gnodet commented on PR #12832:
URL: https://github.com/apache/maven/pull/12832#issuecomment-5435493220
Thanks for the fix — the null guards are a clear improvement over the opaque
NPE. One thing worth addressing in this PR though:
## Root cause: `HashMap` corruption under parallel builds (`-T`)
The most likely real-world trigger for this NPE is **concurrent access to
the `projects` map in `ProjectIndex`**. In
`MojoExecutor.executeForkedExecutions()`, the `projects` map is both read
(`.get()` at line 443) and written (`.put()` at lines 460, 472) — and in
parallel builds, multiple threads do this concurrently through the same cached
`ProjectIndex` instance.
Plain `HashMap` is not thread-safe for concurrent read/write. The internal
structure can silently corrupt (especially during resize), causing `.get()` to
return `null` for a key that **is** in the map. In that scenario, the
`projectId` is valid and present — it's the `HashMap` corruption that makes it
appear missing.
With only the null guards, the build still **fails** in this case — just
with a better message. But it shouldn't fail at all.
### Suggested additional change
In `ProjectIndex.java`, change the `projects` map from `HashMap` to
`ConcurrentHashMap`:
```java
public ProjectIndex(List<MavenProject> projects) {
this.projects = new ConcurrentHashMap<>(projects.size() * 2);
this.indices = new HashMap<>(projects.size() * 2); // read-only after
construction, safe as-is
...
}
```
The `indices` map can stay as `HashMap` — it's never written to after
construction, so it's safe for concurrent reads.
This way:
- **Parallel builds**: `ConcurrentHashMap` prevents the corruption → build
succeeds (root cause fixed)
- **Stale cached index / extensions**: the null guards in `MojoExecutor`
catch genuinely missing keys → clear error (your fix)
Both changes together give a complete fix. What do you think?
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