Introducing FallbackJSBundleLoader

Reviewed By: michalgr

Differential Revision: D4386951

fbshipit-source-id: b1375deee9b3268d414e1b03fa79df50ac4d36cb

ReactAndroid/src/main/java/com/facebook/react/cxxbridge/FallbackJSBundleLoader.java

@@ -0,0 +1,87 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.cxxbridge;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.ListIterator;
+import java.util.Stack;
+
+/**
+ * FallbackJSBundleLoader
+ *
+ * An implementation of {@link JSBundleLoader} that will try to load from
+ * multiple sources, falling back from one source to the next at load time
+ * when an exception is thrown for a recoverable error.
+ */
+public final class FallbackJSBundleLoader extends JSBundleLoader {
+
+  /* package */ static final String RECOVERABLE = "facebook::react::Recoverable";
+
+  // Loaders to delegate to, with the preferred one at the top.
+  private Stack<JSBundleLoader> mLoaders;
+
+  // Reasons why we fell-back on previous loaders, in order of occurrence.
+  private final ArrayList<Exception> mRecoveredErrors = new ArrayList<>();
+
+  /**
+   * @param loaders Loaders for the sources to try, in descending order of
+   *                preference.
+   */
+  public FallbackJSBundleLoader(List<JSBundleLoader> loaders) {
+    mLoaders = new Stack();
+    ListIterator<JSBundleLoader> it = loaders.listIterator(loaders.size());
+    while (it.hasPrevious()) {
+      mLoaders.push(it.previous());
+    }
+  }
+
+  /**
+   * This loader delegates to (and so behaves like) the currently preferred
+   * loader. If that loader fails in a recoverable way and we fall back from it,
+   * it is replaced by the next most preferred loader.
+   */
+  @Override
+  public String loadScript(CatalystInstanceImpl instance) {
+    while (true) {
+      try {
+        return getDelegateLoader().loadScript(instance);
+      } catch (Exception e) {
+        if (!e.getMessage().startsWith(RECOVERABLE)) {
+          throw e;
+        }
+
+        mLoaders.pop();
+        mRecoveredErrors.add(e);
+        // TODO (t14839302): Report a soft error for each swallowed exception.
+      }
+    }
+  }
+
+  private JSBundleLoader getDelegateLoader() {
+    if (!mLoaders.empty()) {
+      return mLoaders.peek();
+    }
+
+    RuntimeException fallbackException =
+      new RuntimeException("No fallback options available");
+
+    // Invariant: tail.getCause() == null
+    Throwable tail = fallbackException;
+    for (Exception e : mRecoveredErrors) {
+      tail.initCause(e);
+      while (tail.getCause() != null) {
+        tail = tail.getCause();
+      }
+    }
+
+    throw fallbackException;
+  }
+}