feat: let back buffer be configurable

Introduces a new config option BACK_BUFFER_LENGTH that defaults to 30,
the previous value.
When set, we trim the back buffer according to the provided value.

src/config.js

@@ -1,6 +1,7 @@
 export default {
   GOAL_BUFFER_LENGTH: 30,
   MAX_GOAL_BUFFER_LENGTH: 60,
+  BACK_BUFFER_LENGTH: 30,
   GOAL_BUFFER_LENGTH_RATE: 1,
   // 0.5 MB/s
   INITIAL_BANDWIDTH: 4194304,

src/segment-loader.js

@@ -53,7 +53,7 @@ export const illegalMediaSwitch = (loaderType, startingMedia, trackInfo) => {
 };
 
 /**
- * Calculates a time value that is safe to remove from the back buffer without interupting
+ * Calculates a time value that is safe to remove from the back buffer without interrupting
  * playback.
  *
  * @param {TimeRange} seekable
@@ -63,15 +63,15 @@ export const illegalMediaSwitch = (loaderType, startingMedia, trackInfo) => {
  * @param {number} targetDuration
  *        The target duration of the current playlist
  * @return {number}
- *         Time that is safe to remove from the back buffer without interupting playback
+ *         Time that is safe to remove from the back buffer without interrupting playback
  */
 export const safeBackBufferTrimTime = (seekable, currentTime, targetDuration) => {
   // 30 seconds before the playhead provides a safe default for trimming.
   //
   // Choosing a reasonable default is particularly important for high bitrate content and
   // VOD videos/live streams with large windows, as the buffer may end up overfilled and
   // throw an APPEND_BUFFER_ERR.
-  let trimTime = currentTime - 30;
+  let trimTime = currentTime - Config.BACK_BUFFER_LENGTH;
 
   if (seekable.length) {
     // Some live playlists may have a shorter window of content than the full allowed back

src/videojs-http-streaming.js

@@ -48,10 +48,11 @@ const Vhs = {
   xhr: xhrFactory()
 };
 
-// Define getter/setters for config properites
+// Define getter/setters for config properties
 [
   'GOAL_BUFFER_LENGTH',
   'MAX_GOAL_BUFFER_LENGTH',
+  'BACK_BUFFER_LENGTH',
   'GOAL_BUFFER_LENGTH_RATE',
   'BUFFER_LOW_WATER_LINE',
   'MAX_BUFFER_LOW_WATER_LINE',

test/configuration.test.js

@@ -152,6 +152,34 @@ QUnit.test('MAX_GOAL_BUFFER_LENGTH set warning and invalid', function(assert) {
   assert.equal(Config.MAX_GOAL_BUFFER_LENGTH, 60, 'default');
 });
 
+QUnit.test('BACK_BUFFER_LENGTH get warning', function(assert) {
+  assert.equal(
+    Vhs.BACK_BUFFER_LENGTH,
+    Config.BACK_BUFFER_LENGTH,
+    'Vhs.BACK_BUFFER_LENGTH returns the default'
+  );
+  assert.equal(this.env.log.warn.calls, 1, 'logged a warning');
+});
+
+QUnit.test('BACK_BUFFER_LENGTH set warning', function(assert) {
+  Vhs.BACK_BUFFER_LENGTH = 10;
+  assert.equal(this.env.log.warn.calls, 1, 'logged a warning');
+
+  assert.equal(Config.BACK_BUFFER_LENGTH, 10, 'returns what we set it to');
+});
+
+QUnit.test('BACK_BUFFER_LENGTH set warning and invalid', function(assert) {
+  Vhs.BACK_BUFFER_LENGTH = 'nope';
+  assert.equal(this.env.log.warn.calls, 2, 'logged two warnings');
+
+  assert.equal(Config.BACK_BUFFER_LENGTH, 30, 'default');
+
+  Vhs.BACK_BUFFER_LENGTH = -1;
+  assert.equal(this.env.log.warn.calls, 2, 'logged two warnings');
+
+  assert.equal(Config.BACK_BUFFER_LENGTH, 30, 'default');
+});
+
 QUnit.test('GOAL_BUFFER_LENGTH_RATE get warning', function(assert) {
   assert.equal(
     Vhs.GOAL_BUFFER_LENGTH_RATE,
@@ -390,7 +418,7 @@ options.forEach((opt) => {
     assert.deepEqual(
       vhs.options_[opt.name],
       opt.test,
-      `${opt.name} should be equal to sourchHandler option`
+      `${opt.name} should be equal to sourceHandler option`
     );
   });
 
@@ -412,7 +440,7 @@ options.forEach((opt) => {
     assert.deepEqual(
       vhs.options_[opt.name],
       opt.test,
-      `${opt.name} should be equal to sourchHandler option`
+      `${opt.name} should be equal to sourceHandler option`
     );
   });
 });