Merge pull request #2708 from binhdvo/skippable

Add API for fetching skippable frame content

lib/decompress/zstd_decompress.c

@@ -380,6 +380,19 @@ unsigned ZSTD_isFrame(const void* buffer, size_t size)
     return 0;
 }
 
+/*! ZSTD_isSkippableFrame() :
+ *  Tells if the content of `buffer` starts with a valid Frame Identifier for a skippable frame.
+ *  Note : Frame Identifier is 4 bytes. If `size < 4`, @return will always be 0.
+ */
+unsigned ZSTD_isSkippableFrame(const void* buffer, size_t size)
+{
+    if (size < ZSTD_FRAMEIDSIZE) return 0;
+    {   U32 const magic = MEM_readLE32(buffer);
+        if ((magic & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) return 1;
+    }
+    return 0;
+}
+
 /** ZSTD_frameHeaderSize_internal() :
  *  srcSize must be large enough to reach header size fields.
  *  note : only works for formats ZSTD_f_zstd1 and ZSTD_f_zstd1_magicless.
@@ -503,7 +516,6 @@ size_t ZSTD_getFrameHeader(ZSTD_frameHeader* zfhPtr, const void* src, size_t src
     return ZSTD_getFrameHeader_advanced(zfhPtr, src, srcSize, ZSTD_f_zstd1);
 }
 
-
 /** ZSTD_getFrameContentSize() :
  *  compatible with legacy mode
  * @return : decompressed size of the single frame pointed to be `src` if known, otherwise
@@ -544,6 +556,37 @@ static size_t readSkippableFrameSize(void const* src, size_t srcSize)
     }
 }
 
+/*! ZSTD_readSkippableFrame() :
+ * Retrieves a zstd skippable frame containing data given by src, and writes it to dst buffer.
+ *
+ * The parameter magicVariant will receive the magicVariant that was supplied when the frame was written,
+ * i.e. magicNumber - ZSTD_MAGIC_SKIPPABLE_START.  This can be NULL if the caller is not interested
+ * in the magicVariant.
+ *
+ * Returns an error if destination buffer is not large enough, or if the frame is not skippable.
+ *
+ * @return : number of bytes written or a ZSTD error.
+ */
+ZSTDLIB_API size_t ZSTD_readSkippableFrame(void* dst, size_t dstCapacity, unsigned* magicVariant,
+                                            const void* src, size_t srcSize)
+{
+    U32 const magicNumber = MEM_readLE32(src);
+    size_t skippableFrameSize = readSkippableFrameSize(src, srcSize);
+    size_t skippableContentSize = skippableFrameSize - ZSTD_SKIPPABLEHEADERSIZE;
+
+    /* check input validity */
+    RETURN_ERROR_IF(!ZSTD_isSkippableFrame(src, srcSize), frameParameter_unsupported, "");
+    RETURN_ERROR_IF(skippableFrameSize < ZSTD_SKIPPABLEHEADERSIZE || skippableFrameSize > srcSize, srcSize_wrong, "");
+    RETURN_ERROR_IF(skippableContentSize > dstCapacity, dstSize_tooSmall, "");
+
+    /* deliver payload */
+    if (skippableContentSize > 0  && dst != NULL)
+        ZSTD_memcpy(dst, (const BYTE *)src + ZSTD_SKIPPABLEHEADERSIZE, skippableContentSize);
+    if (magicVariant != NULL)
+        *magicVariant = magicNumber - ZSTD_MAGIC_SKIPPABLE_START;
+    return skippableContentSize;
+}
+
 /** ZSTD_findDecompressedSize() :
  *  compatible with legacy mode
  *  `srcSize` must be the exact length of some number of ZSTD compressed and/or

lib/zstd.h

@@ -1441,6 +1441,26 @@ ZSTDLIB_API size_t ZSTD_compressSequences(ZSTD_CCtx* const cctx, void* dst, size
 ZSTDLIB_API size_t ZSTD_writeSkippableFrame(void* dst, size_t dstCapacity,
                                             const void* src, size_t srcSize, unsigned magicVariant);
 
