[doc] Add decompressor errata document

Add a document that lists the known bugs in the decoder where valid
frames are rejected, along with the version that they were fixed.

doc/decompressor_errata.md

@@ -0,0 +1,84 @@
+Decompressor Errata
+===================
+
+This document captures known decompressor bugs, where the decompressor rejects a valid zstd frame.
+Each entry will contain:
+1. The last affected decompressor versions.
+2. The decompressor components affected.
+2. Whether the compressed frame could ever be produced by the reference compressor.
+3. An example frame.
+4. A description of the bug.
+
+The document is in reverse chronological order, with the bugs that affect the most recent zstd decompressor versions listed first.
+
+
+Compressed block with 0 literals and 0 sequences
+------------------------------------------------
+
+**Last affected version**: v1.5.2
+
+**Affected decompressor component(s)**: Library & CLI
+
+**Produced by the reference compressor**: No
+
+**Example Frame**: `28b5 2ffd 2000 1500 0000 00`
+
+The zstd decoder incorrectly rejected blocks of type `Compressed_Block` that encodes literals as `Raw_Literals_Block` with no literals, and has no sequences.
+
+This type of block was never generated by the reference compressor.
+
+Additionally, these blocks were disallowed by the spec up until spec version 0.3.2 when the restriciton was lifted by [PR#1689](https://github.com/facebook/zstd/pull/1689).
+
+> 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
+------------------------
+
+**Last affected version**: v1.4.3
+
+**Affected decompressor component(s)**: CLI only
+
+**Produced by the reference compressor**: No
+
+**Example Frame**: `28b5 2ffd a001 0002 0002 0010 000b 0000 00`
+
+The zstd CLI decompressor rejected cases where the first block was an RLE block whose `Block_Size` is 131072, and the frame contains more than one block.
+This only affected the zstd CLI, and not the library.
+
+The example is an RLE block with 131072 bytes, followed by a second RLE block with 1 byte.
+
+The compressor currently works around this limitation by explicitly avoiding producing RLE blocks as the first
+block.
+
+https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L3527-L3535
+
+Tiny FSE Table & Block
+----------------------
+
+**Last affected version**: v1.3.4
+
+**Affected decompressor component(s)**: Library & CLI
+
+**Produced by the reference compressor**: Possibly until version v1.3.4, but probably never
+
+**Example Frame**: `28b5 2ffd 2027 c500 0080 f3f1 f0ec ebc6 c5c7 f09d 4300 0000 e0e0 0658 0100 603e 52`
+
+The zstd library rejected blocks of type `Compressed_Block` whose offset of the last table with type `FSE_Compressed_Mode` was less than 4 bytes from the end of the block.
+
+In more depth, let `Last_Table_Offset` be the offset in the compressed block (excluding the header) that
+the last table with type `FSE_Compressed_Mode` started. If `Block_Content - Last_Table_Offset < 4` then
+the buggy zstd decompressor would reject the block. This occurs when the last serialized table is 2 bytes
+and the bitstream size is 1 byte.
+
+For example:
+* There is 1 sequence in the block
+* `Literals_Lengths_Mode` is `FSE_Compressed_Mode` & the serialized table size is 2 bytes
+* `Offsets_Mode` is `Predefined_Mode`
+* `Match_Lengths_Mode` is `Predefined_Mode`
+* The bitstream is 1 byte. E.g. there is only one sequence and it fits in 1 byte.
+
+The total `Block_Content` is `5` bytes, and `Last_Table_Offset` is `2`.
+
+See the compressor workaround code:
+
+https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L2667-L2682