BlobCache: implement cache serialization
This change adds serialization and deserialization functionality to
BlobCache, conforming to the Flattenable interface.
Change-Id: Ibc99cb1c3d015f363d57d0713eabccec07ff975e
diff --git a/include/utils/BlobCache.h b/include/utils/BlobCache.h
index 11d4246..4f342a2 100644
--- a/include/utils/BlobCache.h
+++ b/include/utils/BlobCache.h
@@ -19,6 +19,7 @@
#include <stddef.h>
+#include <utils/Flattenable.h>
#include <utils/RefBase.h>
#include <utils/SortedVector.h>
#include <utils/threads.h>
@@ -28,10 +29,11 @@
// A BlobCache is an in-memory cache for binary key/value pairs. A BlobCache
// does NOT provide any thread-safety guarantees.
//
-// The cache contents can be serialized to a file and reloaded in a subsequent
-// execution of the program. This serialization is non-portable and should only
-// be loaded by the device that generated it.
-class BlobCache : public RefBase {
+// The cache contents can be serialized to an in-memory buffer or mmap'd file
+// and then reloaded in a subsequent execution of the program. This
+// serialization is non-portable and the data should only be used by the device
+// that generated it.
+class BlobCache : public RefBase, public Flattenable {
public:
// Create an empty blob cache. The blob cache will cache key/value pairs
@@ -58,14 +60,13 @@
void set(const void* key, size_t keySize, const void* value,
size_t valueSize);
- // The get function retrieves from the cache the binary value associated
- // with a given binary key. If the key is present in the cache then the
- // length of the binary value associated with that key is returned. If the
- // value argument is non-NULL and the size of the cached value is less than
- // valueSize bytes then the cached value is copied into the buffer pointed
- // to by the value argument. If the key is not present in the cache then 0
- // is returned and the buffer pointed to by the value argument is not
- // modified.
+ // get retrieves from the cache the binary value associated with a given
+ // binary key. If the key is present in the cache then the length of the
+ // binary value associated with that key is returned. If the value argument
+ // is non-NULL and the size of the cached value is less than valueSize bytes
+ // then the cached value is copied into the buffer pointed to by the value
+ // argument. If the key is not present in the cache then 0 is returned and
+ // the buffer pointed to by the value argument is not modified.
//
// Note that when calling get multiple times with the same key, the later
// calls may fail, returning 0, even if earlier calls succeeded. The return
@@ -77,6 +78,37 @@
// 0 <= valueSize
size_t get(const void* key, size_t keySize, void* value, size_t valueSize);
+ // getFlattenedSize returns the number of bytes needed to store the entire
+ // serialized cache.
+ virtual size_t getFlattenedSize() const;
+
+ // getFdCount returns the number of file descriptors that will result from
+ // flattening the cache. This will always return 0 so as to allow the
+ // flattened cache to be saved to disk and then later restored.
+ virtual size_t getFdCount() const;
+
+ // flatten serializes the current contents of the cache into the memory
+ // pointed to by 'buffer'. The serialized cache contents can later be
+ // loaded into a BlobCache object using the unflatten method. The contents
+ // of the BlobCache object will not be modified.
+ //
+ // Preconditions:
+ // size >= this.getFlattenedSize()
+ // count == 0
+ virtual status_t flatten(void* buffer, size_t size, int fds[],
+ size_t count) const;
+
+ // unflatten replaces the contents of the cache with the serialized cache
+ // contents in the memory pointed to by 'buffer'. The previous contents of
+ // the BlobCache will be evicted from the cache. If an error occurs while
+ // unflattening the serialized cache contents then the BlobCache will be
+ // left in an empty state.
+ //
+ // Preconditions:
+ // count == 0
+ virtual status_t unflatten(void const* buffer, size_t size, int fds[],
+ size_t count);
+
private:
// Copying is disallowed.
BlobCache(const BlobCache&);
@@ -144,6 +176,46 @@
sp<Blob> mValue;
};
+ // A Header is the header for the entire BlobCache serialization format. No
+ // need to make this portable, so we simply write the struct out.
+ struct Header {
+ // mMagicNumber is the magic number that identifies the data as
+ // serialized BlobCache contents. It must always contain 'Blb$'.
+ uint32_t mMagicNumber;
+
+ // mBlobCacheVersion is the serialization format version.
+ uint32_t mBlobCacheVersion;
+
+ // mDeviceVersion is the device-specific version of the cache. This can
+ // be used to invalidate the cache.
+ uint32_t mDeviceVersion;
+
+ // mNumEntries is number of cache entries following the header in the
+ // data.
+ size_t mNumEntries;
+ };
+
+ // An EntryHeader is the header for a serialized cache entry. No need to
+ // make this portable, so we simply write the struct out. Each EntryHeader
+ // is followed imediately by the key data and then the value data.
+ //
+ // The beginning of each serialized EntryHeader is 4-byte aligned, so the
+ // number of bytes that a serialized cache entry will occupy is:
+ //
+ // ((sizeof(EntryHeader) + keySize + valueSize) + 3) & ~3
+ //
+ struct EntryHeader {
+ // mKeySize is the size of the entry key in bytes.
+ size_t mKeySize;
+
+ // mValueSize is the size of the entry value in bytes.
+ size_t mValueSize;
+
+ // mData contains both the key and value data for the cache entry. The
+ // key comes first followed immediately by the value.
+ uint8_t mData[];
+ };
+
// mMaxKeySize is the maximum key size that will be cached. Calls to
// BlobCache::set with a keySize parameter larger than mMaxKeySize will
// simply not add the key/value pair to the cache.