LUCENE-7576: detect when special case automaton is passed to Terms.intersect

lucene/CHANGES.txt

@@ -81,6 +81,10 @@ Bug Fixes
 * LUCENE-7536: ASCIIFoldingFilterFactory used to return an illegal multi-term
   component when preserveOriginal was set to true. (Adrien Grand)
 
+* LUCENE-7576: Fix Terms.intersect in the default codec to detect when
+  the incoming automaton is a special case and throw a clearer
+  exception than NullPointerException (Tom Mortimer via Mike McCandless)
+
 Improvements
 
 * LUCENE-6824: TermAutomatonQuery now rewrites to TermQuery,

lucene/core/src/java/org/apache/lucene/codecs/blocktree/FieldReader.java

@@ -182,6 +182,9 @@ public final class FieldReader extends Terms implements Accountable {
     //System.out.println("intersect: " + compiled.type + " a=" + compiled.automaton);
     // TODO: we could push "it's a range" or "it's a prefix" down into IntersectTermsEnum?
     // can we optimize knowing that...?
+    if (compiled.type != CompiledAutomaton.AUTOMATON_TYPE.NORMAL) {
+      throw new IllegalArgumentException("please use CompiledAutomaton.getTermsEnum instead");
+    }
     return new IntersectTermsEnum(this, compiled.automaton, compiled.runAutomaton, compiled.commonSuffixRef, startTerm, compiled.sinkState);
   }
     

lucene/core/src/java/org/apache/lucene/index/Terms.java

@@ -49,8 +49,12 @@ public abstract class Terms {
    *  provided <code>startTerm</code> must be accepted by
    *  the automaton.
    *
-   * <p><b>NOTE</b>: the returned TermsEnum cannot
-   * seek</p>.
+   *  <p>This is an expert low-level API and will only work
+   *  for {@code NORMAL} compiled automata.  To handle any
+   *  compiled automata you should instead use
+   *  {@link CompiledAutomaton#getTermsEnum} instead.
+   *
+   *  <p><b>NOTE</b>: the returned TermsEnum cannot seek</p>.
    *
    *  <p><b>NOTE</b>: the terms dictionary is free to
    *  return arbitrary terms as long as the resulted visited

lucene/core/src/test/org/apache/lucene/index/TestTermsEnum.java

@@ -998,4 +998,22 @@ public class TestTermsEnum extends LuceneTestCase {
     }
     dir.close();
   }
+
+  // LUCENE-7576
+  public void testIntersectRegexp() throws Exception {
+    Directory d = newDirectory();
+    RandomIndexWriter w = new RandomIndexWriter(random(), d);
+    Document doc = new Document();
+    doc.add(newStringField("field", "foobar", Field.Store.NO));
+    w.addDocument(doc);
+    IndexReader r = w.getReader();
+    Fields fields = MultiFields.getFields(r);
+    CompiledAutomaton automaton = new CompiledAutomaton(new RegExp("do_not_match_anything").toAutomaton());
+    Terms terms = fields.terms("field");
+    String message = expectThrows(IllegalArgumentException.class, () -> {terms.intersect(automaton, null);}).getMessage();
+    assertEquals("please use CompiledAutomaton.getTermsEnum instead", message);
+    r.close();
+    w.close();
+    d.close();
+  }
 }