Introduce NoOpIndexReader to handle missing indexes without errors (#2113)

We stumbled upon errors accessing index from plugins which were not yet initialized. To prevent this errors we use our own No-Op IndexReader for missing indices.

gradle/changelog/missing-indexes.yaml

@@ -0,0 +1,2 @@
+- type: fixed
+  description: Handle missing indexes with own No Op IndexReader ([#2113](https://github.com/scm-manager/scm-manager/pull/2113))

scm-webapp/src/main/java/sonia/scm/search/IndexManager.java

@@ -83,7 +83,11 @@ public class IndexManager {
 
   public IndexReader openForRead(LuceneSearchableType type, String indexName) throws IOException {
     Path path = resolveIndexDirectory(type, indexName);
-    return DirectoryReader.open(FSDirectory.open(path));
+    FSDirectory fsDirectory = FSDirectory.open(path);
+    if (DirectoryReader.indexExists(fsDirectory)) {
+      return DirectoryReader.open(fsDirectory);
+    }
+    return new NoOpIndexReader();
   }
 
   public IndexWriter openForWrite(IndexParams indexParams) {

scm-webapp/src/main/java/sonia/scm/search/NoOpIndexReader.java

@@ -0,0 +1,177 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020-present Cloudogu GmbH and Contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package sonia.scm.search;
+
+import org.apache.lucene.index.CompositeReader;
+import org.apache.lucene.index.Fields;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.StoredFieldVisitor;
+import org.apache.lucene.index.Term;
+import org.apache.lucene.index.Terms;
+import org.apache.lucene.index.TermsEnum;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+
+@SuppressWarnings("java:S2272") // Exception obsolete
+public class NoOpIndexReader extends CompositeReader {
+
+  public NoOpIndexReader() {
+    super();
+  }
+
+  @Override
+  protected List<? extends IndexReader> getSequentialSubReaders() {
+    return Collections.emptyList();
+  }
+
+  @Override
+  public Fields getTermVectors(int docID) {
+    return new Fields() {
+      @Override
+      public Iterator<String> iterator() {
+        return new Iterator<>() {
+          @Override
+          public boolean hasNext() {
+            return false;
+          }
+
+          @Override
+          public String next() {
+            return "";
+          }
+        };
+      }
+
+      @Override
+      public Terms terms(String field) {
+        return new Terms() {
+          @Override
+          public TermsEnum iterator() {
+            return null;
+          }
+
+          @Override
+          public long size() {
+            return 0;
+          }
+
+          @Override
+          public long getSumTotalTermFreq() {
+            return 0;
+          }
+
+          @Override
+          public long getSumDocFreq() {
+            return 0;
+          }
+
+          @Override
+          public int getDocCount() {
+            return 0;
+          }
+
+          @Override
+          public boolean hasFreqs() {
+            return false;
+          }
+
+          @Override
+          public boolean hasOffsets() {
+            return false;
+          }
+
+          @Override
+          public boolean hasPositions() {
+            return false;
+          }
+
+          @Override
+          public boolean hasPayloads() {
+            return false;
+          }
+        };
+      }
+
+      @Override
+      public int size() {
+        return 0;
+      }
+    };
+  }
+
+  @Override
+  public int numDocs() {
+    return 0;
+  }
+
+  @Override
+  public int maxDoc() {
+    return 0;
+  }
+
+  @Override
+  public void document(int docID, StoredFieldVisitor visitor) throws IOException {
+    // do nothing
+  }
+
+  @Override
+  protected void doClose() {
+    // do nothing
+  }
+
+  @Override
+  public IndexReader.CacheHelper getReaderCacheHelper() {
+    return null;
+  }
+
+  @Override
+  public int docFreq(Term term) {
+    return 0;
+  }
+
+  @Override
+  public long totalTermFreq(Term term) {
+    return 0;
+  }
+
+  @Override
+  public long getSumDocFreq(String field) {
+    return 0;
+  }
+
+  @Override
+  public int getDocCount(String field) {
+    return 0;
+  }
+
+  @Override
+  public long getSumTotalTermFreq(String field) {
+    return 0;
+  }
+
+}

scm-webapp/src/test/java/sonia/scm/search/IndexManagerTest.java

@@ -173,6 +173,14 @@ class IndexManagerTest {
     }
   }
 
+  @Test
+  void shouldOpenNoOpIndexReaderForMissingIndex() throws IOException {
+    open(Songs.class, "idx-for-read");
+    try (IndexReader reader = indexManager.openForRead(searchableType, "idx-for-read")) {
+      assertThat(reader).isInstanceOf(NoOpIndexReader.class);
+    }
+  }
+
   private void addDoc(IndexWriter writer, String name) throws IOException {
     Document doc = new Document();
     doc.add(new TextField("hitchhiker", name, Field.Store.YES));