slachiewicz opened a new issue, #13827:
URL: https://github.com/apache/maven/issues/13827
### Question
Should Maven offer plugins a way to run work on another thread inside the
current mojo execution scope, or is the expectation that plugins never let
scoped components be created off the Mojo thread?
### Case
`maven-site-plugin`'s `site:run` serves pages from the JDK HTTP server's own
threads, after `execute()` has started the server. A Doxia parser contributed
by a site dependency, the Asciidoctor one, injects `MavenProject`. Parsers are
looked up per document, so it is instantiated on a server thread and fails with
```
OutOfScopeException: Cannot access Key[type=MavenProject, annotation=[none]]
outside of a scoping block
at MojoExecutionScopeModule.configure(MojoExecutionScopeModule.java:47)
```
`site:site` works, since it renders on the Mojo thread
(apache/maven-site-plugin#987, MSITE-849).
`MojoExecutionScope` keeps its state in a `ThreadLocal` (3.9.x
`org.apache.maven.execution.scope.internal`, 4.x `org.apache.maven.impl.di`).
`SessionScope` does not; it uses a `CopyOnWriteArrayList`, so session-scoped
components work from any thread. Plugins see only
`org.apache.maven.execution.scope` (the annotations and
`WeakMojoExecutionListener`), so they cannot enter or seed the mojo execution
scope themselves.
### What the plugin does now
apache/maven-site-plugin#1350 keeps the server's thread pool for requests
and hands only document rendering to the Mojo thread, which loops over a queue
until the server stops. It works with every Maven version, but it serializes
rendering and any plugin starting a server or a worker pool has to rediscover
it.
### Options
1. **Context propagation API.** In the exported
`org.apache.maven.execution.scope` package (and the Maven 4 API equivalent),
something that captures the current mojo execution scope on the Mojo thread and
returns a wrapper (`Runnable`/`Callable`/`Executor`) that enters the captured
state on whichever thread runs the task and exits it afterwards.
2. **Guidance only.** Document that mojo-execution-scoped objects are only
available on the Mojo thread, and that components meant to be created by
plugins on other threads must not inject them; the producer of the component
(here Doxia Sitetools) should pass what is needed explicitly. That is being
discussed for this case in apache/maven-doxia-sitetools#736.
3. **Something else** that already exists and I missed.
The site plugin needs option 2 or its current workaround anyway while it
supports Maven 3.9. The question is whether option 1 is worth adding for 3.10
or 4.x, or whether option 2 is the intended model.
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