Add null guards (#4542)

While the `@NotNull` decorator informs developers that the correct usage
is that the decorated variables should not be `null`, it does not enforce
such in any way.
This leads to cases where an incorrectly fed `null` could propagate far
away before being detected (such as via `NullPointerException`s).
This commit implements some `null` guards where needed for a fail-fast
treatment of `null` inputs.

opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/OpenGrokThreadFactory.java

@@ -26,6 +26,7 @@
 
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
+import java.util.Objects;
 
 /**
  * ThreadFactory to be used throughout OpenGrok to make sure all threads have common prefix.
@@ -45,7 +46,7 @@ public OpenGrokThreadFactory(String name) {
 
     @Override
     public Thread newThread(@NotNull Runnable runnable) {
-        Thread thread = Executors.defaultThreadFactory().newThread(runnable);
+        Thread thread = Executors.defaultThreadFactory().newThread(Objects.requireNonNull(runnable, "runnable"));
         thread.setName(PREFIX + threadPrefix + thread.getId());
         return thread;
     }

opengrok-indexer/src/main/java/org/opengrok/indexer/history/HistoryReader.java

@@ -26,6 +26,7 @@
 import java.io.Reader;
 import java.io.StringReader;
 import java.util.List;
+import java.util.Objects;
 
 import org.jetbrains.annotations.NotNull;
 import org.opengrok.indexer.util.IOUtils;
@@ -47,7 +48,7 @@ public int read(char @NotNull [] cbuf, int off, int len) throws IOException {
         if (input == null) {
             input = createInternalReader();
         }
-        return input.read(cbuf, off, len);
+        return input.read(Objects.requireNonNull(cbuf, "cbuf"), off, len);
     }
 
     @Override

opengrok-indexer/src/main/java/org/opengrok/indexer/index/IndexCheck.java

@@ -36,6 +36,7 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
@@ -101,7 +102,7 @@ public enum IndexCheckMode {
      * @param configuration configuration based on which to perform the check
      */
     public IndexCheck(@NotNull Configuration configuration) {
-        this(configuration, null);
+        this(configuration, null); // configuration is guarded against null inside this constructor
     }
 
     /**
@@ -111,7 +112,7 @@ public IndexCheck(@NotNull Configuration configuration) {
      *                     on whether projects are enabled in the configuration.
      */
     public IndexCheck(@NotNull Configuration configuration, Collection<String> projectNames) {
-        this.configuration = configuration;
+        this.configuration = Objects.requireNonNull(configuration, "configuration");
         if (projectNames != null) {
             this.projectNames.addAll(projectNames);
         }

opengrok-indexer/src/main/java/org/opengrok/indexer/index/IndexDatabase.java

@@ -2124,7 +2124,7 @@ private static class CountingWriter extends Writer {
 
         @Override
         public void write(@NotNull char[] chars, int off, int len) throws IOException {
-            out.write(chars, off, len);
+            out.write(Objects.requireNonNull(chars), off, len);
             count += len;
         }
 

opengrok-indexer/src/test/java/org/opengrok/indexer/configuration/OpenGrokThreadFactoryTest.java

@@ -0,0 +1,38 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * See LICENSE.txt included in this distribution for the specific
+ * language governing permissions and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at LICENSE.txt.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Portions Copyright (c) 2024, Heewon Lee <heewon.lee@kaist.ac.kr>.
+ */
+package org.opengrok.indexer.configuration;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class OpenGrokThreadFactoryTest {
+    @Test
+    void testNullRunnable() throws Exception {
+        assertThrows(NullPointerException.class, () -> {
+            OpenGrokThreadFactory factory = new OpenGrokThreadFactory("");
+            factory.newThread(null);
+        });
+    }
+}

opengrok-indexer/src/test/java/org/opengrok/indexer/history/HistoryReaderTest.java

@@ -0,0 +1,38 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * See LICENSE.txt included in this distribution for the specific
+ * language governing permissions and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at LICENSE.txt.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Portions Copyright (c) 2024, Heewon Lee <heewon.lee@kaist.ac.kr>.
+ */
+package org.opengrok.indexer.history;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class HistoryReaderTest {
+    @Test
+    void testNullCbuf() throws Exception {
+        assertThrows(NullPointerException.class, () -> {
+            HistoryReader historyReader = new HistoryReader(new History());
+            historyReader.read(null, 0, 0);
+        });
+    }
+}

opengrok-indexer/src/test/java/org/opengrok/indexer/index/IndexCheckTest.java

@@ -255,4 +255,12 @@ void testMultipleTestsWithSameInstance() throws Exception {
             }
         }
     }
+
+    @Test
+    void testNullConfiguration() throws Exception {
+        assertThrows(NullPointerException.class, () -> {
+            new IndexCheck(null);
+            }
+        );
+    }
 }