Fix ClonedIterators not being readable

Since ClonedIterators are not autoStarted, they should be marked as
readable. Otherwise, dependents may never consider them as being
readable.

This lies at the root cause of the following Comunica bugs, where
execution would halt due to an intermediary iterator never becoming
readable:
- comunica/comunica#1168
- comunica/comunica#817

asynciterator.ts

@@ -1796,6 +1796,9 @@ export class ClonedIterator<T> extends TransformIterator<T> {
   constructor(source: AsyncIterator<T>) {
     super(source, { autoStart: false });
     this._reading = false;
+    // As cloned iterators are not auto-started, they must always be marked as readable.
+    if (source)
+      this.readable = true;
   }
 
   protected _init() {

test/ClonedIterator-test.js

@@ -173,8 +173,8 @@ describe('ClonedIterator', () => {
         });
 
         if (index === 0) {
-          it('should not have emitted the `readable` event', () => {
-            getClone()._eventCounts.readable.should.equal(0);
+          it('should have emitted the `readable` event once', () => {
+            getClone()._eventCounts.readable.should.equal(1);
           });
         }
 
@@ -187,8 +187,8 @@ describe('ClonedIterator', () => {
         });
 
         if (index === 0) {
-          it('should not be readable', () => {
-            getClone().readable.should.be.false;
+          it('should be readable', () => {
+            getClone().readable.should.be.true;
           });
 
           it('should return null on read', () => {
@@ -204,7 +204,7 @@ describe('ClonedIterator', () => {
           before(() => { getIterator()._push('a'); });
 
         it('should have emitted the `readable` event', () => {
-          getClone()._eventCounts.readable.should.equal(1);
+          getClone()._eventCounts.readable.should.equal(index === 0 ? 2 : 1);
         });
 
         it('should not have emitted the `end` event', () => {
@@ -231,7 +231,7 @@ describe('ClonedIterator', () => {
           before(() => { getIterator().close(); });
 
         it('should not have emitted anymore `readable` events', () => {
-          getClone()._eventCounts.readable.should.equal(1);
+          getClone()._eventCounts.readable.should.equal(index === 0 ? 2 : 1);
         });
 
         it('should have emitted the `end` event', () => {
@@ -756,6 +756,45 @@ describe('ClonedIterator', () => {
     });
   });
 
+  describe('Cloning an iterator without autoStart', () => {
+    const clones = createClones(() => new TransformIterator(() => new ArrayIterator([1], { autoStart: false })));
+
+    describe('before the first item is read', () => {
+      describeClones(clones, getClone => {
+        it('should be readable', () => {
+          getClone().readable.should.be.true;
+        });
+
+        it('should have emitted the `readable` event', () => {
+          getClone()._eventCounts.readable.should.equal(1);
+        });
+      });
+    });
+
+    describe('after the first item is read', () => {
+      describeClones(clones, getClone => {
+        let item;
+        before(() => { item = getClone().read(); });
+
+        it('should have read the item correctly', () => {
+          item.should.equal(1);
+        });
+
+        it('should not be readable', () => {
+          getClone().readable.should.be.false;
+        });
+
+        it('should be done', () => {
+          getClone().done.should.be.true;
+        });
+
+        it('should not have emitted another `readable` event', () => {
+          getClone()._eventCounts.readable.should.equal(1);
+        });
+      });
+    });
+  });
+
   describe('Cloning an iterator that errors', () => {
     const clones = createClones(() => new AsyncIterator());
     let iterator;