Fix FST not readable error in test

Author: dungba88

lucene/core/src/java/org/apache/lucene/util/fst/FST.java

@@ -417,6 +417,7 @@ public FST(FSTMetadata<T> metadata, DataInput in, FSTStore fstStore) throws IOEx
 
   /** Create the FST with a metadata object and a FSTReader. */
   public FST(FSTMetadata<T> metadata, FSTReader fstReader) {
+    assert metadata != null;
     this.metadata = metadata;
     this.outputs = metadata.outputs;
     this.fstReader = fstReader;

lucene/core/src/java/org/apache/lucene/util/fst/FSTCompiler.java

@@ -176,7 +176,15 @@ private FSTCompiler(
   }
 
   // Get the respective FSTReader of the DataOutput. If the DataOutput is also a FSTReader then we
-  // will use it, otherwise we will return null.
+  // will use it, otherwise we will throw an exception.
+
+  /**
+   * Get the respective {@link FSTReader} of the {@link DataOutput}. If the DataOutput is also a
+   * FSTReader then we will use it, otherwise we will throw an exception.
+   *
+   * @return the DataOutput as FSTReader
+   * @throws IllegalStateException if the DataOutput does not implement FSTReader
+   */
   public FSTReader getFSTReader() {
     if (dataOutput instanceof FSTReader) {
       return (FSTReader) dataOutput;

lucene/core/src/test/org/apache/lucene/util/fst/TestFSTs.java

@@ -1252,7 +1252,6 @@ public void testNonFinalStopNode() throws Exception {
     final Long nothing = outputs.getNoOutput();
     final FSTCompiler<Long> fstCompiler =
         new FSTCompiler.Builder<>(FST.INPUT_TYPE.BYTE1, outputs).build();
-    final FST<Long> fst = fstCompiler.fst;
 
     final FSTCompiler.UnCompiledNode<Long> rootNode =
         new FSTCompiler.UnCompiledNode<>(fstCompiler, 0);
@@ -1285,6 +1284,8 @@ public void testNonFinalStopNode() throws Exception {
 
     fstCompiler.finish(fstCompiler.addNode(rootNode));
 
+    final FST<Long> fst = new FST<>(fstCompiler.fst.metadata, fstCompiler.getFSTReader());
+
     StringWriter w = new StringWriter();
     // Writer w = new OutputStreamWriter(new FileOutputStream("/x/tmp3/out.dot"));
     Util.toDot(fst, w, false, false);

lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingSuggester.java

@@ -586,7 +586,10 @@ public void build(InputIterator iterator) throws IOException {
           fstCompiler.add(scratchInts.get(), outputs.newPair(cost, br));
         }
       }
-      fst = new FST<>(fstCompiler.compile(), fstCompiler.getFSTReader());
+      FST.FSTMetadata<Pair<Long, BytesRef>> fstMetadata = fstCompiler.compile();
+      if (fstMetadata != null) {
+        fst = new FST<>(fstMetadata, fstCompiler.getFSTReader());
+      }
       count = newCount;
 
       // Util.dotToFile(fst, "/tmp/suggest.dot");

lucene/suggest/src/java/org/apache/lucene/search/suggest/fst/WFSTCompletionLookup.java

@@ -126,7 +126,10 @@ public void build(InputIterator iterator) throws IOException {
       previous.copyBytes(scratch);
       newCount++;
     }
-    fst = new FST<>(fstCompiler.compile(), fstCompiler.getFSTReader());
+    FST.FSTMetadata<Long> fstMetadata = fstCompiler.compile();
+    if (fstMetadata != null) {
+      fst = new FST<>(fstMetadata, fstCompiler.getFSTReader());
+    }
     count = newCount;
   }