Add MFile::createIfExists to fix AggDataset relying on null MFile for…

… nonexisting file

cdm/core/src/main/java/thredds/inventory/MFiles.java

@@ -8,6 +8,7 @@
 import java.io.IOException;
 import java.util.ServiceLoader;
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import thredds.filesystem.MFileOS;
@@ -48,4 +49,21 @@ public static MFile create(@Nonnull String location) {
     }
     return new MFileOS(location);
   }
+
+  /**
+   * Create an {@link thredds.inventory.MFile} from a given location if it exists, otherwise return
+   * null.
+   *
+   * @param location location of file (local or remote) to be used to back the MFile object
+   * @return {@link thredds.inventory.MFile}
+   */
+  @Nullable
+  public static MFile createIfExists(String location) {
+    if (location == null) {
+      return null;
+    }
+
+    final MFile mFile = create(location);
+    return mFile.exists() ? mFile : null;
+  }
 }

cdm/core/src/main/java/ucar/nc2/internal/ncml/AggDataset.java

@@ -72,7 +72,7 @@ protected AggDataset(MFile mfile, @Nullable Object spiObject, @Nullable Element
   protected AggDataset(String cacheLocation, String location, @Nullable String id,
       @Nullable EnumSet<Enhance> wantEnhance, @Nullable ucar.nc2.util.cache.FileFactory reader,
       @Nullable Object spiObject, @Nullable Element ncmlElem) {
-    this.mfile = location == null ? null : MFiles.create(location);
+    this.mfile = MFiles.createIfExists(location);
     this.cacheLocation = cacheLocation;
     this.id = id;
     this.enhance = (wantEnhance == null) ? NetcdfDataset.getEnhanceNone() : wantEnhance;