Suchabfrage nach Fehlern in Apache Solr

Wir prüfen noch einmal das Apache-Produkt. Dieses Mal haben wir uns für Solr entschieden, eine Open-Source-Suchserverplattform. Mit Solr können Sie schnell und effizient nach Informationen in Datenbanken und Online-Ressourcen suchen. Bei solch einer komplexen Aufgabe können selbst erfahrene Apache-Entwickler leicht Fehler machen. In diesem Artikel werden wir uns mit solchen Fehlern befassen.

Wer bist du, Solr?

Vor nicht allzu langer Zeit haben wir eines der bekanntesten Apache-Projekte überprüft, die NetBeans IDE. Bei der Überprüfung haben wir viele interessante Warnungen unseres Analysegeräts gefunden. Übrigens sind die Entwickler schnell auf sie aufmerksam geworden und haben vor mir einen Pull-Request gestellt :) Dieses Mal haben wir uns entschieden, einen Blick auf ein weiteres ihrer großen Produkte zu werfen, die Solr-Volltextsuchplattform.

Apache Solr wurde 2006 erstmals eingeführt und bietet eine breite Palette an Funktionen, von dynamischem Clustering und Datenbankintegration bis hin zur Verarbeitung komplex formatierter Dokumente. Diese Suchmaschine ermöglicht Ihnen die schnelle Suche und Analyse von Informationen auf einer Website und bietet außerdem die Möglichkeit, Suchserver auf Linux-Hardware zu hosten. Solr hat...

Um es kurz zu machen: Ich möchte Sie nicht mit zu vielen Details belästigen. Es reicht zu wissen, dass es sich um eine praktische Softwareplattform handelt, die Big Data optimiert. Warum schauen wir uns nicht den Quellcode an und durchsuchen ihn nach etwas Interessantem oder Ungewöhnlichem? Genau das werden wir jetzt tun.

Sie haben hier etwas durcheinander gebracht

Ein Programmierer ist nie vor Tippfehlern sicher, bei denen die Operatoren verwechselt werden. Hier ist einer davon:

public Map<String, List<String>> getIndexFilesPathForSnapshot(
    String collectionName, String snapshotName, String pathPrefix)
    throws SolrServerException, IOException {
  ....

  if (meta != null) {                  // <=
    throw new IllegalArgumentException(
                  "The snapshot named " + snapshotName +
                  " is not found for collection " + collectionName);
  }

  DocCollection collectionState = solrClient.getClusterState() 
                                            .getCollection(collectionName);
  for (Slice s : collectionState.getSlices()) {
    List<CoreSnapshotMetaData> replicaSnaps = 
                     meta.getReplicaSnapshotsForShard(s.getName());  // <=
    ....
  }
  return result;
}

Diese Methode verwendet die Variable meta, um Informationen über die System-Snapshots zu speichern. Es gibt auch eine Überprüfung, ob meta != null im obigen Codefragment vorhanden ist. Wenn ja, wird IllegalArgumentException ausgelöst. Das allein scheint bizarr. Okay, jetzt schauen wir uns den Schleifenkörper an. Hier wird getReplicaSnapshotsForShard aufgerufen. Da meta hier immer null ist, erhalten wir NullPointerException. Es scheint, dass der Entwickler nur einen Tippfehler gemacht und die Operatoren verwechselt hat. Daher sollte eine Ausnahme ausgelöst werden, wenn meta gleich null ist.

Der PVS-Studio-Analysator fungierte als Korrekturleser und meldete einen erkannten Tippfehler:

V6008 Null-Dereferenzierung von „Meta“. SolrSnapshotsTool.java 262

Ich möchte hinzufügen, dass solche Fehler mit vertauschten Operatoren häufiger vorkommen, als Sie vielleicht denken. Ich habe zum Beispiel schon mindestens zwei ähnliche im NetBeans-Projekt gesehen:

private SourcesModel getModel() {
  SourcesModel tm = model.get();
  if (tm == null) {
    tm.sourcePath.removePropertyChangeListener (this);
    tm.debugger.getSmartSteppingFilter ().
    removePropertyChangeListener (this);
  }
  return tm;
}

V6008 Null-Dereferenzierung von „tm“. SourcesModel.java 713

public void propertyChange(PropertyChangeEvent evt) {
  ....
  synchronized (this) {
    artifacts = null;
    if (listeners == null && listeners.isEmpty()) {
      return;
    }
    ....
  }
}

V6008 Null-Dereferenzierung von „Listenern“. MavenArtifactsImplementation.java 613

Möglicherweise haben wir ein neues Tippfehlermuster gefunden. Wir werden weiter beobachten.

