Workaround for "m2e plugin sometimes 'loses' resources"

The Java Builder may delete files from the project output directory that
need to be re-created by the m2e Maven Builder.

With commit 8e5cd49,
any changes to the project output directory were ignored, leading to
unexpected errors when running an application after modifying the
project POM: resources were missing from the target classpath, leading
all sorts of unexpected program behavior.

This change allows marking these changes as relevant, as long as the
following conditions are met:

- The expected resource no longer exists (i.e., it was deleted by
  another builder, plugin or outside process)
- The modification applies to a resource in the classes or test-classes
  folder (i.e., outputLocation/testOutputLocation)

See #1511
See #1275

org.eclipse.m2e.core/META-INF/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %Bundle-Name
 Bundle-SymbolicName: org.eclipse.m2e.core;singleton:=true
-Bundle-Version: 2.4.1.qualifier
+Bundle-Version: 2.4.2.qualifier
 Bundle-Activator: org.eclipse.m2e.core.internal.MavenPluginActivator
 Bundle-Vendor: %Bundle-Vendor
 Bundle-Localization: plugin

org.eclipse.m2e.core/pom.xml

@@ -19,7 +19,7 @@
 	</parent>
 
 	<artifactId>org.eclipse.m2e.core</artifactId>
-	<version>2.4.1-SNAPSHOT</version>
+	<version>2.4.2-SNAPSHOT</version>
 	<packaging>eclipse-plugin</packaging>
 
 	<name>Maven Integration for Eclipse Core Plug-in</name>

org.eclipse.m2e.core/src/org/eclipse/m2e/core/internal/builder/MavenBuilderImpl.java

@@ -29,6 +29,7 @@
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Predicate;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -209,6 +210,11 @@ private boolean hasRelevantDelta(IMavenProjectFacade projectFacade, IResourceDel
     if(project == null || buildOutputLocation == null) {
       return true;
     }
+
+    Predicate<IPath> isOutput = toPrefixPredicate(projectFacade.getOutputLocation());
+    Predicate<IPath> isTestOutput = toPrefixPredicate(projectFacade.getTestOutputLocation());
+    Predicate<IPath> isOutputOrTestOutput = isOutput.or(isTestOutput);
+
     IPath projectPath = project.getFullPath();
     List<IPath> moduleLocations = projectFacade.getMavenProjectModules().stream()
         .map(module -> projectPath.append(module)).toList();
@@ -219,7 +225,11 @@ private boolean hasRelevantDelta(IMavenProjectFacade projectFacade, IResourceDel
         IPath fullPath = delta.getFullPath();
         if(buildOutputLocation.isPrefixOf(fullPath)) {
           //anything in the build output is not interesting for a change as it is produced by the build
-          //lets see if there are more interesting parts...
+          // ... unless a classpath resource that existed before has been deleted, possibly by another builder
+          if(isOutputOrTestOutput.test(fullPath) && !resource.exists()) {
+            hasRelevantDelta.set(true);
+            return false;
+          }
           return true;
         }
         for(IPath modulePath : moduleLocations) {
@@ -237,6 +247,13 @@ private boolean hasRelevantDelta(IMavenProjectFacade projectFacade, IResourceDel
     return hasRelevantDelta.get();
   }
 
+  private static Predicate<IPath> toPrefixPredicate(IPath location) {
+    if(location == null) {
+      return (p) -> false;
+    }
+    return (p) -> location.isPrefixOf(p);
+  }
+
   private List<IIncrementalBuildFramework.BuildContext> setupProjectBuildContext(IProject project, int kind,
       IResourceDelta delta, IIncrementalBuildFramework.BuildResultCollector results, ProjectBuildState buildState)
       throws CoreException {