Avoid infinite recursion if cache file modified underneath cache

This generalizes our "does file still exist" check to also check
that the file is the expected length. If it's not, we don't trust
it. This avoids infinite recursion in CacheDataSource if a cache
file is truncated underneath the cache.

Issue: #6165
PiperOrigin-RevId: 265707928

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -697,9 +697,9 @@ public final class SimpleCache implements Cache {
     }
     while (true) {
       SimpleCacheSpan span = cachedContent.getSpan(position);
-      if (span.isCached && !span.file.exists()) {
-        // The file has been deleted from under us. It's likely that other files will have been
-        // deleted too, so scan the whole in-memory representation.
+      if (span.isCached && span.file.length() != span.length) {
+        // The file has been modified or deleted underneath us. It's likely that other files will
+        // have been modified too, so scan the whole in-memory representation.
         removeStaleSpans();
         continue;
       }
@@ -739,14 +739,14 @@ public final class SimpleCache implements Cache {
   }
 
   /**
-   * Scans all of the cached spans in the in-memory representation, removing any for which files no
-   * longer exist.
+   * Scans all of the cached spans in the in-memory representation, removing any for which the
+   * underlying file lengths no longer match.
    */
   private void removeStaleSpans() {
     ArrayList<CacheSpan> spansToBeRemoved = new ArrayList<>();
     for (CachedContent cachedContent : contentIndex.getAll()) {
       for (CacheSpan span : cachedContent.getSpans()) {
-        if (!span.file.exists()) {
+        if (span.file.length() != span.length) {
           spansToBeRemoved.add(span);
         }
       }