Reset binder service threads' cgroup/priority after command completion

To prevent buggy command implementations from poisoning binder threads'
scheduling class & priority for future command execution, we now reset the
cgroup and thread priority to foreground/normal when a binder service thread
finishes executing the designated command.

Change-Id: Ibc0ab2485751453f6dc96fdb4eb877fd02796e3f

libs/binder/IPCThreadState.cpp

@@ -426,6 +426,21 @@ void IPCThreadState::joinThreadPool(bool isMain)
             result = executeCommand(cmd);
         }
         
+        // After executing the command, ensure that the thread is returned to the
+        // default cgroup and priority before rejoining the pool.  This is a failsafe
+        // in case the command implementation failed to properly restore the thread's
+        // scheduling parameters upon completion.
+        int my_id;
+#ifdef HAVE_GETTID
+        my_id = gettid();
+#else
+        my_id = getpid();
+#endif
+        if (!set_sched_policy(my_id, SP_FOREGROUND)) {
+            // success; reset the priority as well
+            setpriority(PRIO_PROCESS, my_id, ANDROID_PRIORITY_NORMAL);
+        }
+
         // Let this thread exit the thread pool if it is no longer
         // needed and it is not the main process thread.
         if(result == TIMED_OUT && !isMain) {