Recreate poToken generator if current is broken

This will be tried only once, and afterwards an error will be thrown

app/src/main/java/org/schabi/newpipe/util/potoken/PoTokenProviderImpl.kt

@@ -1,5 +1,7 @@
 package org.schabi.newpipe.util.potoken
 
+import android.os.Handler
+import android.os.Looper
 import android.util.Log
 import org.schabi.newpipe.App
 import org.schabi.newpipe.extractor.NewPipe
@@ -22,33 +24,68 @@ object PoTokenProviderImpl : PoTokenProvider {
             return null
         }
 
-        val (poTokenGenerator, visitorData, streamingPot) = synchronized(WebPoTokenGenLock) {
-            if (webPoTokenGenerator == null || webPoTokenGenerator!!.isExpired()) {
-                webPoTokenGenerator = PoTokenWebView.newPoTokenGenerator(App.getApp()).blockingGet()
-                webPoTokenVisitorData = YoutubeParsingHelper
-                    .randomVisitorData(NewPipe.getPreferredContentCountry())
+        return getWebClientPoToken(videoId = videoId, forceRecreate = false)
+    }
 
-                // The streaming poToken needs to be generated exactly once before generating any
-                // other (player) tokens.
-                webPoTokenStreamingPot = webPoTokenGenerator!!
-                    .generatePoToken(webPoTokenVisitorData!!).blockingGet()
+    /**
+     * @param forceRecreate whether to force the recreation of [webPoTokenGenerator], to be used in
+     * case the current [webPoTokenGenerator] threw an error last time
+     * [PoTokenGenerator.generatePoToken] was called
+     */
+    private fun getWebClientPoToken(videoId: String, forceRecreate: Boolean): PoTokenResult {
+        // just a helper class since Kotlin does not have builtin support for 4-tuples
+        data class Quadruple<T1, T2, T3, T4>(val t1: T1, val t2: T2, val t3: T3, val t4: T4)
+
+        val (poTokenGenerator, visitorData, streamingPot, hasBeenRecreated) =
+            synchronized(WebPoTokenGenLock) {
+                val shouldRecreate = webPoTokenGenerator == null || forceRecreate ||
+                        webPoTokenGenerator!!.isExpired()
+
+                if (shouldRecreate) {
+                    // close the current webPoTokenGenerator on the main thread
+                    webPoTokenGenerator?.let { Handler(Looper.getMainLooper()).post { it.close() } }
+
+                    // create a new webPoTokenGenerator
+                    webPoTokenGenerator = PoTokenWebView
+                        .newPoTokenGenerator(App.getApp()).blockingGet()
+                    webPoTokenVisitorData = YoutubeParsingHelper
+                        .randomVisitorData(NewPipe.getPreferredContentCountry())
+
+                    // The streaming poToken needs to be generated exactly once before generating
+                    // any other (player) tokens.
+                    webPoTokenStreamingPot = webPoTokenGenerator!!
+                        .generatePoToken(webPoTokenVisitorData!!).blockingGet()
+                }
+
+                return@synchronized Quadruple(
+                    webPoTokenGenerator!!,
+                    webPoTokenVisitorData!!,
+                    webPoTokenStreamingPot!!,
+                    shouldRecreate
+                )
+            }
+
+        val playerPot = try {
+            // Not using synchronized here, since poTokenGenerator would be able to generate
+            // multiple poTokens in parallel if needed. The only important thing is for exactly one
+            // visitorData/streaming poToken to be generated before anything else.
+            poTokenGenerator.generatePoToken(videoId).blockingGet()
+        } catch (throwable: Throwable) {
+            if (hasBeenRecreated) {
+                // the poTokenGenerator has just been recreated (and possibly this is already the
+                // second time we try), so there is likely nothing we can do
+                throw throwable
+            } else {
+                // retry, this time recreating the [webPoTokenGenerator] from scratch;
+                // this might happen for example if NewPipe goes in the background and the WebView
+                // content is lost
+                Log.e(TAG, "Failed to obtain poToken, retrying", throwable)
+                return getWebClientPoToken(videoId = videoId, forceRecreate = true)
             }
-            return@synchronized Triple(
-                webPoTokenGenerator!!, webPoTokenVisitorData!!, webPoTokenStreamingPot!!
-            )
         }
 
-        // Not using synchronized here, since poTokenGenerator would be able to generate multiple
-        // poTokens in parallel if needed. The only important thing is for exactly one
-        // visitorData/streaming poToken to be generated before anything else.
-        val playerPot = poTokenGenerator.generatePoToken(videoId).blockingGet()
         Log.e(TAG, "success($videoId) $playerPot,web.gvs+$streamingPot;visitor_data=$visitorData")
-
-        return PoTokenResult(
-            webPoTokenVisitorData!!,
-            playerPot,
-            webPoTokenStreamingPot!!,
-        )
+        return PoTokenResult(visitorData, playerPot, streamingPot)
     }
 
     override fun getWebEmbedClientPoToken(videoId: String): PoTokenResult? = null

app/src/main/java/org/schabi/newpipe/util/potoken/PoTokenWebView.kt

@@ -7,6 +7,7 @@ import android.os.Looper
 import android.util.Log
 import android.webkit.JavascriptInterface
 import android.webkit.WebView
+import androidx.annotation.MainThread
 import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers
 import io.reactivex.rxjava3.core.Single
 import io.reactivex.rxjava3.core.SingleEmitter
@@ -275,6 +276,7 @@ class PoTokenWebView private constructor(
     /**
      * Releases all [webView] and [disposables] resources.
      */
+    @MainThread
     override fun close() {
         disposables.dispose()