+/*! ZSTD_readSkippableFrame() :
+ * Retrieves a zstd skippable frame containing data given by src, and writes it to dst buffer.
+ *
+ * The parameter magicVariant will receive the magicVariant that was supplied when the frame was written,
+ * i.e. magicNumber - ZSTD_MAGIC_SKIPPABLE_START.  This can be NULL if the caller is not interested
+ * in the magicVariant.
+ *
+ * Returns an error if destination buffer is not large enough, or if the frame is not skippable.
+ *
+ * @return : number of bytes written or a ZSTD error.
+ */
+ZSTDLIB_API size_t ZSTD_readSkippableFrame(void* dst, size_t dstCapacity, unsigned* magicVariant,
+                                            const void* src, size_t srcSize);
+
+/*! ZSTD_isSkippableFrame() :
+ *  Tells if the content of `buffer` starts with a valid Frame Identifier for a skippable frame.
+ */
+ZSTDLIB_API unsigned ZSTD_isSkippableFrame(const void* buffer, size_t size);
+
+
 
 /***************************************
 *  Memory management

tests/fuzzer.c

@@ -1830,6 +1830,46 @@ static int basicUnitTests(U32 const seed, double compressibility)
     if (memcmp(decodedBuffer, CNBuffer, CNBuffSize / 2) != 0) goto _output_error;
     DISPLAYLEVEL(3, "OK \n");
 
+    /* Simple API skippable frame test */
+    DISPLAYLEVEL(3, "test%3i : read/write a skippable frame : ", testNb++);
+    {
+        U32 i;
+        unsigned readMagic;
+        unsigned long long receivedSize;
+        size_t skippableSize;
+        const U32 skipLen = 129 KB;
+        char* const skipBuff = (char*)malloc(skipLen);
+        assert(skipBuff != NULL);
+        for (i = 0; i < skipLen; i++)
+            skipBuff[i] = (BYTE) ((seed + i) % 256);
+        skippableSize = ZSTD_writeSkippableFrame((BYTE*)compressedBuffer, compressedBufferSize,
+                                        skipBuff, skipLen, seed % 15);
+        CHECK_Z(skippableSize);
+        CHECK_EQ(1, ZSTD_isSkippableFrame(compressedBuffer, skippableSize));
+        receivedSize = ZSTD_readSkippableFrame(decodedBuffer, CNBuffSize, &readMagic, compressedBuffer, skippableSize);
+        CHECK_EQ(skippableSize, receivedSize + ZSTD_SKIPPABLEHEADERSIZE);
+        CHECK_EQ(seed % 15, readMagic);
+        if (memcmp(decodedBuffer, skipBuff, skipLen) != 0) goto _output_error;
+
+        free(skipBuff);
+    }
+    DISPLAYLEVEL(3, "OK \n");
+
+    DISPLAYLEVEL(3, "test%3i : read/write an empty skippable frame : ", testNb++);
+    {
+        unsigned readMagic;
+        unsigned long long receivedSize;
+        size_t skippableSize;
+        skippableSize = ZSTD_writeSkippableFrame((BYTE*)compressedBuffer, compressedBufferSize,
+                                        CNBuffer, 0, seed % 15);
+        CHECK_EQ(ZSTD_SKIPPABLEHEADERSIZE, skippableSize);
+        CHECK_EQ(1, ZSTD_isSkippableFrame(compressedBuffer, skippableSize));
+        receivedSize = ZSTD_readSkippableFrame(NULL, 0, &readMagic, compressedBuffer, skippableSize);
+        CHECK_EQ(skippableSize, receivedSize + ZSTD_SKIPPABLEHEADERSIZE);
+        CHECK_EQ(seed % 15, readMagic);
+    }
+    DISPLAYLEVEL(3, "OK \n");
+
     /* Dictionary and CCtx Duplication tests */
     {   ZSTD_CCtx* const ctxOrig = ZSTD_createCCtx();
         ZSTD_CCtx* const ctxDuplicated = ZSTD_createCCtx();