dashorst opened a new issue, #13590:
URL: https://github.com/apache/maven/issues/13590
### Affected version
4.0.0-rc-7, and 4.0.0-SNAPSHOT 4.0.0-20261016.214020-597
(084c28d756eebbb1cf651ea9b2efd376b724ee01)
### Bug description
With `-b concurrent` and two or more threads,
`maven-compiler-plugin:compile` can get a classpath that misses the module's
dependencies. In the reproducer below this happens on every run. Maven 3.9.16
and the default builder of Maven 4 build the same project fine.
The trigger appears to be a mojo that requires dependency collection or
resolution for a *different* scope and is bound to a phase that the concurrent
builder runs in parallel with `compile`. In our case that is
`flatten-maven-plugin:flatten` (`requiresDependencyCollection = runtime`),
bound to `process-resources`. The build plan shows both steps for the same
project:
```
Phase: after:resources
Goal: org.codehaus.mojo:flatten-maven-plugin:1.8.0:flatten (flatten)
Dependencies (collect): runtime
Phase: compile
Goal: org.apache.maven.plugins:maven-compiler-plugin:3.16.0:compile
(default-compile)
Dependencies (resolve): compile
```
The dependency trees that `-X` logs for the module are complete, but the
classpath handed to javac only contains the runtime-scoped artifacts. In larger
builds it sometimes also lacks compile-scoped artifacts. So the project's
artifact set seems to be replaced by the result of the other step's collection
while `compile` uses it. Because the collection happens before the mojo
executes, skipping the mojo (`-Dflatten.skip=true`) does not help.
#### Reproducer
`pom.xml`:
```xml
<project xmlns="http://maven.apache.org/POM/4.0.0";>
<modelVersion>4.0.0</modelVersion>
<groupId>repro</groupId>
<artifactId>parent</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>pom</packaging>
<modules>
<module>module</module>
</modules>
<properties>
<maven.compiler.release>21</maven.compiler.release>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>flatten-maven-plugin</artifactId>
<version>1.8.0</version>
<executions>
<execution>
<id>flatten</id>
<phase>process-resources</phase>
<goals>
<goal>flatten</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
```
`module/pom.xml`:
```xml
<project xmlns="http://maven.apache.org/POM/4.0.0";>
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>repro</groupId>
<artifactId>parent</artifactId>
<version>1.0-SNAPSHOT</version>
</parent>
<artifactId>module</artifactId>
<dependencies>
<dependency>
<groupId>jakarta.ws.rs</groupId>
<artifactId>jakarta.ws.rs-api</artifactId>
<version>3.1.0</version>
<scope>provided</scope>
</dependency>
</dependencies>
</project>
```
`module/src/main/java/repro/Uses.java`:
```java
package repro;
import jakarta.ws.rs.core.Response;
public class Uses {
Class<?> type = Response.class;
}
```
Run `mvn clean package -b concurrent -T 2`:
```
[ERROR] module/src/main/java/repro/Uses.java:[3,26] package
jakarta.ws.rs.core does not exist
```
Results, 5 runs each (`clean package`, JDK 21, Linux):
| Command | Result |
|---|---|
| Maven 3.9.16, `-T 16` | 5/5 pass |
| Maven 4, default builder, `-T 16` | 5/5 pass |
| Maven 4, `-b concurrent -T 1` | 5/5 pass |
| Maven 4, `-b concurrent -T 2`, `-T 4`, `-T 16` | 5/5 fail |
| Maven 4, `-b concurrent -T 16`, flatten bound to `process-classes` | 5/5
pass |
Some observations from narrowing it down:
- With `clean compile` instead of `clean package` the reproducer passes.
- A single-module project with the same plugin and dependency passes; it
needs the parent/module reactor.
- Removing the flatten plugin makes it pass.
In a 67-module project the same problem surfaced as random modules missing
`provided` and also some compile-scoped dependencies, depending on timing.
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