stagefright: TinyCacheSource to read continuously

If the seek operation of underlying MediaDataSource implementation is
expensive and each read is relatively small (e.g. Live MPEG2 TS
stream), current TinyCacheSource implementation requests seek too
frequently.

Bug: 21764375

Change-Id: Ic620bf027bc4ef48822549c83c5c98109986884a

1 files changed, 19 insertions, 3 deletions

diff --git a/media/libstagefright/CallbackDataSource.cpp b/media/libstagefright/CallbackDataSource.cpp
index 41f0175..de972ce 100644
--- a/media/libstagefright/CallbackDataSource.cpp
+++ b/media/libstagefright/CallbackDataSource.cpp
@@ -109,9 +109,25 @@ ssize_t TinyCacheSource::readAt(off64_t offset, void* data, size_t size) {
     }
 
     // Check if the cache satisfies the read.
-    if (offset >= mCachedOffset && offset + size <= mCachedOffset + mCachedSize) {
-        memcpy(data, &mCache[offset - mCachedOffset], size);
-        return size;
+    if (mCachedOffset <= offset && offset < mCachedOffset + mCachedSize) {
+        if (offset + size <= mCachedOffset + mCachedSize) {
+            memcpy(data, &mCache[offset - mCachedOffset], size);
+            return size;
+        } else {
+            // If the cache hits only partially, flush the cache and read the
+            // remainder.
+
+            // This value is guaranteed to be greater than 0 because of the
+            // enclosing if statement.
+            const ssize_t remaining = mCachedOffset + mCachedSize - offset;
+            memcpy(data, &mCache[offset - mCachedOffset], remaining);
+            const ssize_t readMore = readAt(offset + remaining,
+                    (uint8_t*)data + remaining, size - remaining);
+            if (readMore < 0) {
+                return readMore;
+            }
+            return remaining + readMore;
+        }
     }
 
     // Fill the cache and copy to the caller.