Lassen Sie uns mit dem nächsten Fragment fortfahren. In dieser Klasse hat ein Entwickler verwechselt, was in einer der get-Methoden zurückgegeben werden soll.

public class FunctionQParser extends QParser {
  ....
  boolean parseMultipleSources = true;
  boolean parseToEnd = true;
  ....
  public void setParseMultipleSources(boolean parseMultipleSources) {
    this.parseMultipleSources = parseMultipleSources;
  }

  /** parse multiple comma separated value sources */
  public boolean getParseMultipleSources() {
    return parseMultipleSources;
  }

  public void setParseToEnd(boolean parseToEnd) {
    this.parseToEnd = parseToEnd;
  }

  /** throw exception if there is extra 
      stuff at the end of the parsed valuesource(s). */
  public boolean getParseToEnd() {
    return parseMultipleSources;
  }
  ....
}

Die Klasse dient zum Parsen einer bestimmten geschriebenen mathematischen Funktion. Es gibt zwei Eigenschaften, um das Verhalten des Parsers zu ändern: parseMultipleSources analysiert alle Quellen numerischer Werte und parseToEnd prüft, ob die Funktion mit einer Zeichenfolge bis zum Ende analysiert werden soll.

Sehen wir uns nun die Methoden get und set für diese Felder an. Das Feld parseMultipleSources wird in getParseToEnd zurückgegeben. Der Programmierer hat verwechselt, welches Feld hier zurückgegeben werden soll.

Der Analysator erkennt leicht nicht übereinstimmende zurückgegebene Felder:

V6091 Verdächtige Getter-Implementierung. Stattdessen sollte wahrscheinlich das Feld „parseToEnd“ zurückgegeben werden. FunctionQParser.java 87, FunctionQParser.java 57

Der Tippfehler im folgenden Codefragment kann zu einer NullPointerException führen.

public void stringField(FieldInfo fieldInfo, String value) throws IOException {
  // trim the value if needed
  int len = value != null ? 
             UnicodeUtil.calcUTF16toUTF8Length(value, 0, value.length()) : 0;
  if (value.length() > maxLength) {               // <=    
    value = value.substring(0, maxLength);
  }
  countItem(fieldInfo.name, value, len);
}

Schauen wir uns das genauer an: value wird zuerst mit null verglichen und dann wird in der nächsten Zeile die Methode length() aufgerufen Wert. Aber die Variable kann null sein! Höchstwahrscheinlich hätte der Entwickler die Variable len verwenden sollen, anstatt length() aufzurufen.

Diesen Tippfehler haben wir dank der Analysemeldung gefunden:

V6008 Mögliche Null-Dereferenzierung von „Wert“. IndexSizeEstimator.java 735, IndexSizeEstimator.java 736

Sehen wir uns ein weiteres Codefragment mit einem Tippfehler an:

public Object doWork(Object value) throws IOException {
  ....
  List<?> list = (List<?>) value;
  // Validate all of same type and are comparable
  Object checkingObject = list.get(0);
  for (int idx = 0; idx < list.size(); ++idx) {
    Object item = list.get(0);                         // <=

    if (null == item) {
      throw new IOException(....);
    } else if (!(item instanceof Comparable<?>)) {
      throw new IOException(....);
    } else if (!item.getClass()
                    .getCanonicalName()
                    .equals(checkingObject.getClass()
                                          .getCanonicalName())) {
       throw new IOException(....);
    }
  }
  ....
}

Hier lohnt es sich, auf die for-Schleife zu achten. Wie üblich deklariert der Programmierer die Schleife und die Zählervariable idx, dann erhält er die Anzahl der Elemente in der Liste. Es gibt jedoch ein Problem: Bei jeder Iteration der Schleife wird nur das Element bei 0 verwendet: list.get(0) index.

