Fix back buffer not being flushed when backBufferLength is set to 0 (video-dev#6781)

Author: tmiller02

src/controller/buffer-controller.ts

@@ -1127,7 +1127,7 @@ transfer tracks: ${JSON.stringify(transferredTracks, (key, value) => (key === 'i
         ? config.liveBackBufferLength
         : config.backBufferLength;
 
-    if (Number.isFinite(backBufferLength) && backBufferLength > 0) {
+    if (Number.isFinite(backBufferLength) && backBufferLength >= 0) {
       const maxBackBufferLength = Math.max(backBufferLength, targetDuration);
       const targetBackBufferPosition =
         Math.floor(currentTime / targetDuration) * targetDuration -

tests/unit/controller/buffer-controller-operations.ts

@@ -617,6 +617,14 @@ describe('BufferController with attached media', function () {
         expect(triggerSpy).to.not.have.been.calledWith(Events.BUFFER_FLUSHING);
       });
 
+      it('should execute a remove operation if backBufferLength is set to 0', function () {
+        hls.config.backBufferLength = 0;
+        evokeTrimBuffers(hls);
+        expect(triggerSpy.withArgs(Events.BUFFER_FLUSHING)).to.have.callCount(
+          2,
+        );
+      });
+
       it('should execute a remove operation if flushing a valid backBuffer range', function () {
         evokeTrimBuffers(hls);
         expect(triggerSpy.withArgs(Events.BUFFER_FLUSHING)).to.have.callCount(