Pipelining prevents Java compilation with compileOrder := JavaThenScala config

[guersam]

Original submitted as sbt/sbt#7546, but I think it's a Zinc issue.

steps

1. Checkout https://github.com/guersam/sbt-debug-pipelined-hybrid-project.
2. Run $ sbt run.

problem

It fails due to a missing Java class. The incremental compilation log shows that the Java source code is not compiled.

expectation

It prints:

Hello world!
42

notes

1. Both usePipelining := true and compileOrder := CompilerOrder.JavaThenScala settings are needed to reproduce
2. It's reproducible both in sbt 1.9.9 and 1.10.0-RC2.
3. It happens with both Scala 2 and 3.
4. CI result with sbt 'last compileIncremental' log

[info] welcome to sbt 1.10.0-RC2 (Eclipse Adoptium Java 11.0.22)
[info] loading project definition from /home/runner/work/sbt-debug-pipelined-hybrid-project/sbt-debug-pipelined-hybrid-project/project
[info] loading settings for project root from build.sbt ...
[info] set current project to sbt-pipelining-debug (in build file:/home/runner/work/sbt-debug-pipelined-hybrid-project/sbt-debug-pipelined-hybrid-project/)
[debug] [zinc] IncrementalCompile -----------
[debug] IncrementalCompile.incrementalCompile
[debug] previous = Stamps for: 0 products, 0 sources, 0 libraries
[debug] current source = Set(${BASE}/src/main/java/example/JavaClass.java, ${BASE}/src/main/scala/example/Main.scala)
[debug] > initialChanges = InitialChanges(Changes(added = Set(${BASE}/src/main/java/example/JavaClass.java, ${BASE}/src/main/scala/example/Main.scala), removed = Set(), changed = Set(), unmodified = ...),Set(),Set(),API Changes: Set())
[debug] Full compilation, no sources in previous analysis.
[debug] all 2 sources are invalidated
[debug] Initial set of included nodes: 
[debug] Recompiling all sources: number of invalidated sources > 50.0 percent of all sources
[debug] compilation cycle 1
[info] compiling 1 Scala source and 1 Java source to /home/runner/work/sbt-debug-pipelined-hybrid-project/sbt-debug-pipelined-hybrid-project/target/scala-3.4.1/classes ...
[debug] Returning already retrieved and compiled bridge: /home/runner/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-sbt-bridge/3.4.1/scala3-sbt-bridge-3.4.1.jar.
[debug] [zinc] Running cached compiler 5[9](https://github.com/guersam/sbt-debug-pipelined-hybrid-project/actions/runs/8747771673/job/24006749488#step:5:10)c3d61b for Scala Compiler version 3.4.1
[debug] [zinc] The Scala compiler is invoked with:
[debug] 	-classpath
[debug] 	/home/runner/work/sbt-debug-pipelined-hybrid-project/sbt-debug-pipelined-hybrid-project/target/scala-3.4.1/classes:/home/runner/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-library_3/3.4.1/scala3-library_3-3.4.1.jar:/home/runner/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.[12](https://github.com/guersam/sbt-debug-pipelined-hybrid-project/actions/runs/8747771673/job/24006749488#step:5:13)/scala-library-2.13.12.jar
[debug] Compilation failed
[error] (Compile / compileIncremental) Compilation failed

[guersam]

Without compilerOrder setting it compiles but the incremental compilation log is similar.

[debug] [zinc] IncrementalCompile -----------
[debug] IncrementalCompile.incrementalCompile
[debug] previous = Stamps for: 4 products, 2 sources, 1 libraries
[debug] current source = Set(${BASE}/src/main/java/example/JavaClass.java, ${BASE}/src/main/scala/example/Main.scala)
[debug] > initialChanges = InitialChanges(Changes(added = Set(), removed = Set(), changed = Set(), unmodified = ...),Set(${BASE}/target/scala-2.13/classes/example/JavaClass.class, ${BASE}/target/scala-2.13/classes/example/Main$.class, ${BASE}/target/scala-2.13/classes/example/Main$delayedInit$body.class, ${BASE}/target/scala-2.13/classes/example/Main.class),Set(),API Changes: Set())
[debug] 
[debug] Initial source changes:
[debug]         removed: Set()
[debug]         added: Set()
[debug]         modified: Set()
[debug] Invalidated products: Set(${BASE}/target/scala-2.13/classes/example/JavaClass.class, ${BASE}/target/scala-2.13/classes/example/Main$.class, ${BASE}/target/scala-2.13/classes/example/Main$delayedInit$body.class, ${BASE}/target/scala-2.13/classes/example/Main.class)
[debug] External API changes: API Changes: Set()
[debug] Modified binary dependencies: Set()
[debug] Initial directly invalidated classes: Set()
[debug] Sources indirectly invalidated by:
[debug]         product: Set(${BASE}/src/main/java/example/JavaClass.java, ${BASE}/src/main/scala/example/Main.scala)
[debug]         binary dep: Set()
[debug]         external source: Set()
[debug] all 2 sources are invalidated
[debug] Initial set of included nodes: 
[debug] Recompiling all sources: number of invalidated sources > 50.0 percent of all sources
[debug] compilation cycle 1
[info] compiling 1 Scala source and 1 Java source to /home/guersam/playground/sbt-pipeline-debug/sbt-pipelining-debug/target/scala-2.13/classes ...
[debug] Getting org.scala-sbt:compiler-bridge_2.13:1.10.0-RC2:compile for Scala 2.13.11
[debug] [zinc] Running cached compiler 21021528 for Scala compiler version 2.13.11
[debug] [zinc] The Scala compiler is invoked with:
[debug]         -classpath
[debug]         /home/guersam/playground/sbt-pipeline-debug/sbt-pipelining-debug/target/scala-2.13/classes:/home/guersam/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.11/scala-library-2.13.11.jar
[debug] Scala compilation took 0.213943626 s
[debug] done compiling Scala sources

[guersam]

I think I found the cause:

1. When pipelining is enabled, Zinc triggers Scala compilation regardless of the compile order setting. (source)
2. Only the scala sources are included for compile because the compiler order is not Mixed. (source)