Der PVS-Studio-Analysator hat diesen Fehler erkannt:

V6016 Verdächtiger Zugriff auf ein Element des „Listen“-Objekts durch einen konstanten Index innerhalb einer Schleife. AscEvaluator.java 56

The following example shows two methods with different names. However, they do the same thing.

private static List<Feature> makeFeatures(int[] featureIds) {
  final List<Feature> features = new ArrayList<>();
  for (final int i : featureIds) {
    Map<String, Object> params = new HashMap<String, Object>();
    params.put("value", i);
    final Feature f = Feature.getInstance(solrResourceLoader, 
                       ValueFeature.class.getName(), "f" + i, params);
    f.setIndex(i);
    features.add(f);
  }
  return features;
}

private static List<Feature> makeFilterFeatures(int[] featureIds) {
  final List<Feature> features = new ArrayList<>();
  for (final int i : featureIds) {
    Map<String, Object> params = new HashMap<String, Object>();
    params.put("value", i);
    final Feature f = Feature.getInstance(solrResourceLoader, 
                       ValueFeature.class.getName(), "f" + i, params);
    f.setIndex(i);
    features.add(f);
  }
  return features;
}

The first one creates a list of the Feature class objects. The second one, based on the name, should return a different type or filter these Features. If the FilterFeature type existed in the source code, we could assume that the developers simply made a typo. However, there's no such type. Maybe the method was copied and the developers forgot about it after copying it.

Anyway, this snippet looks very suspicious. And the analyzer proves this:

V6032 It is odd that the body of method 'makeFeatures' is fully equivalent to the body of another method 'makeFilterFeatures'. TestLTRScoringQuery.java 66, TestLTRScoringQuery.java 79

Forgot to check? Got error on track

If your gut tells you that null checks are unnecessary, don't trust it. In the code below, the "extra" check could've prevented NullPointerException.

public static Map<String, Object> postProcessCollectionJSON(
                                            Map<String, Object> collection) {
  final Map<String, Map<String, Object>> shards = collection != null   // <=
         ? (Map<String, Map<String, Object>>)
           collection.getOrDefault("shards", Collections.emptyMap())
         : Collections.emptyMap();
  final List<Health> healthStates = new ArrayList<>(shards.size());
  shards.forEach(
  ....
  );
  collection.put("health", Health.combine(healthStates).toString());   // <=
  return collection;
}

In the beginning of the method, the programmer checks if the collection reference is empty. If that's the case, then shards are derived from the collection. The most interesting thing is that healthStates is added to the collection at the end, regardless of whether the collection reference is empty or not.

Here's the analyzer warning for this code fragment:

V6008 Potential null dereference of 'collection'. ClusterStatus.java 303, ClusterStatus.java 335

And in the next example, the developers made an obvious mistake in the class constructor to support parallel distribution of thread work.

public class ParallelStream extends CloudSolrStream 
                            implements Expressible {
  ....
  private transient StreamFactory streamFactory;

  public ParallelStream(String zkHost, 
                        String collection, 
                        String expressionString, 
                        int workers, 
                        StreamComparator comp
) throws IOException {
    TupleStream tStream = this.streamFactory
                              .constructStream(expressionString);  // <=
    init(zkHost, collection, tStream, workers, comp);
  }  
  ....
}

The error lies in the first line of the constructor body. The streamFactory field is accessed here, but the field isn't initialized. The developers may have forgotten to add some logic in the constructor, or accidently may have written this line.

The PVS-Studio warning:

V6090 Field 'streamFactory' is being used before it was initialized. ParallelStream.java 61

However, they didn't forget to add a check in this method. Although, I think they put it in the wrong place.

private void createNewCollection(final String collection)
 throws InterruptedException {
  ....
  pending.add(completionService.submit(call));
  while (pending != null && pending.size() > 0) {
    Future<Object> future = completionService.take();
    if (future == null) return;
    pending.remove(future);
  }
}

Let's look at the interaction with the pending field: first the programmer called add, then they decided to make a loop in which they gradually removed elements from the method. The most interesting thing is that they checked that pending isn't null in the loop condition. It looks very suspicious, considering that there's no variable zeroing in the loop body. Seems like they should've added a check before calling the add method as well.

The analyzer warning:

V6060 The 'pending' reference was utilized before it was verified against null. AbstractBasicDistributedZkTestBase.java 1664, AbstractBasicDistributedZkTestBase.java 1665

Lost exception

Like many modern languages, Java has exception handling feature. The most important thing is to not lose them, as it happened here.

private void doSplitShardWithRule(SolrIndexSplitter.SplitMethod splitMethod) 
 throws Exception {
  ....
  try {
    ZkStateReader.from(cloudClient)
                 .waitForState(collectionName, 30, 
                             TimeUnit.SECONDS,
                             SolrCloudTestCase.activeClusterShape(1, 2));
  } catch (TimeoutException e) {
    new RuntimeException("Timeout waiting for
                          1shards and 2 replicas.", e);     // <=
  }
  ....
}

The error lies in the catch block: the developers created the RuntimeException object there, even added a link to the current intercepted TimeoutException and a message. But they forgot to write the throw keyword. So, the exception is never thrown.

