1/*
2 ** Copyright 2011, The Android Open Source Project
3 **
4 ** Licensed under the Apache License, Version 2.0 (the "License");
5 ** you may not use this file except in compliance with the License.
6 ** You may obtain a copy of the License at
7 **
8 **     http://www.apache.org/licenses/LICENSE-2.0
9 **
10 ** Unless required by applicable law or agreed to in writing, software
11 ** distributed under the License is distributed on an "AS IS" BASIS,
12 ** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 ** See the License for the specific language governing permissions and
14 ** limitations under the License.
15 */
16
17#ifndef ANDROID_BLOB_CACHE_H
18#define ANDROID_BLOB_CACHE_H
19
20#include <stddef.h>
21
22#include <utils/Flattenable.h>
23#include <utils/RefBase.h>
24#include <utils/SortedVector.h>
25#include <utils/threads.h>
26
27namespace android {
28
29// A BlobCache is an in-memory cache for binary key/value pairs.  A BlobCache
30// does NOT provide any thread-safety guarantees.
31//
32// The cache contents can be serialized to an in-memory buffer or mmap'd file
33// and then reloaded in a subsequent execution of the program.  This
34// serialization is non-portable and the data should only be used by the device
35// that generated it.
36class BlobCache : public RefBase {
37
38public:
39
40    // Create an empty blob cache. The blob cache will cache key/value pairs
41    // with key and value sizes less than or equal to maxKeySize and
42    // maxValueSize, respectively. The total combined size of ALL cache entries
43    // (key sizes plus value sizes) will not exceed maxTotalSize.
44    BlobCache(size_t maxKeySize, size_t maxValueSize, size_t maxTotalSize);
45
46    // set inserts a new binary value into the cache and associates it with the
47    // given binary key.  If the key or value are too large for the cache then
48    // the cache remains unchanged.  This includes the case where a different
49    // value was previously associated with the given key - the old value will
50    // remain in the cache.  If the given key and value are small enough to be
51    // put in the cache (based on the maxKeySize, maxValueSize, and maxTotalSize
52    // values specified to the BlobCache constructor), then the key/value pair
53    // will be in the cache after set returns.  Note, however, that a subsequent
54    // call to set may evict old key/value pairs from the cache.
55    //
56    // Preconditions:
57    //   key != NULL
58    //   0 < keySize
59    //   value != NULL
60    //   0 < valueSize
61    void set(const void* key, size_t keySize, const void* value,
62            size_t valueSize);
63
64    // get retrieves from the cache the binary value associated with a given
65    // binary key.  If the key is present in the cache then the length of the
66    // binary value associated with that key is returned.  If the value argument
67    // is non-NULL and the size of the cached value is less than valueSize bytes
68    // then the cached value is copied into the buffer pointed to by the value
69    // argument.  If the key is not present in the cache then 0 is returned and
70    // the buffer pointed to by the value argument is not modified.
71    //
72    // Note that when calling get multiple times with the same key, the later
73    // calls may fail, returning 0, even if earlier calls succeeded.  The return
74    // value must be checked for each call.
75    //
76    // Preconditions:
77    //   key != NULL
78    //   0 < keySize
79    //   0 <= valueSize
80    size_t get(const void* key, size_t keySize, void* value, size_t valueSize);
81
82
83    // getFlattenedSize returns the number of bytes needed to store the entire
84    // serialized cache.
85    size_t getFlattenedSize() const;
86
87    // flatten serializes the current contents of the cache into the memory
88    // pointed to by 'buffer'.  The serialized cache contents can later be
89    // loaded into a BlobCache object using the unflatten method.  The contents
90    // of the BlobCache object will not be modified.
91    //
92    // Preconditions:
93    //   size >= this.getFlattenedSize()
94    status_t flatten(void* buffer, size_t size) const;
95
96    // unflatten replaces the contents of the cache with the serialized cache
97    // contents in the memory pointed to by 'buffer'.  The previous contents of
98    // the BlobCache will be evicted from the cache.  If an error occurs while
99    // unflattening the serialized cache contents then the BlobCache will be
100    // left in an empty state.
101    //
102    status_t unflatten(void const* buffer, size_t size);
103
104private:
105    // Copying is disallowed.
106    BlobCache(const BlobCache&);
107    void operator=(const BlobCache&);
108
109    // A random function helper to get around MinGW not having nrand48()
110    long int blob_random();
111
112    // clean evicts a randomly chosen set of entries from the cache such that
113    // the total size of all remaining entries is less than mMaxTotalSize/2.
