Howdy, I would just reflect on a funny fact (for me at least):
This whole problem (being solved on this thread), stems from one single thing: the unsolicited install that Maven 4 does (cf this to user invoking `mvn install` explicitly). Moreover, this unsolicited install happens, for one thing, to make the resume `-r` feature work. And the true irony is, that this resume feature is backed and advertised by folks, whose mantra is "do not `mvn install` but `mvn verify`" (as install "pollutes' your local repository", whatever that means). That mantra can be now extended with "... as Maven 4 will install it, even if you did not ask for it" :) The more I think about it, the more I find this funny. Thanks T On Tue, 4 Aug 2026 at 14:56, Guillaume Nodet <[email protected]> wrote: > > I like the .mvn/target/project-local-repo which solves the problem, > and also provides a good location where plugins could move some > temporary data files without being disturbed by the clean plugin. I'm > thinking about the flatten plugin, the release plugin, and probably > more. > > Le lun. 3 août 2026 à 16:47, Slawomir Jaranowski > <[email protected]> a écrit : > > > > Hi, > > > > On Mon, 3 Aug 2026 at 14:09, Guillaume Nodet <[email protected]> > > wrote: > > > > > > 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? > > > > maybe .mvn/target/project-local-repo > > it should be ignored by git > > > > > - 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 > > > > > > > > -- > > Sławomir Jaranowski > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: [email protected] > > For additional commands, e-mail: [email protected] > > > > > -- > ------------------------ > Guillaume Nodet > > --------------------------------------------------------------------- > To unsubscribe, e-mail: [email protected] > For additional commands, e-mail: [email protected] > --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
