

swift package js --product MyProduct    # first build
swift package js --product MyProduct    # immediate rebuild, no changes
# BridgeJS.swift recompiles, binary relinks

Fix

Compare content before writing. If the existing file has identical content, skip the write.

private func writeIfChanged(_ content: String, to url: URL) throws {
    let existing = try? String(contentsOf: url, encoding: .utf8)
    guard existing != content else { return }
    try content.write(to: url, atomically: true, encoding: .utf8)
}

Applied to both BridgeJS.swift and BridgeJS.json. The --always-write flag still works - it controls whether the file gets created at all (even when empty), just doesn't force a rewrite when content matches.

Question

Was the unconditional write intentional? Wondering if there's an edge case I'm not seeing, if so, lets just close this one 🙏🏻

[kateinoigakukun]

It makes sense to me, thanks!