I believe that the problem lies in DeflateStream>>deflateBlock

Especially the lines that compute lastIndex:

    [blockPosition < position] whileTrue:[
        (position + MaxMatch > writeLimit)
            ifTrue:[lastIndex := writeLimit - MaxMatch]
            ifFalse:[lastIndex := position].

Because the expectations below is:
deflateBlock: lastIndex chainLength: chainLength goodMatch: goodMatch
"Continue deflating the receiver's collection from blockPosition to lastIndex.
Note that lastIndex must be at least MaxMatch away from the end of collection"

And because lastIndex and position are 0 based.
While writeLimit is a size (65536).

I think that the code should rather be:

    [blockPosition < position] whileTrue:[
        (position + MaxMatch + 1 > writeLimit)
            ifTrue:[lastIndex := writeLimit - MaxMatch - 1]
            ifFalse:[lastIndex := position].

My failing example pass with above patch.

I'm about to commit it.
Before I do so, are there some extensive tests for zlib?

2017-03-31 21:13 GMT+02:00 Nicolas Cellier <nicolas.cellier.aka.nice@gmail.com>:

Hi,
some remember that from time to time, especially when the ZipPlugin is disabled, the DeflateStream fallback code is failing with a 'subscript is out of bounds: 65537'

I have a reproducible example with quite a large file

    | zip file |
    zip := (FileStream fileNamed: 'snapshot.bin.gz') binary contentsOfEntireFile asString.
    file := zip unzipped.
    file zipped unzipped = file

which gives me:

ByteString(Object)>>error:
ByteString(Object)>>errorSubscriptBounds:
ByteString(Object)>>at:
ByteString>>at:
ByteString>>byteAt:
GZipWriteStream(DeflateStream)>>updateHashAt:
GZipWriteStream(DeflateStream)>>insertStringAt:
GZipWriteStream(DeflateStream)>>deflateBlock:chainLength:goodMatch:
GZipWriteStream(ZipWriteStream)>>deflateBlock:chainLength:goodMatch:
GZipWriteStream(DeflateStream)>>deflateBlock
GZipWriteStream(DeflateStream)>>next:putAll:startingAt:
GZipWriteStream(DeflateStream)>>nextPutAll:
ByteString(String)>>zipped

When the plugin is enabled, the example works.
So I've been tracking the differences between DeflatePlugin and DeflateStream fallback code for some times without success.

What if I add a bound check in the DeflatePlugin?
Then the plugin equally fails. I mean it reads past zipCollection bounds.

So there's something bad in the implementation.
Why does the example seem to work is still mysterious to me.