[MSHADE-329] Concurrent writes of dependency-reduced-pom.xml adds same exclusion ad infinitum

**[Håkon Hallingstad](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hakonhall)** opened **[MSHADE-329](https://issues.apache.org/jira/browse/MSHADE-329?redirect=false)** and commented

I have a multi-threaded `mvn install` that seems to halt but ends up using 200% CPU, in about 50% of the invocations. The `mvn` command used is:

```
mvn -T1C -nsu -Dmaven.source.skip -Dmaven.javadoc.skip -Dmaven.test.skip install -rf :MODULE
```

Using `mvnDebug` I have found out that there are 2 running Java threads, each writing `dependency-reduced-pom.xml` in two different modules `A` and `B`, respectively. These files seems to become several MB large, before they're deleted and then written again, and so forth.

I have looked into one of the threads, and there is a `rewriteDependencyReducedPomIfWeHaveReduction` in ShadeMojo with a `loopCounter` with value 2735, that just keeps increasing. Presumably there is something like one dependency-reduced-pom.xml written per iteration.

From the source code it seems this can only happen if https://github.com/apache/maven-shade-plugin/blob/master/src/main/java/org/apache/maven/plugins/shade/mojo/ShadeMojo.java#L1172 is hit at least that number of times, meaning the ShadeMojo adds that many exclusions, which seems to correspond to hamcrest-core:

```

 
    junit
    junit
     4.12
    test
   
     
        hamcrest-core
        org.hamcrest
     
     
        hamcrest-core
        org.hamcrest
     
     ...
```

```
grep hamcrest-core dependency-reduced-pom.xml | wc -l
2735
```

 

The same exclusion is added `loopCounter` times in `updateExcludesInDeps`.

There is no problem running `mvn` in single-thread mode.

---

**Affects:** 3.2.1

**Attachments:**
- [mshade-329-thread-dump.txt](https://issues.apache.org/jira/secure/attachment/13018805/mshade-329-thread-dump.txt) (_34.15 kB_)

**Issue Links:**
- [MSHADE-124](https://issues.apache.org/jira/browse/MSHADE-124) Need better plan for getting dependency-reduced-pom.xml out of basedir

- [FLINK-14318](https://issues.apache.org/jira/browse/FLINK-14318) JDK11 build stalls during shading

1 votes, 5 watchers

Start with ShadeMojo.java, especially rewriteDependencyReducedPomIfWeHaveReduction and updateExcludesInDeps, then reproduce with the reported parallel mvn install command or inspect mshade-329-thread-dump.txt. Check how concurrent writes to dependency-reduced-pom.xml cause repeated hamcrest-core exclusions; done means the parallel build no longer loops or produces unbounded duplicate exclusions.

Written by the indexing model from the issue text.