Reusing a compressor

[JonasIsensee]

This was found here: https://github.com/JuliaIO/JLD2.jl/issues/599

When using a ZstdFrameCompressor or a ZstdCompressor, you reproducably get a segfault when trying to reuse the same compressor instance:

I don't know if this can be made to just work, but I hope that this can at least be caught in time and throw an error instead.

using CodecZstd, TranscodingStreams

compressor = ZstdFrameCompressor()
x = rand(UInt8, 4*10^7);
TranscodingStreams.initialize(compressor)
ret1 = transcode(compressor, x);
TranscodingStreams.finalize(compressor)

# compress again using the same compressor
TranscodingStreams.initialize(compressor) # segfault happens here!
# ret2 = transcode(compressor, x);
# TranscodingStreams.finalize(compressor)

[1706176] signal (11.1): Segmentation fault
in expression starting at REPL[7]:1
ZSTD_CCtx_reset at /home/jisensee/.julia/artifacts/4c45bf9c8292490acd9463bbfbf168277d9720b6/lib/libzstd.so (unknown line)
ZSTD_initCStream at /home/jisensee/.julia/artifacts/4c45bf9c8292490acd9463bbfbf168277d9720b6/lib/libzstd.so (unknown line)
ZSTD_initCStream at /home/jisensee/.julia/packages/CodecZstd/KmRP9/src/LibZstd_clang.jl:277 [inlined]
initialize! at /home/jisensee/.julia/packages/CodecZstd/KmRP9/src/libzstd.jl:60 [inlined]
initialize at /home/jisensee/.julia/packages/CodecZstd/KmRP9/src/compression.jl:82

[nhz2]

Maybe the initialize and finalize methods in this package can be removed, and instead initialize on construction and finalize using a finalizer?

[nhz2]

@mkitti It looks like the finalize method cannot be made into a noop, and any use of a codec after calling finalize can lead to seg faults, so JLD2 will need to move those initialize and finalize calls.

#71 adds a finalizer to prevent memory leaks even if finalize is never called after compressing.

[mkitti]

No Julia call should ever segfault. Could we detect the condition and throw an error before segfaulting?

I'm also thinking we should completely redo the JLD2 compression mechanjsm...

[nhz2]

Yes, that requires adding checks for null pointers everywhere the codec is used.

[nhz2]

With #72 I get

julia> using CodecZstd, TranscodingStreams

julia> compressor = ZstdFrameCompressor()
ZstdFrameCompressor(level=3)

julia> x = rand(UInt8, 4*10^7);

julia> TranscodingStreams.initialize(compressor)

julia> ret1 = transcode(compressor, x);

julia> TranscodingStreams.finalize(compressor)

julia> TranscodingStreams.initialize(compressor)
ERROR: codec use after free
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] initialize(codec::ZstdCompressor)
   @ CodecZstd ~/github/CodecZstd.jl/src/compression.jl:83
 [3] top-level scope
   @ REPL[7]:1

The benchmarks are unaffected as well.

[JonasIsensee]

I'm also thinking we should completely redo the JLD2 compression mechanjsm...

Absolutely, I already have some ideas. The current version was an experiment that worked slightly too well for its own good.