Added a SensorEventQueue, a circular buffer meant for reading with one thread

and polling a subhal with another. The writing thread gets access to pointers
in the internal buffer. This design avoids a memcpy on write when the multihal
fetches subhal events using poll().

Unit-tests include multithreaded reading and writing lots of events, in
random-sized chunks.

This is not used by the multihal yet. That will be a different CL.

Change-Id: I58418d69eebebeb96befb08ba3aed080f0f08551

modules/sensors/SensorEventQueue.h

@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef SENSOREVENTQUEUE_H_
+#define SENSOREVENTQUEUE_H_
+
+#include <hardware/sensors.h>
+#include <pthread.h>
+
+/*
+ * Fixed-size circular queue, with an API developed around the sensor HAL poll() method.
+ * Poll() takes a pointer to a buffer, which is written by poll() before it returns.
+ * This class can provide a pointer to a spot in its internal buffer for poll() to
+ * write to, instead of using an intermediate buffer and a memcpy.
+ *
+ * Thread safety:
+ * Reading can be done safely after grabbing the mutex lock, while poll() writing in a separate
+ * thread without a mutex lock. But there can only be one writer at a time.
+ */
+class SensorEventQueue {
+    int mCapacity;
+    int mStart; // start of readable region
+    int mSize; // number of readable items
+    sensors_event_t* mData;
+    pthread_cond_t mDataAvailableCondition;
+    pthread_cond_t mSpaceAvailableCondition;
+    pthread_mutex_t mMutex;
+
+public:
+    SensorEventQueue(int capacity);
+    ~SensorEventQueue();
+    void lock();
+    void unlock();
+    void waitForSpaceAndLock();
+    void waitForDataAndLock();
+
+    // Returns length of region, between zero and min(capacity, requestedLength). If there is any
+    // writable space, it will return a region of at least one. Because it must return
+    // a pointer to a contiguous region, it may return smaller regions as we approach the end of
+    // the data array.
+    // Only call while holding the lock.
+    // The region is not marked internally in any way. Subsequent calls may return overlapping
+    // regions. This class expects there to be exactly one writer at a time.
+    int getWritableRegion(int requestedLength, sensors_event_t** out);
+
+    // After writing to the region returned by getWritableRegion(), call this to indicate how
+    // many records were actually written.
+    // This increases size() by count.
+    // Only call while holding the lock.
+    void markAsWritten(int count);
+
+    // Gets the number of readable records.
+    // Only call while holding the lock.
+    int getSize();
+
+    // Returns pointer to the first readable record, or NULL if size() is zero.
+    // Only call this while holding the lock.
+    sensors_event_t* peek();
+
+    // This will decrease the size by one, freeing up the oldest readable event's slot for writing.
+    // Only call while holding the lock.
+    void dequeue();
+};
+
+#endif // SENSOREVENTQUEUE_H_