feat: Handle zero iterator results

jest.config.js

@@ -11,10 +11,10 @@ export default {
   coverageReporters: ['json-summary', 'text'],
   coverageThreshold: {
     global: {
-      lines: 70.65,
-      statements: 70.77,
-      branches: 66.66,
-      functions: 76.05,
+      lines: 71.03,
+      statements: 71.14,
+      branches: 67,
+      functions: 76.76,
     },
   },
   transform: {

src/generator.ts

@@ -118,7 +118,9 @@ export default class Generator extends EventEmitter<Events> {
   }
 
   private async flush(): Promise<void> {
-    await this.runBatch(this.$thisList);
+    if (this.$thisList.length > 0) {
+      await this.runBatch(this.$thisList);
+    }
   }
 }
 

src/iterator.ts

@@ -37,7 +37,7 @@ export default class Iterator extends EventEmitter<Events> {
       const error = (e: unknown): Error =>
         new Error(
           `The Iterator did not run successfully, it could not get the results from the endpoint ${
-            this.source
+            this.source?.value
           } (offset: ${this.offset}, limit ${this.query.limit}): ${
             (e as Error).message
           }`
@@ -62,6 +62,14 @@ export default class Iterator extends EventEmitter<Events> {
         });
         stream.on('end', () => {
           this.totalResults += resultsPerPage;
+          if (this.totalResults === 0) {
+            this.emit(
+              'error',
+              error(
+                new Error(`no results for query:\n ${this.query.toString()}`)
+              )
+            );
+          }
           this.offset += this.query.limit;
           if (resultsPerPage < this.query.limit!) {
             this.emit('end', this.totalResults);

test/iterator.test.ts

@@ -11,6 +11,7 @@ import removeDirectory from '../src/utils/removeDir.js';
 import {NamedNode} from '@rdfjs/types';
 import getSPARQLQuery from '../src/utils/getSPARQLQuery.js';
 import getEndpoint from '../src/utils/getEndpoint.js';
+import File from '../src/file.js';
 chai.use(chaiAsPromised);
 
 describe('Iterator Class', () => {
@@ -111,24 +112,9 @@ describe('Iterator Class', () => {
         Query.from(getSPARQLQuery(stageConfig.iterator.query, 'select')),
         getEndpoint(new Stage(pipeline, stageConfig))
       );
-      const emittedEvents: {event: string; bindings?: NamedNode}[] = [];
-      async function runIteratorWithPromise(): Promise<boolean> {
-        return new Promise((resolve, reject) => {
-          iterator.addListener('data', bindings => {
-            emittedEvents.push({event: 'data', bindings});
-          });
-          iterator.addListener('end', () => {
-            emittedEvents.push({event: 'end'});
-            resolve(true);
-          });
-          iterator.addListener('error', error => {
-            reject(error);
-          });
-          iterator.run();
-        });
-      }
 
-      await runIteratorWithPromise();
+      const emittedEvents = await runIterator(iterator);
+
       chai.expect(emittedEvents).to.have.lengthOf(154);
       chai.expect(emittedEvents[0].event).to.equal('data');
       chai.expect(emittedEvents[0].bindings?.termType).to.equal('NamedNode');
@@ -139,6 +125,22 @@ describe('Iterator Class', () => {
         .expect(emittedEvents[emittedEvents.length - 1].event)
         .to.equal('end');
     });
+
+    it('throws error if query returns no results', async () => {
+      const query = Query.from(
+        getSPARQLQuery(
+          'SELECT $this WHERE { $this a <http://example.com/no-matches> }',
+          'select'
+        )
+      );
+      const iterator = new Iterator(
+        query,
+        new File('file://static/tests/iris-small.nt')
+      );
+      await expect(async () => await runIterator(iterator)).rejects.toThrow(
+        'no results for query:\n SELECT ?this WHERE { ?this <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.com/no-matches>. }'
+      );
+    });
   });
 });
 
@@ -175,3 +177,22 @@ describe('Query', () => {
     );
   });
 });
+
+async function runIterator(
+  iterator: Iterator
+): Promise<{event: string; bindings?: NamedNode}[]> {
+  const emittedEvents: {event: string; bindings?: NamedNode}[] = [];
+  return new Promise((resolve, reject) => {
+    iterator.on('data', bindings => {
+      emittedEvents.push({event: 'data', bindings});
+    });
+    iterator.on('end', () => {
+      emittedEvents.push({event: 'end'});
+      resolve(emittedEvents);
+    });
+    iterator.on('error', error => {
+      reject(error);
+    });
+    iterator.run();
+  });
+}