elharo opened a new pull request, #12932:
URL: https://github.com/apache/maven/pull/12932
Fixes #10598 (MNG-8174)
### Problem
When a POM property references itself (e.g. `<prop>${prop}</prop>`) and that
property is also used inside a plugin configuration attribute such as
`<filter token="key" value="${prop}"/>`, invoking the model interpolation
throws
```
java.lang.NullPointerException: Attribute value can not be null
at org.codehaus.plexus.util.xml.Xpp3Dom.setAttribute(Xpp3Dom.java:179)
at
...StringVisitorModelInterpolator$ModelVisitor.visit(StringVisitorModelInterpolator.java:727)
```
A recursive expression cycle is detected by the interpolator, which then
returns `null` for the unresolved expression. `visit(Xpp3Dom)` in
`StringVisitorModelInterpolator` unconditionally called
`dom.setAttribute(attr, val)` (and `dom.setValue(val)`) even when the
interpolated value was `null`, causing the NPE. The `visit(Properties)`
method already guarded against `null` in the identical situation.
### Fix
When interpolation of an Xpp3Dom content value or attribute returns `null`
(which happens on a recursive expression cycle), leave the original value
untouched instead of writing `null` into the DOM. This mirrors the existing
guard in `visit(Properties)`.
### Test
Added `testRecursiveExpressionCycleInPluginConfiguration` to
`AbstractModelInterpolatorTest`, which builds a model with a
self-referencing property and a plugin configuration attribute referencing
it. Before the fix the test fails with the NPE above; after the fix the
interpolation completes, reports the recursive-expression-cycle error and
the attribute keeps its original value.
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