fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

doc/decompressor_errata.md

@@ -12,6 +12,26 @@ Each entry will contain:
 The document is in reverse chronological order, with the bugs that affect the most recent zstd decompressor versions listed first.
 
 
+No sequence using the 2-bytes format
+------------------------------------------------
+
+**Last affected version**: v1.5.5
+
+**Affected decompressor component(s)**: Library & CLI
+
+**Produced by the reference compressor**: No
+
+**Example Frame**: see zstd/tests/golden-decompression/zeroSeq_2B.zst
+
+The zstd decoder incorrectly expects FSE tables when there are 0 sequences present in the block
+if the value 0 is encoded using the 2-bytes format.
+Instead, it should immediately end the sequence section, and move on to next block.
+
+This situation was never generated by the reference compressor,
+because representing 0 sequences with the 2-bytes format is inefficient
+(the 1-byte format is always used in this case).
+
+
 Compressed block with a size of exactly 128 KB
 ------------------------------------------------
 
@@ -32,6 +52,7 @@ These blocks used to be disallowed by the spec up until spec version 0.3.2 when
 
 > A Compressed_Block has the extra restriction that Block_Size is always strictly less than the decompressed size. If this condition cannot be respected, the block must be sent uncompressed instead (Raw_Block).
 
+
 Compressed block with 0 literals and 0 sequences
 ------------------------------------------------
 
@@ -51,6 +72,7 @@ Additionally, these blocks were disallowed by the spec up until spec version 0.3
 
 > A Compressed_Block has the extra restriction that Block_Size is always strictly less than the decompressed size. If this condition cannot be respected, the block must be sent uncompressed instead (Raw_Block).
 
+
 First block is RLE block
 ------------------------
 
@@ -72,6 +94,7 @@ block.
 
 https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L3527-L3535
 
+
 Tiny FSE Table & Block
 ----------------------
 

doc/educational_decoder/zstd_decompress.c

@@ -1018,12 +1018,7 @@ static size_t decode_sequences(frame_context_t *const ctx, istream_t *in,
     // This is a variable size field using between 1 and 3 bytes. Let's call its
     // first byte byte0."
     u8 header = IO_read_bits(in, 8);
-    if (header == 0) {
-        // "There are no sequences. The sequence section stops there.
-        // Regenerated content is defined entirely by literals section."
-        *sequences = NULL;
-        return 0;
-    } else if (header < 128) {
+    if (header < 128) {
         // "Number_of_Sequences = byte0 . Uses 1 byte."
         num_sequences = header;
     } else if (header < 255) {
@@ -1034,6 +1029,12 @@ static size_t decode_sequences(frame_context_t *const ctx, istream_t *in,
         num_sequences = IO_read_bits(in, 16) + 0x7F00;
     }
 
+    if (num_sequences == 0) {
+        // "There are no sequences. The sequence section stops there."
+        *sequences = NULL;
+        return 0;
+    }
+
     *sequences = malloc(num_sequences * sizeof(sequence_command_t));
     if (!*sequences) {
         BAD_ALLOC();

doc/zstd_compression_format.md

@@ -16,7 +16,7 @@ Distribution of this document is unlimited.
 
 ### Version
 
-0.3.9 (2023-03-08)
+0.4.0 (2023-06-05)
 
 
 Introduction
@@ -650,15 +650,16 @@ __`Number_of_Sequences`__
 
 This is a variable size field using between 1 and 3 bytes.
 Let's call its first byte `byte0`.
-- `if (byte0 == 0)` : there are no sequences.
-            The sequence section stops there.
-            Decompressed content is defined entirely as Literals Section content.
-            The FSE tables used in `Repeat_Mode` aren't updated.
 - `if (byte0 < 128)` : `Number_of_Sequences = byte0` . Uses 1 byte.
 - `if (byte0 < 255)` : `Number_of_Sequences = ((byte0 - 0x80) << 8) + byte1`. Uses 2 bytes.
             Note that the 2 bytes format fully overlaps the 1 byte format.
 - `if (byte0 == 255)`: `Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00`. Uses 3 bytes.
 
+`if (Number_of_Sequences == 0)` : there are no sequences.
+            The sequence section stops immediately,
+            FSE tables used in `Repeat_Mode` aren't updated.
+            Block's decompressed content is defined solely by the Literals Section content.
+
 __Symbol compression modes__
 
 This is a single byte, defining the compression mode of each symbol type.
@@ -1698,6 +1699,7 @@ or at least provide a meaningful error code explaining for which reason it canno
 
 Version changes
 ---------------
+- 0.4.0 : fixed imprecise behavior for nbSeq==0, detected by Igor Pavlov
 - 0.3.9 : clarifications for Huffman-compressed literal sizes.
 - 0.3.8 : clarifications for Huffman Blocks and Huffman Tree descriptions.
 - 0.3.7 : clarifications for Repeat_Offsets, matching RFC8878

lib/common/zstd_internal.h

@@ -366,13 +366,13 @@ typedef struct {
 
 /*! ZSTD_getcBlockSize() :
  *  Provides the size of compressed block from block header `src` */
-/* Used by: decompress, fullbench (does not get its definition from here) */
+/*  Used by: decompress, fullbench */
 size_t ZSTD_getcBlockSize(const void* src, size_t srcSize,
                           blockProperties_t* bpPtr);
 
 /*! ZSTD_decodeSeqHeaders() :
  *  decode sequence header from src */
-/* Used by: decompress, fullbench (does not get its definition from here) */
+/*  Used by: zstd_decompress_block, fullbench */
 size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
                        const void* src, size_t srcSize);
 

lib/decompress/zstd_decompress_block.c

@@ -706,11 +706,6 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
 
     /* SeqHead */
     nbSeq = *ip++;
-    if (!nbSeq) {
-        *nbSeqPtr=0;
-        RETURN_ERROR_IF(srcSize != 1, srcSize_wrong, "");
-        return 1;
-    }
     if (nbSeq > 0x7F) {
         if (nbSeq == 0xFF) {
             RETURN_ERROR_IF(ip+2 > iend, srcSize_wrong, "");
@@ -723,6 +718,11 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
     }
     *nbSeqPtr = nbSeq;
 
+    if (nbSeq == 0) {
+        /* No sequence : section ends immediately */
+        return (size_t)(ip - istart);
+    }
+
     /* FSE table descriptors */
     RETURN_ERROR_IF(ip+1 > iend, srcSize_wrong, ""); /* minimum possible size: 1 byte for symbol encoding types */
     {   symbolEncodingType_e const LLtype = (symbolEncodingType_e)(*ip >> 6);