114    void clean();
115
116    // isCleanable returns true if the cache is full enough for the clean method
117    // to have some effect, and false otherwise.
118    bool isCleanable() const;
119
120    // A Blob is an immutable sized unstructured data blob.
121    class Blob : public RefBase {
122    public:
123        Blob(const void* data, size_t size, bool copyData);
124        ~Blob();
125
126        bool operator<(const Blob& rhs) const;
127
128        const void* getData() const;
129        size_t getSize() const;
130
131    private:
132        // Copying is not allowed.
133        Blob(const Blob&);
134        void operator=(const Blob&);
135
136        // mData points to the buffer containing the blob data.
137        const void* mData;
138
139        // mSize is the size of the blob data in bytes.
140        size_t mSize;
141
142        // mOwnsData indicates whether or not this Blob object should free the
143        // memory pointed to by mData when the Blob gets destructed.
144        bool mOwnsData;
145    };
146
147    // A CacheEntry is a single key/value pair in the cache.
148    class CacheEntry {
149    public:
150        CacheEntry();
151        CacheEntry(const sp<Blob>& key, const sp<Blob>& value);
152        CacheEntry(const CacheEntry& ce);
153
154        bool operator<(const CacheEntry& rhs) const;
155        const CacheEntry& operator=(const CacheEntry&);
156
157        sp<Blob> getKey() const;
158        sp<Blob> getValue() const;
159
160        void setValue(const sp<Blob>& value);
161
162    private:
163
164        // mKey is the key that identifies the cache entry.
165        sp<Blob> mKey;
166
167        // mValue is the cached data associated with the key.
168        sp<Blob> mValue;
169    };
170
171    // A Header is the header for the entire BlobCache serialization format. No
172    // need to make this portable, so we simply write the struct out.
173    struct Header {
174        // mMagicNumber is the magic number that identifies the data as
175        // serialized BlobCache contents.  It must always contain 'Blb$'.
176        uint32_t mMagicNumber;
177
178        // mBlobCacheVersion is the serialization format version.
179        uint32_t mBlobCacheVersion;
180
181        // mDeviceVersion is the device-specific version of the cache.  This can
182        // be used to invalidate the cache.
183        uint32_t mDeviceVersion;
184
185        // mNumEntries is number of cache entries following the header in the
186        // data.
187        size_t mNumEntries;
188    };
189
190    // An EntryHeader is the header for a serialized cache entry.  No need to
191    // make this portable, so we simply write the struct out.  Each EntryHeader
192    // is followed imediately by the key data and then the value data.
193    //
194    // The beginning of each serialized EntryHeader is 4-byte aligned, so the
195    // number of bytes that a serialized cache entry will occupy is:
196    //
197    //   ((sizeof(EntryHeader) + keySize + valueSize) + 3) & ~3
198    //
199    struct EntryHeader {
200        // mKeySize is the size of the entry key in bytes.
201        size_t mKeySize;
202
203        // mValueSize is the size of the entry value in bytes.
204        size_t mValueSize;
205
206        // mData contains both the key and value data for the cache entry.  The
207        // key comes first followed immediately by the value.
208        uint8_t mData[];
209    };
210
211    // mMaxKeySize is the maximum key size that will be cached. Calls to
212    // BlobCache::set with a keySize parameter larger than mMaxKeySize will
213    // simply not add the key/value pair to the cache.
214    const size_t mMaxKeySize;
215
216    // mMaxValueSize is the maximum value size that will be cached. Calls to
217    // BlobCache::set with a valueSize parameter larger than mMaxValueSize will
218    // simply not add the key/value pair to the cache.
219    const size_t mMaxValueSize;
220
221    // mMaxTotalSize is the maximum size that all cache entries can occupy. This
222    // includes space for both keys and values. When a call to BlobCache::set
223    // would otherwise cause this limit to be exceeded, either the key/value
224    // pair passed to BlobCache::set will not be cached or other cache entries
225    // will be evicted from the cache to make room for the new entry.
226    const size_t mMaxTotalSize;
227
228    // mTotalSize is the total combined size of all keys and values currently in
229    // the cache.
230    size_t mTotalSize;
231
232    // mRandState is the pseudo-random number generator state. It is passed to
233    // nrand48 to generate random numbers when needed.
234    unsigned short mRandState[3];
235
236    // mCacheEntries stores all the cache entries that are resident in memory.
237    // Cache entries are added to it by the 'set' method.
238    SortedVector<CacheEntry> mCacheEntries;
239};
240
241}
242
243#endif // ANDROID_BLOB_CACHE_H
244