Merge branch '6.x' into ccr-6.x

* 6.x:
  Remove unnecessary exception for engine constructor
  Update docs about `script` parameter (#27010)
  Do not set SO_LINGER on server channels (#26997)
  Fix inconsistencies in the rest api specs for *_script (#26971)
  fix inconsistencies in the rest api specs for cat.snapshots (#26996)
  Add docs on full_id parameter in cat nodes API
  Add removal of types to the 6.0 breaking changes
  Create weights lazily in filter and filters aggregation (#26983)
  [TEST] Add test that replicates versioned updates with random flushes
  Use internal searcher for all indexing related operations in the engine
  Use separate searchers for "search visibility" vs "move indexing buffer to disk (#26972)
  Reformat paragraph in template docs to 80 columns
  Clarify settings and template on create index
  Fix reference to TcpTransport in documentation
  Allow Uid#decodeId to decode from a byte array slice (#26987)
  Fix a typo in the similarity docs (#26970)

core/src/main/java/org/elasticsearch/action/termvectors/TermVectorsResponse.java

@@ -305,9 +305,9 @@ private void buildFieldStatistics(XContentBuilder builder, Terms curTerms) throw
         long sumDocFreq = curTerms.getSumDocFreq();
         int docCount = curTerms.getDocCount();
         long sumTotalTermFrequencies = curTerms.getSumTotalTermFreq();
-        if (docCount > 0) {
-            assert ((sumDocFreq > 0)) : "docCount >= 0 but sumDocFreq ain't!";
-            assert ((sumTotalTermFrequencies > 0)) : "docCount >= 0 but sumTotalTermFrequencies ain't!";
+        if (docCount >= 0) {
+            assert ((sumDocFreq >= 0)) : "docCount >= 0 but sumDocFreq ain't!";
+            assert ((sumTotalTermFrequencies >= 0)) : "docCount >= 0 but sumTotalTermFrequencies ain't!";
             builder.startObject(FieldStrings.FIELD_STATISTICS);
             builder.field(FieldStrings.SUM_DOC_FREQ, sumDocFreq);
             builder.field(FieldStrings.DOC_COUNT, docCount);

core/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -90,7 +90,7 @@
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
-import java.util.function.Function;
+import java.util.function.BiFunction;
 
 public abstract class Engine implements Closeable {
 
@@ -465,8 +465,9 @@ public enum SyncedFlushResult {
         PENDING_OPERATIONS
     }
 
-    protected final GetResult getFromSearcher(Get get, Function<String, Searcher> searcherFactory) throws EngineException {
-        final Searcher searcher = searcherFactory.apply("get");
+    protected final GetResult getFromSearcher(Get get, BiFunction<String, SearcherScope, Searcher> searcherFactory,
+                                              SearcherScope scope) throws EngineException {
+        final Searcher searcher = searcherFactory.apply("get", scope);
         final DocIdAndVersion docIdAndVersion;
         try {
             docIdAndVersion = VersionsAndSeqNoResolver.loadDocIdAndVersion(searcher.reader(), get.uid());
@@ -494,23 +495,40 @@ protected final GetResult getFromSearcher(Get get, Function<String, Searcher> se
         }
     }
 
-    public abstract GetResult get(Get get, Function<String, Searcher> searcherFactory) throws EngineException;
+    public abstract GetResult get(Get get, BiFunction<String, SearcherScope, Searcher> searcherFactory) throws EngineException;
+
 
     /**
      * Returns a new searcher instance. The consumer of this
      * API is responsible for releasing the returned searcher in a
      * safe manner, preferably in a try/finally block.
      *
+     * @param source the source API or routing that triggers this searcher acquire
+     *
      * @see Searcher#close()
      */
     public final Searcher acquireSearcher(String source) throws EngineException {
+        return acquireSearcher(source, SearcherScope.EXTERNAL);
+    }
+
+    /**
+     * Returns a new searcher instance. The consumer of this
+     * API is responsible for releasing the returned searcher in a
+     * safe manner, preferably in a try/finally block.
+     *
+     * @param source the source API or routing that triggers this searcher acquire
+     * @param scope the scope of this searcher ie. if the searcher will be used for get or search purposes
+     *
+     * @see Searcher#close()
+     */
+    public final Searcher acquireSearcher(String source, SearcherScope scope) throws EngineException {
         boolean success = false;
          /* Acquire order here is store -> manager since we need
           * to make sure that the store is not closed before
           * the searcher is acquired. */
         store.incRef();
         try {
-            final SearcherManager manager = getSearcherManager(); // can never be null
+            final SearcherManager manager = getSearcherManager(source, scope); // can never be null
             /* This might throw NPE but that's fine we will run ensureOpen()
             *  in the catch block and throw the right exception */
             final IndexSearcher searcher = manager.acquire();
@@ -536,6 +554,10 @@ public final Searcher acquireSearcher(String source) throws EngineException {
         }
     }
 
+    public enum SearcherScope {
+        EXTERNAL, INTERNAL
+    }
+
     /** returns the translog for this engine */
     public abstract Translog getTranslog();
 
@@ -768,7 +790,7 @@ public final boolean refreshNeeded() {
               the store is closed so we need to make sure we increment it here
              */
             try {
-                return getSearcherManager().isSearcherCurrent() == false;
+                return getSearcherManager("refresh_needed", SearcherScope.EXTERNAL).isSearcherCurrent() == false;
             } catch (IOException e) {
                 logger.error("failed to access searcher manager", e);
                 failEngine("failed to access searcher manager", e);
@@ -1306,7 +1328,7 @@ public void release() {
         }
     }
 
-    protected abstract SearcherManager getSearcherManager();
+    protected abstract SearcherManager getSearcherManager(String source, SearcherScope scope);
 
     /**
      * Method to close the engine while the write lock is held.