"poison" DefaultFileSystemHolder.FS on hosted

Author: zapster

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/runtimeinit/FileSystemProviderRuntimeInitSupport.java

@@ -38,6 +38,7 @@
 import com.oracle.svm.core.jdk.JDKInitializedAtRunTime;
 import com.oracle.svm.core.util.BasedOnJDKFile;
 import com.oracle.svm.core.util.VMError;
+import com.oracle.svm.util.ReflectionUtil;
 
 /**
  * This file contains substitutions that are required for initializing {@link FileSystemProvider} at
@@ -79,11 +80,19 @@ final class Target_java_io_DefaultFileSystem_RunTime {
 @BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-25+25/src/java.base/unix/classes/java/io/DefaultFileSystem.java#L39-L41")
 @BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-25+25/src/java.base/windows/classes/java/io/DefaultFileSystem.java#L39-L41")
 class DefaultFileSystemHolder {
-    static final Target_java_io_FileSystem_RunTime FS;
+    static final Object FS;
     static {
         if (SubstrateUtil.HOSTED) {
-            // unused - layered images might want to initialize during image build
-            FS = null;
+            /*
+             * Should be unused, but layered images might want to initialize it during image build.
+             * We set it to the real default file system instead of null to guard against
+             * unintentional usages. In run-time init mode, we don't allow FileSystems in the image
+             * heap, so we would fail the image build with an exception if it happens, which helps
+             * to detect problems.
+             */
+            var defaultFileSystem = ReflectionUtil.lookupClass("java.io.DefaultFileSystem");
+            var getFileSystem = ReflectionUtil.lookupMethod(defaultFileSystem, "getFileSystem");
+            FS = ReflectionUtil.invokeMethod(getFileSystem, null);
         } else {
             FS = Target_java_io_DefaultFileSystem_RunTime.getFileSystem();
         }
@@ -94,7 +103,7 @@ class DefaultFileSystemAccessor {
     @SuppressWarnings("unused")
     static Target_java_io_FileSystem_RunTime get() {
         VMError.guarantee(DefaultFileSystemHolder.FS != null, "DefaultFileSystemHolder.FS is null");
-        return DefaultFileSystemHolder.FS;
+        return SubstrateUtil.cast(DefaultFileSystemHolder.FS, Target_java_io_FileSystem_RunTime.class);
     }
 }