android: Add ChildProcessCrashObserver.java

ChildProcessCrashObserver.java serves a similar purpose as
CrashDumpManager.java did (which it will replace), except without the
need (or ability) to check for a minidump on the filesystem. Instead of
a minidump path, the process ID of the crashed child is passed to the
callback which it can use to identify the correct crash dump.

Bug: crashpad:30
Change-Id: I718e653acfa35b14ebaa17d0e8c76804f9856dfb
Reviewed-on: https://chromium-review.googlesource.com/1150202Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Joshua Peraza <jperaza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577935}

components/crash/android/BUILD.gn

@@ -5,7 +5,8 @@
 import("//build/config/android/rules.gni")
 
 _jni_sources =
-    [ "java/src/org/chromium/components/crash/browser/CrashDumpManager.java" ]
+    [ "java/src/org/chromium/components/crash/browser/ChildProcessCrashObserver.java",
+      "java/src/org/chromium/components/crash/browser/CrashDumpManager.java" ]
 
 generate_jni("jni_headers") {
   sources = _jni_sources

components/crash/android/java/src/org/chromium/components/crash/browser/ChildProcessCrashObserver.java

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.components.crash.browser;
+
+import org.chromium.base.Log;
+import org.chromium.base.ThreadUtils;
+import org.chromium.base.annotations.CalledByNative;
+
+/**
+ * A Java-side bridge for notifying an observer when a child process has crashed.
+ *
+ * Crashpad writes a minidump for a child process in the time between the child receives a crash
+ * signal and when it exits. After the child process exits, Crashpad should have completed writing
+ * a minidump to its CrashReportDatabase and this class's childCrashed() is called, executing any
+ * registered callback.
+ *
+ * It is expected that the callback will be used to handle the newly created dump by, e.g. attaching
+ * a logcat and scheduling it for upload.
+ */
+public class ChildProcessCrashObserver {
+    private static final String TAG = "ChildCrashObserver";
+
+    /**
+     * An interface for registering a callback to be executed when a child process crashes.
+     */
+    public interface ChildCrashedCallback { public void childCrashed(int pid); }
+
+    /**
+     * The globally registered callback for responding to child process crashes, or null if no
+     * callback has been registered yet.
+     */
+    private static ChildCrashedCallback sCallback;
+
+    /**
+     * Registers a callback for responding to child process crashes. May be called at most once, and
+     * only on the UI thread.
+     *
+     * @param callback The callback to trigger when a child process has exited due to a crash.
+     */
+    public static void registerCrashCallback(ChildCrashedCallback callback) {
+        ThreadUtils.assertOnUiThread();
+        assert sCallback == null;
+        sCallback = callback;
+    }
+
+    /**
+     * Notifies any registered observer that a child process has exited due to an apparent crash.
+     */
+    @CalledByNative
+    public static void childCrashed(int pid) {
+        if (sCallback == null) {
+            Log.w(TAG, "Ignoring crash observed before a callback was registered...");
+            return;
+        }
+        sCallback.childCrashed(pid);
+    }
+}