Re: [DISCUSS] Fix for #12646 — moving pro ject-local-repo out of target/

Hi,

I strongly think, we should keep in somewhere in the project folder (so 
no m2 etc.) to keep it visible and show that it's temp. Similar to 
Slawek: Having a "/target" will keep it out of git, with common 
gitignores, so what about something like "project-local-repo/target" - 
so it's clear on high level what the folder is (and not hided in 
optional configuration .mvn folder) and due /target it's not added to git.

Matthias

Am 03.08.2026 um 14:09 schrieb Guillaume Nodet:
> Hi all,
>
> I'd like to get your input on the fix for GH-12646 — a race condition with
> project-local-repo during parallel clean install builds.
>
> The problem
>
> When the root pom.xml has a parent that is also part of the reactor (e.g. a
> super-pom), MultiThreadedBuilder schedules the parent first, then runs the
> root project and child modules concurrently. Since clean and install are
> placed into the same TaskSegment, there is no barrier between them — the
> root project's maven-clean-plugin can delete target/ while sibling modules
> are concurrently writing artifacts into target/project-local-repo, causing
> the build to fail.
>
> Approaches considered
>
> 1. Lock-based synchronization (ReentrantReadWriteLock in ReactorReader)
>
> Acquire a write lock when the project owning target/ enters its clean
> phase, and a read lock when installing artifacts. This prevents the crash
> (concurrent access) but does not guarantee ordering — a module could
> install artifacts before the root's clean starts, only to have them wiped.
> It also adds complexity to ReactorReader for what is fundamentally an
> architectural problem.
>
> 2. Move project-local-repo to ~/.m2/ (user home)
>
> E.g. ~/.m2/local-repository/${projectName}_${hash}, similar to how IntelliJ
> stores project caches. This avoids both the race and the git concern, but:
> - Hard links (already used by ReactorReader) cannot cross filesystem
> boundaries — if ~/.m2/ is on a different volume, every artifact becomes a
> full copy
> - Stale directories accumulate when projects are deleted/moved, with no
> cleanup mechanism
> - CI containers often share ~/.m2/ across builds, causing unwanted
> accumulation
> - Loses project locality (harder to inspect/debug)
>
> 3. Move project-local-repo to .mvn/project-local-repo (chosen)
>
> Move the directory from target/project-local-repo to
> .mvn/project-local-repo. Since maven-clean-plugin only deletes target/, the
> race becomes structurally impossible. ReactorReader fully owns the
> lifecycle — per-GAV cleanup when a project enters its clean phase, install
> on project success. The existing hard-link optimization continues to work
> since both directories are on the same filesystem.
>
> The trade-off is that .mvn/project-local-repo needs to be gitignored. This
> follows the Gradle convention where .gradle/ at the project root is
> universally in .gitignore templates. Maven could also document this
> convention or consider auto-appending the entry.
>
> The fix itself is minimal — the core change is a single line in
> getProjectLocalRepo(), and the rest is removing the lock infrastructure
> (net -40 lines).
>
> 4. Keep in target/ but use maven-clean-plugin fast mode
>
> The fast clean option (maven.clean.fast=true, since plugin 3.2) atomically
> renames target/ instead of recursively deleting it. This would likely avoid
> the race, but it's opt-in and not the default — users hitting the bug would
> need to know about it.
>
> FWIW, a PR has been raised on m-clean-p master branch (4.x) to refactor the
> fast cleaner (fixing problems) and make it the default.
>
> Open questions
>
> - Is .mvn/ the right location, or should we consider a different
> project-root directory?
> - Should Maven auto-add .mvn/project-local-repo to .gitignore, or document
> it as a convention?
> - Are there other considerations I'm missing?
>
> PR: https://github.com/apache/maven/pull/12650
> Issue: https://github.com/apache/maven/issues/12646
>
> Looking forward to your thoughts.
>
>   Cheers
> Guillaume Nodet
>