Fix native memory leak caused by small HWUI path cache creation

HWUI calculates the texture size as w*h*bpp. In some cases, the
calculated path cache is small, but the actual memory allocated
in driver is 4k/8k/16k, much bigger than HWUI calculates.

Example: a 5*65 alpha texture, HWUI think it is 5*65*1 = 325 bytes,
but driver allocates 8K. An app can allocates up to 32M path textures,
which actually consumes 32M*(8*1024/325) = 806M memory.

Here we limit the number of path texture in the cache to 256, it
should be a pretty generous global limit.

Change-Id: I890819b73bb0b7f63e96bc3d9d0ff9469c16838c

libs/hwui/PathCache.cpp

@@ -138,7 +138,7 @@ static void drawPath(const SkPath *path, const SkPaint* paint, SkBitmap& bitmap,
 
 PathCache::PathCache():
         mCache(LruCache<PathDescription, PathTexture*>::kUnlimitedCapacity),
-        mSize(0), mMaxSize(MB(DEFAULT_PATH_CACHE_SIZE)) {
+        mSize(0), mMaxSize(MB(DEFAULT_PATH_CACHE_SIZE)), mTexNum(0) {
     char property[PROPERTY_VALUE_MAX];
     if (property_get(PROPERTY_PATH_CACHE_SIZE, property, nullptr) > 0) {
         INIT_LOGD("  Setting %s cache size to %sMB", name, property);
@@ -202,6 +202,7 @@ void PathCache::removeTexture(PathTexture* texture) {
                         "the cache in an inconsistent state", size);
             }
             mSize -= size;
+            mTexNum--;
         }
 
         PATH_LOGD("PathCache::delete name, size, mSize = %d, %d, %d",
@@ -228,7 +229,7 @@ void PathCache::purgeCache(uint32_t width, uint32_t height) {
 }
 
 void PathCache::trim() {
-    while (mSize > mMaxSize) {
+    while (mSize > mMaxSize || mTexNum > DEFAULT_PATH_TEXTURE_CAP) {
         mCache.removeOldest();
     }
 }
@@ -299,6 +300,7 @@ void PathCache::generateTexture(SkBitmap& bitmap, Texture* texture) {
 
     texture->setFilter(GL_LINEAR);
     texture->setWrap(GL_CLAMP_TO_EDGE);
+    mTexNum++;
 }
 
 ///////////////////////////////////////////////////////////////////////////////

libs/hwui/PathCache.h

@@ -306,6 +306,12 @@ private:
 
     bool mDebugEnabled;
 
+    /**
+     * Driver allocated 4k/8k/16k memory for small path cache,
+     * limit the number of PathTexture in case occupy too much memory in hardware.
+     */
+    uint32_t mTexNum;
+
     sp<PathProcessor> mProcessor;
 
     Vector<uint32_t> mGarbage;

libs/hwui/Properties.h

@@ -249,6 +249,9 @@ enum DebugLevel {
 #define DEFAULT_TEXT_BLACK_GAMMA_THRESHOLD 64
 #define DEFAULT_TEXT_WHITE_GAMMA_THRESHOLD 192
 
+// cap to 256 to limite paths in the path cache
+#define DEFAULT_PATH_TEXTURE_CAP 256
+
 ///////////////////////////////////////////////////////////////////////////////
 // Misc
 ///////////////////////////////////////////////////////////////////////////////