gnodet opened a new pull request, #328:
URL: https://github.com/apache/maven-clean-plugin/pull/328
## Summary
Refactors the fast clean (`-Dmaven.clean.fast=true`) `BackgroundCleaner` to
fix the MCLEAN-102 performance regression and three regressions introduced by
the per-module refactoring in PR #286.
### What this fixes
| # | Bug | Root Cause | Fix |
|---|------|------------|-----|
| 1 | **MCLEAN-102** — fast mode 50% slower on 12-core Windows |
`System.gc()` stop-the-world per file + per-file retry sleeps from background
thread | Batch retry: walk once without retry, sleep once (250ms), retry all
failures |
| 2 | **Thread proliferation** — 500 threads in 500-module reactor |
Per-module `BackgroundCleaner`, each with its own `ExecutorService` |
Session-scoped instance via `SessionData.computeIfAbsent()` |
| 3 | **500 session listeners** | Per-module
`session.registerListener(this)` | Single listener registered in constructor |
| 4 | **Leftover cleanup lost** | `init()` scan removed in PR #286 when
singleton was dropped | Restored: `scanForLeftovers()` queues leftover dirs for
background deletion |
### Changes
- **`BackgroundCleaner.java`** (major rewrite):
- `SessionData.Key<BackgroundCleaner>` + `getOrCreate()` factory — one
instance, one thread, one listener per session
- `deleteInBackground()` replaces `deleteSilently()` — uses
`SimpleFileVisitor` with batch retry instead of creating a `Cleaner` copy that
uses per-file retry with `System.gc()`
- `scanForLeftovers()` — scans fast directory for dirs left by killed
previous builds
- `fastDelete()` is `synchronized` for parallel build safety (`-T`)
- **`Cleaner.java`** (one-line change):
- `setWritable()`: `private static` → `static` (package-private) so
`BackgroundCleaner` can reuse it for force-delete in background
- **`CleanMojo.java`** (one-line change):
- `new BackgroundCleaner(...)` → `BackgroundCleaner.getOrCreate(...)`
### Before vs After (500-module reactor)
| Metric | Before | After |
|--------|--------|-------|
| Background threads | **500** | **1** |
| Session listeners | **500** | **1** |
| `System.gc()` calls (Windows) | thousands | **0** |
| Per-file sleeps (background) | 50–1050ms × N | **0** |
| Batch sleeps | 0 | **1 × 250ms per directory** |
| Leftover cleanup | ❌ | ✅ |
### Design notes
- **Foreground path unchanged**: the traditional `Cleaner.tryDelete()` keeps
`System.gc()` + per-file retry — it runs in the main thread where it can't
cause stop-the-world on other threads, and it may genuinely need to release
JVM-held file handles.
- **Batch retry respects `retryOnError`**: if user sets
`-Dmaven.clean.retryOnError=false`, no retry happens even in background.
- **Remaining failures deferred to next build**: any files that survive the
batch retry will be cleaned up by the leftover scan on the next `fast=true`
build.
## Test plan
- [x] `mvn verify` passes (15 tests, 0 failures)
- [ ] Manual test on Windows with large reactor to verify MCLEAN-102 fix
- [ ] Manual test: kill build mid-clean, verify leftovers cleaned on next run
- [ ] Manual test with `-T 4C` parallel build to verify session-scoped
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