The Lost and Found Bureau, in the form of our analyzer, found the lost exception and notified us about it:

V6006 The object was created but it is not being used. The 'throw' keyword could be missing. ShardSplitTest.java 773

How arithmetic errors interfere with testing

Does testing make software less buggy? Well, humans are the ones who write tests, and they can't help but make mistakes. This is what happened in the following example.

Public class SpellCheckCollatorTest extends SolrTestCaseJ4 {
  private static final int NUM_DOCS_WITH_TERM_EVERYOTHER = 8;
  private static final int NUM_DOCS = 17;
  ....
  @Test
  public void testEstimatedHitCounts() {
    ....
    for (int val = 5; val <= 20; val++) {
      String hitsXPath = xpathPrefix + "long[@name='hits']"; 

      if (val <= NUM_DOCS_WITH_TERM_EVERYOTHER) {
        int max = NUM_DOCS;
        int min = (/* min collected */ val) / 
                  (/* max docs possibly scanned */ NUM_DOCS);
        hitsXPath += "[" + min + " <= . and . <= " + max + "]";
      } 
    ....
    }
  }
  ....
}

A string containing the min variable, which in turn is the result of dividing val by NUM_DOCS, is written to hitsXPath here. Looking closer, you can see that the maximum and minimum values of val in this fragment are 8 and 5. The NUM_DOCS value is always 17. In all cases, min is zero in integer division. Most likely, the programmer forgot to convert division arguments to real numbers and change the type of the min variable.

We found this error using a brand-new diagnostic rule in the PVS-Studio analyzer:

V6113 The '(val) / (NUM_DOCS)' expression evaluates to 0 because the absolute value of the left operand 'val' is less than the value of the right operand 'NUM_DOCS'. SpellCheckCollatorTest.java 683

Danger of checking objects by reference

The class bellow describes the equalsTo comparison method.

private static class RandomQuery extends Query {
  private final long seed;
  private float density;
  private final List<BytesRef> docValues;
  ....
  private boolean equalsTo(RandomQuery other) {
    return seed == other.seed && 
           docValues == other.docValues && 
           density == other.density;
  }
}

Comparisons of the seed and density fields almost don't cause any questions (except, perhaps, for the density field that is a real number), because the values directly written into them are considered. However, since this field has a reference type, the docValues comparison via '==' looks very dubious. This check considers only the address and not the internal state of the object.

With such defect you can miss the case when two different lists store the same values because the lists are the same, but the references are different. It seems that when the developers named the equalsTo method, they hardly meant that it should compare references rather than the internal state of objects.

The PVS-Studio analyzer warning:

V6013 Objects 'docValues' and 'other.docValues' are compared by reference. Possibly an equality comparison was intended. TestFieldCacheSortRandom.java 341

Suspicious lack of synchronization

You won't find an error in the next fragment, but it's still potentially there. How's that possible? Take a look at this code and find out why.

public abstract class CachingDirectoryFactory extends DirectoryFactory {
  ....
  private static final Logger log = LoggerFactory.getLogger(....);
  protected Map<String, CacheValue> byPathCache = new HashMap<>();
  protected IdentityHashMap<Directory, CacheValue> byDirectoryCache = 
                                                  new IdentityHashMap<>();
  ....

  private void removeFromCache(CacheValue v) {
    log.debug("Removing from cache: {}", v);
    byDirectoryCache.remove(v.directory);
    byPathCache.remove(v.path);
  }
}

We won't be able to understand what's wrong here until we look at all the uses of the byDirectoryCache variable. In all other methods, the interaction occurs in the synchronized block. However, in the removeFromCache method, the programmer removes the collection elements outside of the synchronized blocks.

The analyzer detected this suspicious fragment:

V6102 Inconsistent synchronization of the 'byDirectoryCache' field. Consider synchronizing the field on all usages. CachingDirectoryFactory.java 92, CachingDirectoryFactory.java 228

At this point, one could say there's an error here, and the race condition could happen. However, it turns out that all removeFromCache calls are also enclosed in synchronized blocks. So, this is mostly a false positive that could be suppressed.

