Performance optimization of StreamSource

lib/browser/sources/StreamSource.js

@@ -53,10 +53,20 @@ export default class StreamSource {
     return this._reader.read().then(({ value, done }) => {
       if (done) {
         this._done = true
-      } else if (this._buffer === undefined) {
-        this._buffer = value
       } else {
-        this._buffer = concat(this._buffer, value)
+        const chunkSize = len(value)
+
+        // If all of the chunk occurs before 'start' then drop it and clear the buffer.
+        // This greatly improves performance when reading from a stream we haven't started processing yet and 'start' is near the end of the file.
+        // Rather than buffering all of the unused data in memory just to only read a chunk near the end, rather immidiately drop data which will never be read.
+        if (this._bufferOffset + len(this._buffer) + chunkSize < start) {
+          this._buffer = undefined
+          this._bufferOffset += chunkSize
+        } else if (this._buffer === undefined) {
+          this._buffer = value
+        } else {
+          this._buffer = concat(this._buffer, value)
+        }
       }
 
       return this._readUntilEnoughDataOrDone(start, end)

test/spec/test-browser-specific.js

@@ -703,6 +703,61 @@ describe('tus', () => {
           'tus: failed to upload chunk at offset 11, caused by Error: upload was configured with a size of 100 bytes, but the source is done after 11 bytes, originated from request (method: PATCH, url: http://tus.io/uploads/foo, response code: n/a, response text: n/a, request id: n/a)',
         )
       })
+
+      it('should upload data correctly when using a non zero starting offset', async () => {
+        const reader = makeReader('hello world', 1)
+
+        const testStack = new TestHttpStack()
+        const options = {
+          httpStack: testStack,
+          uploadUrl: "http://tus.io/uploads/fileid",
+          retryDelays: [],
+          chunkSize: 6,
+          uploadLengthDeferred: true,
+          onSuccess() {}
+        }
+
+        const upload = new tus.Upload(reader, options)
+        upload.start()
+
+        let req = await testStack.nextRequest()
+        expect(req.url).toBe('http://tus.io/uploads/fileid')
+        expect(req.method).toBe('HEAD')
+        expect(req.requestHeaders['Tus-Resumable']).toBe('1.0.0')
+
+        // Respond with a non zero offset to test that the stream that is created for the reader returns the correct data and ignores the data in the stream before the offset.
+        req.respondWith({
+          status: 200,
+          responseHeaders: {
+            "Upload-Offset": 6,
+          },
+        })
+
+        req = await testStack.nextRequest()
+        expect(req.url).toBe('http://tus.io/uploads/fileid')
+        expect(req.method).toBe('PATCH')
+        expect(req.requestHeaders['Upload-Offset']).toBe(6)
+        expect(req.requestHeaders['Content-Type']).toBe('application/offset+octet-stream')
+        expect(req.body.length).toBe(5)
+        expect(req.body).toEqual(["w","o","r","l", "d"])
+
+        req.respondWith({
+          status: 204,
+          responseHeaders: {
+            'Upload-Offset': 11,
+          },
+        })
+
+        req = await testStack.nextRequest()
+
+        expect(req.url).toBe('http://tus.io/uploads/fileid')
+        expect(req.method).toBe('PATCH')
+        expect(req.requestHeaders['Upload-Offset']).toBe(11)
+        expect(req.requestHeaders['Content-Type']).toBe('application/offset+octet-stream')
+        expect(req.body).toBe(null)
+
+        await options.onSuccess.toBeCalled
+      })
     })
 
     describe('resolving of URIs', () => {