I've found my problem:

SerializeToZeroCopyStream doesn't return any error because all the stream 
does is accumulate the input in a buffer before compression, so it doesn't 
even try to write the result yet.
The stream return error codes when calling Flush() and Close(). If I call 
those then I detect that there is an error from Zlib. But because I was 
relying on the stream destructor to flush, I was not getting any error.

On Tuesday, November 4, 2014 7:16:33 PM UTC+13, Antoine Bouthors wrote:
> Hi,
> I am trying to serialize messages to and from arrays using zlib 
> compression.
> Attached is a trivial example of what I'm trying to do.
> The code asserts at line 64: the messages read from the buffer is empty.
> My problem is: there is no indication whatsoever from protobuf that 
> something went wrong, either during encoding or decoding. I can only detect 
> something went wrong by comparing the decoded result against the original, 
> which is a bit of a pain.
> There are variations around that theme. If I add these two lines after 
> t.set_a():
>     t.set_b( 42 );
>     t.set_c( 42 );
> Then serialization still passes but deserialization fails. It doesn't tell 
> my why, though. Nothing is ever sent to the log handler.
> If I increase the size of the buffer prior to encoding, then everything 
> goes through fine and the assertion passes. Evidently, zlib has issues with 
> small buffers and sometimes the compressed result ends up being bigger than 
> the input. That's something I can understand, but it leaves me with a bunch 
> of questions:
>  - Why am I not getting any error on serialization when trying to 
> serialize into a buffer that's too small?
>  - Why am I not getting any error on deserialization when trying to parse 
> from a buffer that was too small?
>  - What should the size of my buffer be to guarantee that serialization 
> will pass? Even adding 1KB to it, I still have problems on some production 
> cases.
> Thanks,
> A.

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