Although, we still can enhance this code, because there's a potential issue here. For example, when you need to use this method again, you may simply forget to enclose it in a synchronized block. Even though other methods have additional checks that the object exists in the byDirectoryCache collection, an unsynchronized call may delete an element already after the check. As a result, unnecessary actions are performed in another thread with a non-existent element of the collection, which can lead to errors in the program logic.

To protect ourselves from this, we can simply add the synchronized keyword to the removeFromCache method. So, even though there's no real error here, the static analyzer still urges us to write cleaner code.

By the way, we just recently released an article on the pitfalls of using synchronization.

Is it possible to create a few classes named the same?

In this fragment, the programmer didn't consider that classes can be renamed or declared with the same name in different packages.

private static String getFieldFlags(IndexableField f) {
  IndexOptions opts = (f == null) ? null : f.fieldType().indexOptions();

  StringBuilder flags = new StringBuilder();
  ....
  flags.append((f != null && f.getClass()
                              .getSimpleName()
                              .equals("LazyField"))  // <=
                                   ? FieldFlag.LAZY.getAbbreviation(): '-');
  ....
  return flags.toString();
}

This is an obviously unnecessary operation that may result in an error. The f variable has the getClass method called, which returns the object type, then gets and checks the name without specifying packages. All in all, there's no error here right now. However, it can arise for two reasons.

The first one is that there may be classes with the same name in different packages. In this case, it's unclear what kind of LazyField is required, and the program will run in a different way than intended.

The second one is related to changing the class name. If the name is changed, the code won't run as intended at all. And searching for all such strings in a huge code base is very difficult. Even if you resort to searching, it's something you can just forget about.

It'd be much safer to use the instanceof operator:

flags.append(f instanceof LazyDocument.LazyField
             ? FieldFlag.LAZY.getAbbreviation(): '-');

In this case, we wouldn't need to check for null, and the code would be much shorter. The chance of an error would also decrease, if there are classes with the same name in different packages, or if the name of the class changes.

The analyzer detected a potential error and issued a warning:

V6054 Classes should not be compared by their name. LukeRequestHandler.java 247

What about documentation, though?

As a final fragment, we'll look at the following code:

@Override
public UpdateCommand clone() {
  try {
    return (UpdateCommand) super.clone();
  } catch (CloneNotSupportedException e) {
    return null;                         // <=
  }
}

Let's see what's wrong with it, because everything seems fine at first glance. The analyzer informs us that returning null in the clone method is a bad idea:

V6073 It is not recommended to return null from 'clone' method. UpdateCommand.java 97

Why is it not recommended to return null from clone? It's time to consult the Java documentation:

Returns:

a clone of this instance.

Throws:

CloneNotSupportedException - if the object's class does not support the Cloneable interface. Subclasses that override the clone method can also throw this exception to indicate that an instance cannot be cloned.

The exception here indicates that the object can't be cloned. The method should return only a copy of the current object and nothing else. But why the analyzer doesn't recommend returning null from clone? It's all about further use of the code. If you constantly deviate from the recommendations in the documentation, it's difficult to catch non-standard situations.

Let's imagine a scenario where we want to use the UpdateCommand class, but the source code is unavailable, and we can't decompile it. Or we're just lazy. We can only use the built-in library with this class and focus on the interface. Our program needs us to use the clone method, so we write the following code:

try {
  UpdateCommand localCopy = field.clone(); 
  System.out.println(localCopy.toString();
} catch (CloneNotSupportedException e) {
  System.out.println("Could not clone the field"); 
}

In this code, we try to catch the CloneNotSupportedException, but we can't because the exception is a NullPointerException that causes the program to crash when calling localCopy.ToString(). This comes as a complete surprise to the developer. Deviating from official recommendations can be annoying, so it's better to always follow them

Conclusion

Let's stop here and take another look at the errors we found. Most of them are the result of carelessness, but there are some that require additional thought. For example, comparing class names without considering packages, or returning null instead of throwing an exception in the clone method.

Without special development tools like static analyzers, such bugs are difficult to find, especially in projects as large as Apache Solr. If you'd like to search for such non-obvious errors in your project, you may try our static analyzer here.

By the way, Solr isn't the only Apache product we checked:

• 21 bugs in 21st version of Apache NetBeans
• Big / Bug Data: analyzing the Apache Flink source code
• Apache Hadoop code quality: production vs test

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