ADFA-4128 (9/11): quickbuild:daemon — the incremental compile service

[fryanpan]

Part 9/11 of the stacked split of #1669 (requested by Akash). Base: feature/ADFA-4128-qb-08-core-orchestration. Stack overview + review mechanics: PR 1 (#1713). Terms are defined in quickbuild/README.md (lands in PR 1).

This is where the speed comes from: keeping a compiler warm between edits, so a save costs seconds instead of a full cold build.

flowchart LR
    core[":quickbuild:core (PRs 5-8)"] -- "line-delimited JSON on stdin/stdout<br/>(:quickbuild:protocol, PR 3)" --> svc
    subgraph d["<b>This PR: :quickbuild:daemon — separate JVM child process</b>"]
        svc["DaemonService<br/>exception backstop on every op<br/><i>DaemonService.kt</i>"] --> kt["IncrementalCompiler<br/>Kotlin Build Tools API, warm caches<br/><i>IncrementalCompiler.kt</i>"]
        svc --> jv["JavaCompileStep<br/>ABI fingerprint: does a .java edit<br/>force a Kotlin recompile?<br/><i>JavaCompileStep.kt</i>"]
        svc --> dx["FinalStripper + DexTool (d8)<br/><i>FinalStripper.kt</i>"]
        svc --> lk["aapt2 relink<br/>kill-on-timeout<br/><i>Aapt2Link.kt</i>"]
    end
    sdk["device SDK toolchain<br/>aapt2, d8.jar, android.jar"] -.-> d
    classDef thisPrBox fill:#dbeafe,stroke:#93c5fd,color:#1e3a5f
    classDef inPr fill:#ffffff,stroke:#64748b,color:#000
    class d thisPrBox
    class svc,kt,jv,dx,lk inPr

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What to review

• DaemonService.kt — exception backstop; a throwing handler never kills the daemon. Line-by-line.
• IncrementalCompiler.kt, JavaCompileStep.kt — warm caches; ABI fingerprint decides Kotlin recompiles.
• FinalStripper.kt — strips final so generated proxies can subclass user classes.
• Aapt2Link.kt — relink killed on timeout so a hung linker cannot wedge.

How this PR Was Tested

• 25 test files, including the OfflineGuard network check.
• Toolchain-guarded tests would skip green without an SDK; analyze.yml forces failure.
• [verified 2026-08-21] At this cut: :quickbuild:daemon:test green with PRs 1–9 applied — 25 test files (24 suites; TestSdk is the toolchain guard, not a suite), 193 tests, 0 failures, 0 errors. 0 skipped, so the SDK-guarded aapt2/d8/Compose tests genuinely ran rather than skipping green. Coverage 97.4% line / 87.9% branch.
• End-to-end evidence: PR 11. No JVM test runs the real daemon jar.

Coverage (JaCoCo at the stack tip, single run):

Package	Line	Branch	Note
…quickbuild.daemon	100.0%	80.9%	socket lifecycle branches
…quickbuild.daemon.compile	96.1%	88.3%
…quickbuild.daemon.dex	99.0%	57.1%	d8-invocation variants need the real tool
…quickbuild.daemon.protocol	95.2%	97.0%
…quickbuild.daemon.res	98.2%	95.7%
NON-UI TOTAL	97.4%	87.9%	895 lines, 431 branches

11 source files in the diff, all 11 measured.

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